O’scope Fun

My first FDIM was memorable for a variety of reasons, but the most exciting part for me was participation in the the Buildathon. The Buildthon, hosted by Rex Harper W1REX of QRPme unleashes 40 or so builders with soldering irons in a hotel ballroom. From a home brew perspective, that’s about as exciting as it gets. QRPme is the source for a large number of ingenious kits, and of course the famous MePads which facilitate Manhattan style scratch building.

Builders at the Buildathon purchase a small board kit, hopefully small enough that it can be assembled in a few hours in the trenches. Ususally this is something like a keyer, or a small CW audio oscillator, but this year the project was a bit different.

The focus this year was one of the pocket-sized Chinese portable oscilloscopes that are proliferating on the net these days. These scopes are single channel and typically have bandwidth of about 200 KHz, and cost between $20 and $40 dollars. You can find them in kit form, or assembled, with alligator clip leads or with inexpensive 1x-10x bnc scope leads. Wow, an o’scope for $20! – but what can you do with it?

That is the question that is addressed by this year’s Buildathon.

This year’s project was an departure from the norm for Buildathons. Instead of just a simple board kit, it combined an inexpensive piece of test equipment with a “trainer” board which supplied interesting signals to examine.

QRPMe O’sope Fun Board with DSO 150 scope

The kit from QRPme, called the O’scope Fun Board, contains several circuits which produce interesting signals you can look at with your $20 scope. The board provides a number of separate modules on a single board, along with standard pin headers that allow their interconnection. The list of functions, in order of increasing complexity:

  • a small piezo speaker
  • audio frequency oscillator at around 750 Hz
  • a microphone and preamp
  • an audio amplifier with volume control
  • a configurable 555 timer circuit
  • a single chip microprocessor, which outputs ASCII characters in a loop at 2400 baud

and some support functions:

  • a regulated 5v power supply (for the microprocessor chip)
  • a section of built-in MePads available for adding your own components/circuits

Ample opportunity to have fun with a small oscilloscope.

I had attended one of Rex’s Buildathons in the past, held at the 2012 LobsterCon, and have fond memories of rummaging on picnic tables for kit parts and soldering using battery powered soldering irons by the light of Coleman laterns at 2:00AM. In short, it was a blast.

So as soon as the announcement for this year’s Buildathon was made, I signed up. Then I thought: I’m a pretty handy builder, maybe it would be fun to assist at the event. I emailed Rex, and in short order was added to a list of eight “beta-builders” and general event helpers. Our kit orders would be fulfilled before FDIM and shipped to us directly, so that we could assemble the kit, make suggestions on the build and instructions, and generally become “experts” in advance so as to be most helpful during the event.

Along with seven other helpers, I received a beta version of the kit a couple of weeks before FDIM. We all quickly got to work building the boards, and proofing the construction directions. Since the board is intended as a training tool, we also tried out all of the circuits and experimented with the scopes to determine what worked well.

The boards we got were very “beta”, and had manual re-work done by Rex before sending them out to us. My build of the preview board went smoothly, and took about 90 minutes. I am a slow, methodical builder, doing a lot of checks along the way, so I think most folks could finish the kit in less time. Everything worked, and the only change I suggested was to increase the value of resistor in series with the white LED on the board, which was blindingly bright using the 1k resistor supplied.

After completing my board, I decided to add a few components to the MePad section to allow its use as a morse code practice oscilator, by routing the output of the audio oscillator to a 1/8th inch stereo jack, and then to the input of the audio amplifier. I added additional header pins to allow the connections to be made with jumper wires.

O’scope Fun Board implementation of CW practice oscillator

This worked fine, and allowed me to demonstrate a cool feature of the the DSO 150: you can turn off the trigger function altogether, and stream the signal to the display. By setting the timebase to about a second, I could see my CW wave forms go by as I keyed the circuit, as you can see here:

Moving on to checking out the small scope, the first thing I did was to make a list of suggested starting settings for each of the test circuits, as it was fairly easy to hook up the scope and see nothing because the vertical amplitude, horizontal time base, or the triggering was not set correctly.

Next up was a request from Rex to produce better documentation for the scope, since the instruction sheet included with the device could be cryptic to someone not familiar with oscilloscopes to begin with. One of the other helpers found an excellent online resource for the scope (more on this below), but I put together a quick “cheat sheet” listing all of the keys and corresponding functions. We wound up not using this, but I’ve uploaded it as a link on this site for anyone who is curious.

The scope we got is a Jyetech DSO 150 Shell scope. These can be found on eBay, Amazon, Alibaba and a bunch of other places. The units we got are from Circuit Specialists, and come assembled.

This little scope seems handy enough to me that I decided to take it to the next step, and add a rechargeable L-ion battery too it, eliminating the need to deal with the dangling 9v battery on a pigtail (which has the habit of dropping off just when you get the probe attached where you need it.)

You can find a number of YouTube videos on adding a rechargeable battery to the DSO 150, but the best solution I found was a kit produced by gianlucaarenadesign. You can find these on eBay. It is an ingenious solution: he provides a nicely 3D printed replacement back for the DSO 150, with room for a battery and requisite circuitry.

Replacement back for the DSO 150 scope with L-ion battery, charge controller and buck converter

The kit comes with the back, a charge controller for the L-ion battery, and a DC-DC buck converter to raise the 3.7v battery to the 9 volts needed by the scope. Also included is a sliding on/off switch (needed because the buck converted will eventually drain the battery if left connected, even if the scope is not turned on), and a cute transparent plastic “light pipe” to make the charging LEDs visible on the side of the case.

