Note:
While this article describes a modification of the Pico Keyer to reduce RF susceptibility, it also talks about general methods to minimize/reduce RFI-related issues for both portable and "base" stations: This specific topic is covered near the end of this blog entry.
POTA operation
Over the past several years I've done a bit of POTA (Parks On The Air) operating, racking up "about" 1000 contacts as an activator in a number of parks - usually as an "activator", and mostly on CW. Typically, I have operated from a campsite using a portable antenna - usually the JPC-7 loaded dipole (discussed in this blog entry) or the JPC-12 loaded vertical (discussed here) - but I have also used an end-fed half-wave and a simple dipole on occasion - and even the Yaesu ATAS-100 on my vehicle.
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| Figure 1: Operating CW POTA from US-0004, Arches National Park in Utah Click on the image for a larger version. |
In the recent past there has been a revolution in portable power sources in that a LiFePO4 battery - which can supply 20-ish amps - is both light enough to be practical and fairly inexpensive. For those instances where I may be staying at one location for several days the advent of inexpensive solar to maintain the power budget - and the solar controllers can be made to be RF quiet to make it compatible with HF operation (see this article). With this in mind it's practical to operate the transmitter at 100 watts much of the time, something that makes it as easy as possible for those who wish to work me. Despite the ability to run 100 watts, I have occasionally operated QRP (5 watts or less) - again, usually on CW.
A memory keyer
Having used a number of different radios for POTA operation (Yaesu FT-100 and FT-817, Icom IC-706MK2G and even a RockMite) - none of them with a memory keyer - I decided that an "Upgrade" was in order so I got the Ham Gadgets "Ultra Pico Keyer" (Link here). This device is small, powered by a single CR2032 lithium coin cell and costs about US$40 as a kit (not including shipping) including a (partially) 3-D printed case. For portable use, I couple it with the "Outdoor Pocket Double Paddle" (with magnets!) from CW Morse (link).
This is a nice, little device in that it provides a consistent interface to the user - no matter which radio you might use - and it has a number of message memories (up to eight), perfect for an activity like POTA where a message (e.g. "CQ POTA") may be repeated many, many times during the course of the operation.
Getting "stuck"
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| Figure 2: The Ham Gadgets "Pico Keyer" (left) along with the CW Morse Outdoor Paddle. Click on the image for a larger version. |
While the Ultra Pico Keyer works as advertised, I did notice a problem on the first trip out while using a portable antenna: It would get "stuck".
Clearly, this was an RF susceptibility issue - verified by reducing transmit power and observing that it no longer happened. In short, at 5 watts there was usually no issue, but at 100 watts the radio would stay keyed continuously after the first Morse element whether it was sent from a stored message or via the paddle: While it was "stuck", I could still hear the sidetone - via the keyer's internal speaker - sending the message or what was keyed via the paddle indicating that it was not the microcontroller that had crashed but the circuitry that keyed the radio that was the problem.
Further testing showed that when the unit got "stuck" due to RF and simply unplugging the paddle from the back of the keyer would cause it to release (get "un-stuck"). The fact that this happened using a portable antenna provided further evidence of potential RF sensitivity.
Analyzing the problem
As I'm wont to do, I decided to take a look at the Pico Keyer's schematic to see if there was something about its design and construction that might make it more susceptible to RF interference - and I was surprised at what I found. Here's the diagram found in the manual that is freely available online on the web site (link):
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| Figure 3: Annotated diagram of the Pico Keyer with RF current paths shown. The components in question are Q1 and Q2, in the upper-right corner. The lines highlighted in yellow are those through which RF currents will flow (between the radio chassis and the paddle/cable) if no bypass capacitor is installed. The added capacitor is shown below the "OUTPUT" jack near the upper-right with the resulting RF current path around Q1/Q2 shown in magenta. Click to get a larger image. |
While there are protection capacitors on the paddle input (C1, C2) my eye was immediately drawn to the output keying (upper-right) where I was, at first, confused as to the arrangement with an N-channel MOSFET in both the keying line and the "common" (ring) of the "OUTPUT" connector (e.g. Q1 and Q2) - but then I remembered that the manual stated that this device would key both positive and negative voltages, explaining the "unusual" arrangement.
While admittedly clever, I could immediately see a susceptibility issue here - the problematic RF current path highlighted in yellow in Figure 3, above: The "OUTPUT" jack more or less will "float" compared to the "ground" of the keyer itself, which is also connected to the "ground" lead of the cable to the paddle as well as the external paddle itself. This configuration almost guarantees that there will be at least some RF current flowing from the radio and through the keyer's output circuit for several reasons:
- If you are using this in a portable situation, the radio will surely have some RF on its chassis. As noted in the final section of this blog entry, it's almost impossible to prevent all RF current from getting onto the feedline - even if you do use a common-mode RF choke and a very nearby antenna is likely to immerse the radio and its interconnecting gear in a rather strong radio-frequency field.
- The paddle and the cable that connects it to the keyer should be considered as part of an antenna - and this situation is made worse if one is sitting at, say, a metal table and also if you, the operator, place your hand at/near the paddle/cable, further encouraging a "through" path for RF.
What this means is that there will be at least some RF current flowing from the radio chassis, through the keyer and then, as indicated by the yellow-highlighted lines - via transistor Q2 (and Q1) and then through the cable to the paddle. I didn't really investigate the exact mechanism by which RF current through this path was causing the keying line to get "stuck" - but here are a couple of possibilities.
- RF may be coupling from the drain of Q2 into its gate - and subsequently into Q1's gate as well, which is tied in parallel with it with the peaks of the RF voltage turning on the FET. Even if RF through the FET was causing it to conduct only on half of the RF cycle, this would surely be enough to key the radio. It's also possible that the transistor was turned, on average, only "partially" on by the RF energy - not enough to completely shunt out the RF, but enough to key the radio.
- The RF could also be getting into the output pin of the microcontroller via the FET, causing its totem pole output to get "stuck" on while it was present.
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| Figure 4: The added capacitor(s) can be seen soldered between the "sleeve" pins of the "OUTPUT" and "PADDLE" jacks, on the bottom of the board. As you can see, I've made this modification to both of my Pico Keyers! Click on the image for a larger version. |
When this occurred, I happened to be on a POTA activation, but I had my "electronic toolbox" in the car which included a number of useful items such as a soldering iron and a smattering of useful electronic components (a some common resistors, capacitors, etc.). Grabbing a 1000pF capacitor, I connected one end to the "sleeve" (ground) pin of the "PADDLE" jack and the other end to the "sleeve" of the "OUTPUT" jack - effectively providing a bypass to RF energy on Q2's drain to the circuit "ground" to eliminate any RF voltage potential between the cable connecting the radio and that going to the paddle.
This modification completely solved the problem: It is my opinion that this capacitor should be supplied with the kit. Additionally, Q2 could be eliminated completely and its source/drain leads jumpered if negative keying is not needed. See Footnote 1
Since the topic of "RF on the rig" was already broached, the rest of this article will describe how to reduce it. It's worth noting that the susceptibility of the memory keyer was such that even with the measures described below, it was affected at 100 watts.
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Suppressing RF on the gear and connecting cables
Some readers of this may immediately say "You are obviously doing something wrong with your set-up if there's enough RF on your gear to cause a problem".
The problem of RF going somewhere other than out the antenna has been known for many decades and is sometimes referred to as "Hot Mic", a situation where there is enough RF on the radio - and the microphone - that the operator can even get an RF burn from touching the gear. When this happens RF can get into the radio itself and cause undesired operation (malfunctions, distorted audio, etc.) but accessories connected to the radio - most notably sound interfaces, computers and even keyers - can be adversely affected.
While in the case above there was apparently some RF present on the gear to cause a problem, there isn't anywhere near enough to cause issues with the radio itself, and the radio+microphone (when running SSB) seemed immune. Some types of antennas - typically ground-plane verticals, random-wires and end-fed half-wave antennas can, by their nature, put RF on the feedline - and thus the radios - unless extra steps are taken to minimize this problem in addition to properly installing/configuring the antenna, namely:
- Common-mode choke on the feedline. Typically placed near the antenna, this usually consists of coaxial cable wound on a ferrite toroid - typically 6-12 turns on an FT240 or FT140 core with either Mix 31 or Mix 43 as the material - the latter being generally more useful/preferred for portable operations where the higher bands (40 meters and up) are most likely to be used. Sometimes operators wish to have the feedline itself act as part of the counterpoise/ground - something that can risk a "hot mic" situation and in this case placing the common-mode choke farther along the coax - often near the radio - is the better choice. (Some operators will put a choke at the antenna and near the radio.)