You have to provide the L-ion battery, some wire, and in my case a JST two wire connector to run from the buck converter to the main board. (Caution: the connectors available on Amazon have varying color coding, check the polarity you need on the board, and ignore the wire color code, which has a 50/50 chance of being wrong.) Optionally you can add a small diode at the output of the converter, I had one so I put it in.

All is explained in the excellent single sheet instruction which come with the kit. It’s an easy upgrade, and cost about $14 shipped from Italy. Yes I know, sounds like an expensive upgrade to a $20 scope, but if you really intend on using this thing portable I think it’s worth it.

So are these little scopes just a toy, or are they useful?

Certainly as a teaching tool for introducing someone to the uses of an oscilloscope you cannot beat the pricetag. Beyond that though, I think these instruments could be very useful additions to your toolkit. Certainly for troubleshooting audio circuits you are all set, plenty of bandwidth. Also, if you do any building with Arduino or Rasberry Pi projects, this scope can be very handy looking a states of GPIO pins and PWM output.

The fact that it is the size and weight of a small DMM means it can go in the tool bag, where bench scope stays at home on, well, the bench… So it probably won’t replace a good bench scope, but all in all a useful, and fun tool.
73
de N2HTT

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Hello, Antuino!

Having just come back from my first Four Days in May, hosted by QRP-ARCI, I have a lot to talk about. My first FDIM really exceeded my expectations; true, the Dayton Hamvention is an impressive event, but the activities and meeting the people at FDIM really made this first trip memorable. If you are interested in QRP or homebrewing gear, I can’t recommend it strongly enough.

There were a number of events I want to tell you about, but in this post I want to describe a new piece of test gear I brought back, which will really change the way I work with home brewed radios. This test instrument is called the Antuino, presented by Ashhar Farhan of uBitx fame. It places within reach of the amateurs’ budget circuit analysis of amplitude vs. frequency, formerly the domain of very expensive service monitors and network analyzers.

Antuino (labeling is my addition)

Antuino (labeling is my addition)

The Antuino is powered by an Arduino Nano v3, and consists of these functional blocks:

  • an SWR bridge
  • a calibrated receiver
  • an si5351 signal generator
  • an Arduino Nano
  • a 128×64 LCD graphical display, and encoder/pushbutton.

Using these blocks, under the control of the Nano, the Antuino provides the following functionality

  • a graphical antenna analyzer, plotting SWR vs. frequency
  • a scalar network analyzer, which lets you plot the frequency response of passive circuits, such as filters
  • a relative power analyzer, showing received signal strength vs. frequency

If you have ever wanted to check for spurious emmissions from your homebrew transmitter, check the response of a low pass filter, or just figure out the 2:1 SWR bandwidth of you antenna, this is the instrument for you. And all, in the true uBitx tradition, it will do it at about a $100 price point. You can’t beat that for functionality per dollar.

We all got excited about the Antuino when Ashhar gave a short presentation on it during the Thursday talks at FDIM, the the excitement really peaked when we found out that there were a large number of first production run instruments available for sale at Vendor Night, Thursday evening. A large line formed, but there were enough to go around, and I felt quite lucky to have been able to snag one. My understanding is that they will go on sale to the general public on June first, from the HF Signals website.

Of course, these are first run units, so they were not without a few minor warts out of the gate, (like the uBitx had.) There is virtually no documentation online yet, other than the schematic and the source code which is open source and hosted on github. (Ashhar mentioned that a software update will be available shortly providing some minor improvements.) The intent is to make the platform easily hackable, and I already have some ideas I want to try out.

Antuino is supplied in a plain black case without any markings or labels (I added my own), but applying power brings up the LCD display which tells the entire story. The key piece of information needed is that the SMA female connector near the centerline of the device is used for all inputs, the one near the top of the case is the output of the signal generator, used only for the SNA function. There is a mini-USB jack for programming the Nano, a 2.1mm barrel jack for input power, 9 – 12v, and a power switch on the top of the case. The rest of the UI is in the software.

There was a minor hardware issue with these first run units that a lot of folks ran into, myself included. The case contains a 6-cell AA battery tray – a full set of batteries will power the unit for about 4 hours. However we could’t get the unit to run from the batteries. Removing the board and resoldering the pins of the barrel jack above and below the board, and also the pins of the connector from the battery tray to the board solved all battery operation issues. I’m using NiMH batteries in mine, and it is very convenient to not have a power connection trailing the instrument.

My first Antuino experiments were hotel room-based, and consisted of checking the SWR of a 2 meter HT antenna (yes, the frequency range of the si5351 extends up 2m) and scanning the FM broadcast band with the HT antenna attached. The SWR plot looked believable if not very interesting, and there were definitely some peaks in the FM band that appear to correspond to strong local stations. But more interesting testing had to wait until I got home.

SWR sweep of a 2 meter HT antenna

SWR sweep of a 2 meter HT antenna

FM Broadcast Band

FM Broadcast Band

Sweeping your antenna for SWR is very simple:

  • use the encoder to move the highlight to the segments of the frequency display and select your center frequency for the scan by pressing the encoder to enter the field, turning the encoder to select the value, and pressing again to exit the field.
  • move the highlight to SWR and press the encoder to select.
  • move the highlight to the frequence range and select the width of scan
  • at this point the bar display shows the instantaneous value being measured
  • move the highligh to the PLOT button (it has a double frame when selected) and press the encoder to start the scan
Home screen setup for sweeping 80 meter SWR

Home screen setup for sweeping 80 meter SWR

The setup screen is replaced with a chart showing SWR vs. frequency. Press the encoder again to exit the chart back to the home screen. Here is a scan of my G5RV on 80 meters.

80 meter SWR plot

80 meter SWR plot

Using the SNA (Scalar Network Analyzer) function is also straightforward. Connect the upper SMA connector to the input of the circuit you wish to scan, and the output of the circuit to the sense input (central SMA connector.) Using the home screen, set you frequency, select the SNA function and bandwidth, and press PLOT.

Home screen setup to plot filter response

Home screen setup to plot filter response

The resulting plot will show the response of your circuit in dbm over the frequency range. Here is a plot of a low pass filter designed for 40 meters, which I use with my WISPR transmitter:

Low pass filter plot

Low pass filter plot

Now we get to the calibrated receive function of the Antuino. The key thing here that you have to be careful not to overload the receiver, you cannot connect your transmitter directly to it. Above 1mw input the receiver is swamped and basically does not display any signal. Higher power levels could damage the receiver. I tried to use a resistor network with a dummy load as a sniffer, but I could’t get the signals down into a readable range. I suspect there was a lot of leakage in addition to the voltage on the resistor network. What worked very well was about a foot of plain wire stuck into the female connector and draped over the sniffer and the dummy load I was transmitting into. This brought the signals down into a measureable range.

Dummy load sniffer with wire pickup

Dummy load sniffer with wire pickup

Although it is tempting to set the scanning range to a large value, like +/- 25MHz to look for spurious emissions from a transmitter, I found that this did not work very well, as the signal measured at the primary frequency was very low compare to that measured at say +/- 500kHz. My guess is that this is because there are probably a fixed number of steps in the graph, and at the wide scan you are probably stepping right over the fundemental frequency you are interested in. This theory could be confirmed by looking at the code, which I have not done yet.

A better approach is to use a relatively narrow range around the fundemental, make note of the peak value, and then perform the scan at the second, third etc. harmonic frequencies and compare results. A little cumbersome, but it works.

Home screen setup for power scan at 7050 kHz

Home screen setup for power scan at 7050 kHz

Home screen setup for power scan at 7050 kHz

Home screen setup for power scan at 7050 kHz

Here is the plot from my SKY-SDR at 7.050 MHz, and also at 14.1 MHz.

At 14.1 MHz the second scan looks close to 40db down compared to the first, which is what we would hope for.

Right out of the gate, the Antuino is proving to be an interesting and useful tool in the shack, and I’m sure it will only improve as the software evolves and folks come up with new and novel applications for it.

Keep your eye on the HF Signals website, and till next time

73
de N2HTT

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Goodbye, old Paint

I can’t believe that I haven’t made a post since the middle of August. The latter part of summer was absorbed by major family events, and the time just evaporated. Now as winter is approaching, and things have settled down a bit, I’m starting to return to my project bench. The primary project on the bench since summer has been a scratch-built transmitter using two 955 acorn triodes, which is nearly done and currently in the debugging stage.

I’ve got the 955 transmitter to the point where it’s doing something, but not exactly what I had in mind. At this stage of the project I would normally pull out my trusty BK Precision 20 MHz dual channel oscilloscope and start poking around, so that is exactly what I did. Hmm.. oscillation, that’s good, frequency, way off, not so good… – and then, suddenly, nothing. No trace, just the horizontal sweep. I tried different probes, different channels, different signal sources, all to no avail. My old friend was kaput.

I’ve had that scope for about 15 years. It was an eBay purchase, about $80 shipped, and at the time it was a really big deal, being my first oscilloscope ever. The 20 MHz bandwidth was just adequate for working on 40 meters and down, at 20 meters you could see a signal, but no real detail.

BK Precision 20MHz dual channel oscilloscope, doing its thing

BK Precision 20MHz dual channel oscilloscope, doing its thing

However all the bits worked, both channels, and it became an indispensable tool on the bench for scratch building, troubleshooting, and general electronic poking about.

So, when I had thoroughly convinced myself of its demise, two thoughts flashed through my head:

  • I can’t be without a scope right now, how will I ever complete the transmitter?
  • No way a am I going to try to fix this thing or spend a nickel trying to get it repaired. You probably can’t get parts for it anymore anyway.

Nope, if I’m going to replace it, I’m going to get a real scope like they talk about on Soldersmoke. One of those fancy digital scopes that does everything. I mean, how often do you upgrade your oscilloscope? This probably is my big chance.

I took a look on the web, and in pretty short order zeroed in on the clear choice (or at least it seems so to me) for a hobbyist level digital scope: the Rigol DS1054z. There are numerous enthusiastic reviews to be found on YouTube, such as this overview, and this in-depth review. Pretty much everyone seemed to agree that in terms of features and performance per dollar, you can’t beat this scope. I’m not going to detail the positives here, you can check out the links, but basically a 4-channel, 50Mhz scope with a large 7 inch display is pretty deluxe.

It took a couple of days to convince myself this was the right choice, and a couple of days after that an alarmingly large box from Amazon arrived. No worries though, the scope is actually only about the size of a large-ish lunch box. It was just very well packed.

It was a few days before I had the time to sit down and get acquainted with my new scope. I watched a bunch of YouTube videos to get familiar with the front panel operation for typical tasks. It became pretty obvious that I had traded my skateboard for a muscle car – this device will do any task I might ever need, and a whole bunch more that I won’t. Debugging 40 meter QRP tube transmitters – no problem.

One of the features of the new scope that absolutely captivated me is the ability to provide a frequency domain display via an FFT function. I have built a lot of scratch built QRP transmitters, and have dutifully added low pass filters to the outputs to insure adequate spectral purity. But up till now I have never been able to look at the output of transmitters to see if they meet the FCC requirements. This was the first thing I had to try.