- Use of a "balanced" antenna. A balanced antenna like a dipole is generally more likely to induce less RF current on its feedline than a purely end-fed antenna (a vertical is included) as it contains its own counterpoise - but having a perfectly-balanced antenna is not really possible and the feedline itself will usually participate in conducting/radiating RF along with the antenna to some degree. A high-impedance antenna like an end-fed half-wave can sometimes reduce the probability of RF currents on the gear, but note that current can peak at every odd-numbered quarter-wave interval along the feedline and if the radio happens to be at one of these current nodes, issues are more likely to arise: Placing a common-mode choke at a current node can help.
- Counterpoise/ground plane at the radio. If you are operating in a metal vehicle it's less likely that RFI will be a problem as one is likely to be surrounded (e.g. shielded) - plus the fact that the shield of the coaxial cable feeding the antenna can be electrically bonded to its chassis. Barring being in a Faraday cage like a vehicle, having a counterpoise connected at the radio (particularly if it's 1/4 wave long at the operating frequency - and if there is more than one of them) this can siphon off some of the RF that might be present owing to its lower impedance. The use of a common-mode choke prior to the counterpoise at the radio will help to raise the impedance of the conducted RF and will usually improve the efficacy of a counterpoise/ground plane.
- Ferrites only go so far. At HF, a simple "snap on" choke will probably do very little for the simple fact that there does not exist a common ferrite material that will offer a reasonable degree of choking impedance at, 14 MHz with just one turn (e.g. wire passed through it). What is required is that multiple turns of a conductor be passed through the device (snap-on choke, toroid, etc.) as the impedance/inductance is proportional to the square of the number of turns. Even so, there's a practical limit as to the choking impedance of a piece of wire around a ferrite (probably in the hundreds of Ohms for a "casually-wound" device). As in the case of the keyer, I chose to use a capacitor, instead: It is a tiny, inexpensive device able to fit inside the keyer rather than a large lump in a cable and it directly addresses the issue at hand by making the circuit intrinsically RF-tolerant. In other words, it's the correct component for the job!
- Place the antenna far away from the radio. As noted, this isn't always practical - or even desirable during portable operation. In my opinion, equipment used with a radio transceiver should already have a modicum of resistance to stray RF energy so that even small/moderate amounts of RF on the gear will not cause any problems.
If you are operating portable, there's one thing that you probably aren't going to get very faraway from: The antenna itself. Almost by definition, portable operating implies being near the antenna owing to the need to have a feedline of manageable length and also due to practicalities of not wanting to lug a long feedline along or taking up more real estate than necessary. What this means is that it's likely that you and your radio will be immersed in a rather strong RF field - and this also means that anything made out of anything that is conductive (the radio, power cables, microphones, interconnect cables to your paddle and keyer - and even you) are likely to intercept RF energy this will get into everything.
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Footnote
- A 1000pF capacitor has theoretical impedance of about 23 ohms at 7 MHz and it did the job here, but a 10000pF (e.g. 0.01uF or 10nF - ideally about 2.3 ohms at 7 MHz) capacitor would to just fine as well. For positive keying (which is what likely what any modern radio uses) values as large as 0.1uF (100nF) would work as well - but this large of a value may cause issues with radios that use negative keying (e.g. high-impedance lines on some vintage radios).
If you never plan to use a radio with negative keying, you could simply short together the source and drain leads of Q2 together to reduce RF susceptibility.
This kit is actually supplied with an "extra" capacitor: The user can select between a 0.01uF (10nF) and a 0.047uF (47nF) capacitor (C3) on the "headphone" jack to set the loudness. As I installed the 0.047uF capacitor, I had the 0.01uF left over. Unfortunately, the specific capacitor supplied was thick enough that it prevent the board from sitting in the bottom of the case, raising it up and preventing the lid from fitting properly. I could have probably connected this capacitor to the same circuit points on the top side of the board, but as I was home when I made this modification to my second keyer I simply found a lower-profile capacitor that didn't interfere with the board clearance.
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This page stolen from ka7oei.blogspot.com
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