For my test subject, I chose a Michigan Mighty Mite I built a few years ago (because it was featured on Soldersmoke BTW), to which I had added a 7-pole LPF from the recipe by George Dobbs, G3RJV. I have actually had this rig on the air, but was it okay? Now I know…

The waveform coming off the output link is pretty crufty, as you can see in the photo below:

Very not good looking transmitter output, before filter

Very not good-looking transmitter output, before filter

but the output of the LPF looks pretty good:

Pristine signal, post low pass filter

Pristine signal, post low pass filter

Now, to the magic of FFT. The unfiltered signal looks harmonic rich, and those spikes are only about 10db down – not good

The dirty secrets of the unfiltered signal revealed

The dirty secrets of the unfiltered signal revealed

But the output of the filter is very clean:

Signal cleaned up, and measured with cursor feature

Signal cleaned up, and measured with cursor feature

Only a small spike at the third harmonic. Using the cursor feature of the scope (yes, it has cursors! <arm pump/>) I measured the 3rd harmonic at 42db down – just making the FCC spectral purity regs. I can sleep at night again, knowing my MMM will not be polluting the airwaves, when and if I ever put it on the air again.

One controversial aspect of the DS1054z is the fact that the scope hardware can support more features than the firmware allows; you can field upgrade you scope to higher specs by purchasing a license key from Rigol. The extended capabilities include among other things: 100Mhz bandwidth, more capture memory, and software decoding of communication protocols like RS-232 and I2C. That latter sounds interesting to me, as I have done a bunch of Arduino sketches using I2C – it could come in handy.

Rigol DS1054Z showing option count-down

Rigol DS1054Z showing option count-down

The scope ships with all the extended features enabled for the first 40 hours of operation, so you get hooked on using them. Then they expire and turn off. The license keys from Rigol are quite expensive, you could nearly double the cost of the scope by purchasing a key.

Here’s the controversial part: it is very easy to hack the scope. A quick web search will return numerous YouTube’s, blog articles, and links on how this is done. I’m not going to provide any links here, if you are interested, go look. But lot’s of folks just unlock all features (you will hear about this a lot on YouTube how-to videos concerning teh DS1054z), but there is a down side: hacking the scope voids the 3 year warranty.

So I’ve decided not to hack – but to wait. If I find one of the extended features is a can’t-live-without, or it’s three years from now and the warranty is over, then maybe. But for now, what the firmware allows is more than enough scope for my needs.

And what will happen to my old BK? Well I thought about it, and decided that the odds were low (and the returns not worth it) that someone might want to buy a broken 30-year-old scope at a hamfest. I certainly don’t want to undertake troubleshooting it myself, and it would not be worth investing any money in a professional repair. So with the absolute minimum amount of sentiment I could muster, I brought the slightly rusted but beloved instrument to that big recycling center in the sky. (Well, actually near Oneonta)

Good bye, old Paint, and

73,
de N2HTT

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Automating the Eclipse

We did not include plans to travel to view the total eclipse taking place this August, it just didn’t work out with other family obligations scheduled for late this summer. So I was really excited to learn about the Solar Eclipse QSO Party, a neat way to participate in some crowd-sourced science from our location in the Northeast, where we will only see a partial eclipse. I made plans to take the day off (Monday August 21) and operate from the alt-QTH in New York’s Southern Tier. QRP operation and simple wire antennas.

RBN, here I come.

The folks running the experiment/event, HamSci, have a very nice resource page for the SEQP. Among other things, it notes that the very popular logging program, N1MM logger, has been updated to include a contest template for the SEQP. N1MM is a very capable program, but one that I have not used in the past. However for this event I downloaded it, installed it, and got it up and running with a minimum of fuss. My original intent was to use it for logging only, performing all other functions manually. Reviewing all the capabilities that could be mine if I interfaced my rig (Elecraft KX3) was sorely tempting, and I began to play with hooking the rig up to the computer. From there it was a slippery slope indeed…

Elecraft provides a USB cable for CAT operation and firmware updates for the KX3, along with a very nice utility program. I have used these tools for updating the rig, but not for actual operation, although the utility program provides CW and digital mode capability.

Knowing that this functionality works, I carefully reviewed the N1MM documentation for interface instructions. They seemed almost too simple. With a few mouse clicks, I had my KX3 connected in SO1V (single operator one VFO) mode, reporting band and frequency changes to the N1MM window. Easy. But wouldn’t it be nice to have all those N1MM macro buttons working?

Checking the N1MM documentation, and then some general web searching revealed the following pieces of information:

  • the KX3 does not support the DTR/RTS signals needed for hardware CW keying on the ACC1 serial port used for CAT operations [CAT is an acronym for computer control of a transceiver over a serial port. Couldn’t find the origin of the acronym on the web – n2htt].
  • Computer driven CW from the Elecraft Utility program is accomplished through the CAT commands KY and KYW, which instruct the radio to send the accompanying ASCII text as Morse code
  • the N1MM documentation specifically states that CW operation via CAT commands is not supported – they suggest the use of third-party hardware interfaces for CW

Dang. I don’t have a third-party interface handy. I did have a hardware interface that I had built a while back, that used USB to serial port conversion to toggle the DTR line for keying my Yaesu FT-897. I played with this using the KX3 ACC2 input configured for PTT, but could not get it to go.

Some of the web results I found seemed to indicate that despite N1MM documentation to the contrary, limited support of CAT based CW was possible with the KX3 (thanks K4MTX). I then spent most of a perfectly lovely Saturday indoors in front of computer screen, and in the end got it to work… pretty much.

N1MM logger hooked up to the KX3

N1MM logger hooked up to the KX3

Sparing you the exquisite details of everything I tried, here is what the problems are that must be overcome:

  • the Elecraft KY command can only handle a 26 character message. The KYW command allows concatenating as many messages as you like seamlessly, so that is not a radio side limitation.
  • A problem occurs when an N1MM macro, like {EXCH}, expands out to make your message longer than 26 characters. When this occurs, your attempt to send the message results in nothing. No error, no CW, just silence.
  • N1MM polls the radio periodically for status, and if the radio does not respond in time, N1MM reports a timeout and presents a dialog asking if you wish to continue talking to the rig. Say yes. This was occurring a lot during long CAT controlled CW messages, the EXCH macro was particularly bad. (N1MM provides a setting to increase the timeout by 50 or 100%, but even at 100% the time-out still occurred.)

The solution for making (unsupported) CAT driven CW work with the KX3 is to do the following:

  • wrap each N1MM macro in its own KYW command including a trailing space
  • break up the macros into as small chunks as possible to minimize time-outs

My macro setup requires three button presses to respond with an exchange, as I separated the call signs (yours and his) out from the exchange macro. Slightly clumsy but it works.
Here then is my macro file for the KX3 and N1MM supporting CAT driven CW:

------------------ clip here -----------------------------
###################
# RUN Messages 
###################
F1 CQ SE,{CAT1ASC KYWCQ CQ SE ;KYW{MYCALL} ;KYW{MYCALL} ;KYWK;}
F2 EXCH,{CAT1ASC KYW {SENTRSTCUT} ;KYW{EXCH} ;KYW{MYCALL} BK;}
F3 TU,{CAT1ASC KYW{TU {MYCALL};}
F4 {MYCALL},{CAT1ASC KYW{MYCALL};}
F5 His Call,{CAT1ASC KYW{CALL};}
F6 Repeat,{CAT1ASC KYW{SENTRSTCUT};KYW{EXCH};KYW{EXCH};}
F7 Grid?,{CAT1ASC KYW GRID?;};
F8 Agn?,{CAT1ASC KYWAGN?;};
F9 Rpt?,{CAT1ASC KYWRPT?;};
F10 Call?,{CAT1ASC KYWCL?;};
F11 Spare,
F12 Wipe,{WIPE}
###################
# S&P Messages 
###################
F1 QRL?,{CAT1ASC KYWQRL? DE {MYCALL};}
F2 EXCH,{CAT1ASC KYW{SENTRSTCUT} ;KYW{EXCH} ;KYW{MYCALL} BK;}
F4 {MYCALL},{CAT1ASC KYW{MYCALL};}
F5 His Call,{CAT1ASC KYW{CALL};}
F6 Repeat,{CAT1ASC KYW{SENTRSTCUT};KYW{EXCH};KYW{EXCH};}
F7 Grid?,{CAT1ASC KYW GRID?;};
F8 Agn?,{CAT1ASC KYWAGN?;};
F9 Rpt?,{CAT1ASC KYWRPT?;};
F10 Call?,{CAT1ASC KYWCL?;};
F11 Spare,
F12 Wipe,{WIPE}
------------------ clip here -----------------------------

Happy eclipse QSOing!
73,
de N2HTT

 

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Roll your own

It’s the height of summer, and we are enjoying a week’s vacation at the alt-QTH in New York’s beautiful Southern Tier. The weather is warm and dry, with occasional afternoon thunder showers keep everything green, and the pace is slow and peaceful. I usually bring up lots of radio projects to occupy my time, and this year was no exception.

I have always been very interested in portable QRP operation, and have been building out a couple of portable kits for that purpose. The most recent features the MTR-4B trail-friendly rig, combined with a SOTABeams 30 foot fiberglass mast, all of which fits easily in a day pack. I have long been a fan of end-fed half-wave (EFHW) antennas, and have a couple made up on 40 meters – with a small tuner these wires can be used on multiple bands. They require only one point of support, virtually no feed line, and are lightweight, good performers.

Just a couple of weeks ago, I came across a blog post by Michael, G0POT, where he describes his experience building a trapped EFHW for three bands, 20m, 30m, and 40m. This antenna can be used without a tuner on those bands, which is a hiking plus – one less piece of gear to carry.  The key to the antenna’s success is a 1:60 impedance transformer, or unun, which can be built in a small box. This becomes the rig-end termination of the antenna. Michael provides links to PA3HHO’s blog, with good pictorial instructions on how to wind this unun on a ferrite core. The resulting antenna is lightweight, needs no additional accessories, and performs well – it has become my portable favorite.

Now we come to the vacation story. I bought two toroids to make the unun, and still had the second one with me. I also have an old 1:4 balun built from a kit that I never use. Sitting on the deck looking for something to do, I decided to re-purpose the box and fittings from the balun to house a second 1:60 unun. These things just seem too useful not to have a spare on hand.

Well no problem, I brought tools and materials to build with, but unlike the primary QTH there is no well stocked bench here. And now, with the demise of the local Radio Shack store, no handy place to pick up that odd resistor you don’t happen to have.
Well, the odd resistor was in fact the issue. When I had made the first unun, I tested with my antenna analyzer and a 3.3 kOhm resistor, just to verify that I was seeing around 50 ohms (plus or minus) with the resistor across the output terminals. Just a sanity check, I’m never quite sure I’ve counted the turns correctly. I did not have the resistor with me in the kit…

Of course I could just build the thing and check it out with an EFHW attached to see how it performs, but it got me thinking. If I were stranded on a deserted island with a rig that needed a 3.3 kOhm resistor to become functional to call for help, and had only typical household items available (okay so not too deserted an island. Think abandoned resort with no Radio Shack nearby) could I fix the rig?

I asked myself the “what are resistors made of?” question. Precision metallic oxide films seemed beyond the realm of homebrew possibility, but what about good old carbon? We have lots of old crappy pencils lurking in the kitchen drawers, so I tracked one down that didn’t look too pristine, cut the eraser end off, and measured the resistance of the lead – only 16 Ohms, very low, but definitely promising.

To increase the resistance, I figured you have to reduce the cross-section of the material, so I tried making a dark pencil mark on a 3×5 index card and measured the resistance – about 10 Megohms. This could work.

My lovely XYL, Janette KC2LLA, supplied a very soft drawing pencil (more graphite, less clay in the lead), and I started systematically filling in dark lines of pencil lead on the 3×5 card, checking the resistance as I went. It continued to decrease to about 100 kOhms, then kinda stalled as I added more graphite. It occurred to me that the point of contact, (the meter leads) might be having an effect, so I added two alligator clips across the stripe, and the resistance dropped dramatically.

With the alligator clips in place, adding more graphite continued to reduce the resistance, but with less and less effect as the width of the graphite area moved out beyond the end of the alligator clips. It seemed to bottom out at around 6 kOhms – at this point Jan had a brilliant idea: cut the graphite area in two, and connect the two pieces in parallel.

Graphite covered index card pieces, ready for parallel hookup

Graphite covered index card pieces, ready for parallel hookup

I placed the two cut pieces, graphite sides facing out, between the clips and voila – 3 kOhms! By moving the position of the clips slightly I was able to dial in 3000 Ohms pretty precisely.

3k on the button!

3k on the button!

After assuring myself that the resistance reading was stable and repeatable, I proceeded to build the unun. Testing with the home-made resistor showed that the windings were fine.
The unun went in the box, and a re-check with my paper resistor showed good results.

Testing the 1:60 unun with a paper resistor!

Testing the 1:60 unun with a paper resistor!

It’s unlikely that the paper resistor will find a permanent home in my test equipment, but it was a fun and interesting experiment in kitchen physics that worked surprisingly well.

73,
de N2HTT

 

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Nice guys.

I’m very excited about operating out-of-doors this season. It seems to be in the air: the May 2017 issue of QST is featuring portable operating, with a nice assortment of hiker-ish tools adorning the cover. It wasn’t that bad a winter as winters go, but apparently we are all aching to get outside and get a little fresh air. And play radio, of course.

So for the last few weeks I have been getting together my kit to try my hand at SOTA (Summits on the Air). Basically, take a hike, climb a hill, set up a state-of-the-art QRP station, work a pile-up, pack up and go home. And, post lot’s of photos…

In addition to the physical fitness aspects of this ham radio niche (I’m working on that – actually, that’s part of my sudden fascination with hiking), there’s the challenge of setting up a station in any and all conditions. And, the toys are just the best.

I recently purchased a trail-friendly QRP rig, the MTR-4B from LNR Precision. I have only just started to use it, but I am really liking this little rig. Really essential QRP radio, nothing extra but everything you could want. I’ll probably do a write-up on it in another post, but in this post I want to discuss a critical accessory that took me a while to figure out. The fiberglass, telescoping, antenna mast.

Naturally, operating en plein air requires an antenna, and those pesky things generally need to be elevated into the air, the higher the better. There are two basic models for portable antenna supports:
“Throw a wire into a tree”
and
“The lightweight telescoping fiberglass push-up mast”

You really need to be prepared to employ either when you arrive at your summit. Of course, you may find a perfect, conveniently placed tree to throw a wire up into. (I wrote a blog post about this a while back.) Then again, you may not. Or, you may not want to, because you are in park land where you are not permitted to attach anything to the trees. In that case, the second choice is the way to go.

There are readily available, inexpensive fishing poles (called “squid poles” or “crappie poles”) that can be used to support an antenna at a reasonable height. These are typically 16 – 25 feet long when extended, but they suffer from two drawbacks: first, being intended for fishing, they are extremely flexible especially at the tip, and the weight of a wire antenna can cause the end to bend over sharply, and even break under the right (wrong) conditions. The other disadvantage is that collapsed, these things are about 4 1/2 feet long, or longer. You could pretend it was a hiking staff, but it seems just a cumbersome thing to carry on a hike up a hill.

No, what you want is a purpose-made fiberglass mast designed with hiking in mind. The one I have is from SOTABeams, extends to about 33 feet, and collapses down to about 26 inches. It weighs about three pounds, not a trivial load while hiking, but definitely worth its weight in altitude. This mast will easily support an end-fed half wave dipole (my favorite wire antenna) at a point a couple of feet from the top without bending over. Perfect… except it needs to be guyed in place.

Here’s the rub: it’s a beautiful day, you’ve hiked to the top of your local summit ready to deploy your station, but you are alone. Everyone had something better to do than take a hike to place where they could hold your mast up while you set up the guy lines. So now you are trying to somehow balance the mast against a rock or your daypack while trying to get the guy lines snug, running around in a circle over and over. It can’t be done. I know.

So it became necessary to figure out how to put up a 30 foot mast with four guy lines alone, and I think I’ve figured it out. All you need to do is a little one-time prep work before you head out, and you’ll be able to set up your mast alone in just a couple of minutes without even breaking a sweat. The answer makes use of simple trigonometry. (If simple trigonometry is not available in your area, regular trigonometry will do.)

The guy line scenario usually consists of four (but you can get away with three) guy lines attached to the mast some ways up the pole, extending out at a 45 degree angle, and anchored in the ground. The method I’m about to describe will work for any guying system, but since I purchased a guying kit with my mast from SOTABeams, I’ll describe how it works with that. My kit consisted of a guying ring (actually a plastic square with a round hole that fits around mast, plus 4 attachment holes), and a generous length of fluorescent yellow lightweight cord. You divide the cord into 4 equal parts, and tie each part into one of the attachment holes using a bowline knot. This is the starting point for my single-handed guying technique.

But first, a word about knots. This technique relies on using specific knots, and I’ll tell you what they are, but I’m not going to explain how to tie them. Look them up on Youtube, you can find much better descriptions than I could give. But do use these specific knots, it matters.

Okay here goes: the one-time preparation you need to do to your guy lines.

  1. Place the guy ring on your mast, and measure how high up from the base it rests. You can do this with no wires or lines attached by extending the mast out horizontally on the ground. This measurement is very important, so make sure the guy ring is sitting snug on the mast before you make the measurement. With my mast and setup, the ring is 10 feet 6 inches above the base of the mast.
  2. For each guy line, measure a distance equal to the height of the ring from the point where the line attaches to the ring. At this point, take a bight of the line (a bight just means fold the line and make an open loop) and tie an overhand knot, to create a permanent loop in the guy line at that point. After this step, I had a permanent loop 10 feet 6 inches from the attachment point in each of my guy lines.

    A Permanent Loop (bight tied in an overhand knot)

    A Permanent Loop (bight tied in an overhand knot)

  3. Here’s where the trigonometry comes in: each guy line when taut forms a triangle, with one point being the guying ring, one point the base of the mast, and one the point of attachment to the ground. We want the angle at the ground attachment point to be 45 degrees, and that means that the ground attachment point will be exactly as far away from the base of the mast as the guy ring is up the mast. (10 feet 6 inches in my case).
    And we can also compute the length of the guy line from the ground attachment to the guy ring, it will be 1.4 times the height of the guy ring (trigonometry) or in my case 14.85 feet.

    Detail drawing explaining trigonometry

    Detail drawing explaining trigonometry

    Do this calculation for your guy ring height, and round the answer up the next foot, because we want a little slack. I used 15 feet as my value. Measure this distance from the attachment point of the guy line, and tie another permanent loop at this point. Do this for each guy line.

So now you have 4 guy lines, each with two permanent loops tied in them at the same distances.

Guy line with two permanent loops tied in

Guy line with two permanent loops tied in

You are now ready to put up your mast alone. Here’s how to do it:

  1. Select the spot where the base of the mast will go, and stake the guy ring on the ground through each of the attachment loops. Stretch each line out straight in the direction it needs to go. For four lines, each opposite guy should line up straight.

    The Stakeout

    The Stakeout

  2. For each guy in turn, take the stake from the guy ring loop, and push it into the ground where the first permanent loop is. You don’t want to put the stake through the loop, just next to it using the loop as a location marker.

    Use the first loop to position the stake.

    Use the first loop to position the stake.

  3. For each guy, place the second permanent loop around the stake and anchor it. We’re done with the first loop.
  4. Place the mast on the spot where it will go, and start to extend it through the guy ring. Don’t forget to attach your antenna after you’ve got the mast going through the guy ring. Really.

    Starting the mast up

    Starting the mast up

  5. Extend the mast a section a time. Ignore the guy lines, pay attention to the antenna wire which is going all sorts of places you don’t want it to.

When the mast is fully extended, you can let go. The guys will be loose, but have enough tension to hold the mast upright, although leaning at an angle.

Self-standing, but leaning over

Self-standing, but leaning over

Now it’s time to adjust the guys for proper tension.

To make a tensioning mechanism we’re going to use two knots: a slip loop, and a trucker’s hitch. Go look them up, and then come back here. (Here is a nice YouTube video that demonstrates all the knots I have mentioned.)

Okay, start with guy line opposite the direction the mast is leaning. We will do this on all guys in turn, always moving next to the guy line opposite the lean.

  1. Tie a slip loop in the guy line about three or four feet up from the stake.
  2. Grab the line, and slip the permanent loop off the anchor. (Don’t let go of the line.) Loop the standing part of the line around the stake so it can move freely, and pull it up so there is a little tension on the line.
    Pull the free end of the guy line through the slip loop, and pull it to tension it the way you want. Tie a trucker’s hitch around both parts of the line to secure it.

    Guy line tensioned and secured with trucker's hitch

    Guy line tensioned and secured with trucker’s hitch

Voila, a beautiful, self-standing mast you put up all by yourself. You can tweak the tension on the guy lines until the mast is perfectly vertical. Now what was it I was going to do with this mast… oh, yes..

Beautifuly guyed mast, which we needed for some reason..

Beautifuly guyed mast, which we needed for some reason..

I know this guying process sounds a little complex, but believe me after you have done it fifty or sixty times it will become second nature. And speaking of nature, what could be better than enjoying the outdoors and ham radio at the same time, hanging around with a bunch of nice guys.
73, de
N2HTT

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A cootie?

Settling in for the long winter season, I have lots of bench projects waiting in the queue. Some have time constraints and are higher priority, such as getting my HX-1681 back on the air in time for this years Novice Rig Round-up (0000 UTC on FEB 18 through 2359 UTC on FEB 26, 2017). Others, like finishing the one tube regen I started, or repairing half a dozen broken items, will rise to the top of the list in due time. And there are some new home brew ideas I am eager to start, definitely including another QRP tube transmitter this year.
So with all these worthy items awaiting attention, how do I spend the little bench time I had available over the last two weeks? I built a cootie key, of course.

The "Depot" cootie key

The “Depot” cootie key

A cootie key, or sideswiper as they are also called, is a key that is operated with a horizontal motion of the hand, rather than the up-and-down motion of a straight key. It has two contacts, left and right, and unlike a paddle a key closure on either side closes the same circuit. Cooties go way back; they were the first attempt to solve the problem of “glass arm” (what we call a repetitive strain injury today) which was the frequent result of too much straight key activity. It is said that the first cooties were made by enterprising telegraphers who mounted two straight keys base to base on their sides, with the keys wired in parallel. Later, commercially made cooties appeared, touting less fatigue and faster keying speed as advantages.

In the early 20th Century the “bug” or semi-automatic key made its commercial debut. Since that key automatically completes the dits, the telegrapher does less work, and soon the bug pretty much completely supplanted the cootie key and they more-or-less disappeared. Today the advent of inexpensive electronic keyers and the paddle, which uses two separate circuits for the dot and dash, have pretty much relegated bugs and cooties to the attic, to be used only by aficionados of retro and antique gear.

But there is one other group of hams who have traditionally embraced the cootie: those seeking a key for next to no cash outlay. Cooties can be made out of any piece of springy steel and two contact points. Hacksaw blades are commonly used, but I have seen pictures of cooties made from steak knives jammed point first into a block of wood! This is a technology that lends itself to improvisation, and that is what ham radio is all about.

Sending with a cootie, or side-swiping, is tricky business. Basically, you start each character on the same side of the key (I’m right-handed, I start with my right index finger first), and then alternate thumb and finger creating the elements of the character. The timing is all on you, much as it is with a straight key. I found the technique difficult at first, but with some practice still find it difficult. My inclination is to start each character with whatever finger was left over after completing the last, but I can see that method quickly leads to disorganization and madness. The always-start-on-the-same-side approach makes sense if you think about each character as a separate unit, always formed the same way. Harder to learn, but I think you get a better sounding result.

I have actually owned a very fancy cootie key for some time now, made by Llaves Telegraphicas Artesanas (LTA) in Spain. The gentleman who made these keys, Guillermo Janer, EA6YG, has been a silent key since 2008, but you can still see a very nice page describing the keys he made at Morse Express.

LTA cootie, on a nice chunk of granit countertop

LTA cootie, on a nice chunk of granite countertop

This key is beautifully crafted, but really awkward to use. With the pivot in the center the throw is short, and the stiff brass arm has no give upon making contact, making for a very jerky feel when keying. After several tries to get used to it, I gave up and it sits very handsomely on the shelf where I display my key collection.

I got interested in cooties again when just recently one came up for sale on a club swapmeet listing. This was a cootie made by Vizkey. It gets excellent reviews, and I got intrigued. The Vizkey version is based on the venerable hacksaw blade which simultaneously provides the contacts and the spring tension for the key. These keys are a bit pricey though, and I wondered if there was a way to experiment with cooties that would require less commitment.

On to steak knives and blocks of wood.. I researched inexpensive home brew options, and came up with this really nice design by Mike Maynard, KC4ICY. His Depot Key, so named because all the parts can be obtained from, well, you know. Anyway it looked like a nice inexpensive way to play with a cootie, and being based on a hacksaw blade I figured it would be pretty much the authentic side-swiping experience. Mike estimates that his version of the cootie can be built for about $10. I think the actual number is closer to $20, but I didn’t have any old hacksaw blades handy and the Depot only had packages of two available. I did have most of the rest of the hardware on hand, as well as the wooden base (leftover from the external VXO project) and the wood finishing supplies (leftover from the Bayou Jumper.) I bought the angle irons, the hacksaw blade, and the 1/8 stereo plug which I was out of at the moment. And the buttons for the finger pieces.

My first attempt at finger pieces was to cut out two rectangles from a heavy polyethylene container. They looked awful, so I went searching through the craft department of our local Wal-Mart, and found a package of six really cheap coat buttons for $2. The buttons had flat backs so they fit together nicely around the hacksaw blade, and a #6 bolt fit the hole perfectly.

Coat button finger pieces

Coat button finger pieces

Construction was very straightforward. I cut the hacksaw blade to length using a pair of tin snips (don’t forget the safety goggles when working with springy steel), and drilled a hole near the cut end to accommodate the fingerpiece attachment. The blade was a little tough to drill, but no big deal really, just go slow and perhaps use a drop of oil on the bit. I used a wire brush in my Dremel tool to remove the paint from the hacksaw blade, as it was a rather garish white and yellow color scheme.

In a departure from Mike’s design, I added a bolt across the top of the angles holding the contacts, so I would only have to wire one side. This eliminates the possibility of using the key as a single lever paddle, but adds some rigidity to the contact posts.

Strain relief

Strain relief

The only other feature I added was a staple and a dab of hot glue to act as a strain relief for the cable, which is a short length of RG-174 coax. I put a piece of non-skid foam backing, (the kind used under rugs) on the underside, held in place with a few pieces of double-sided scotch tape. Although key is very light, this arrangement holds it in place just fine, and there is no need to hold the key with your other hand while keying.

Non-slip rug backing

Non-slip rug backing

So how does it play? Not bad, actually. The springiness of the blade, and the soft landing with a little give on the contacts is very pleasant. It makes a little noise when keying, but not nearly as bad as the LTA key, which clanks dramatically with each stroke.

I’ve been practicing off the air with it, and while I like the feel and the side-to-side keying, I still sound pretty awful. There is a tendency to run the dits together, and shorten the dahs. Based on internet advice, I set the contacts pretty far apart – this is supposed to help avoid the running together.

There are many online resources for on the art of sideswipery – the SideSiperNet website lists nets and other information, and has a gallery of keys posted by cootie afficianados that is very interesting to browse through. You can also find a short essay on the practice of side-swiping at Morse Express. The also feature another commercial cootie, the GHD-501 (see the bottom of the page), a pricey but beautiful instrument.

I like my homely little cootie, and will continue to play with it – maybe even venture out onto the air someday if my sending with it ever becomes comprehensible. I did get to satisfy my side-swiping itch without investing in an expensive instrument – dodged the bullet on that one.

73,
de N2HTT

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