High-voltage Antenna Length Relay

There are many ways to design a multi-band antenna. Each has its good and bad points, and all have been discussed on this blog multiple times. Common deficiencies of multi-band antennas are SWR bandwidth on the lowest bands, loss in the loading elements and pattern peculiarities on one or more bands (multiple lobes and nulls).

Examples include:

• Traps
• Fans (parallel elements from a common feed point)
• Adjustable matching network, in the shack or at the feed point
• Frequency sensitive transmission line sections
• Relays
• Motorized, adjustable length elements
  

All can be complex to home brew due to finicky design and construction. It is no surprise that most hams that need or want a multi-band antenna choose a commercial product in which all the complexities have, hopefully, been solved by someone else.

Perfection is impossible so tradeoffs are necessary. You choose an antenna where the tradeoffs are acceptable for your operating interests and what fits within your property and support structures. Many hams make their choice by price and size. Performance claims are either believed, ignored or deemed acceptable. Yet it is possible to avoid many of the deficits of multi-band antennas. That requires careful design, an understanding of antenna and network theory and practice, and test equipment.

It has been quite a long time since I last wrote an article about antenna design. I most often write about what I'm doing, of which there's an awful lot, and for the past year I spent little time designing antennas. I was recently motivated to investigate a multi-band challenge I have in my station. Although you may not have the same type of antenna in mind, the following discussion may be useful.

One of my objectives is to make the 160 meter mode of my 3-element 80 meter vertical yagi more effective without compromising 80 meter performance. Currently I switch in a loading coil and L-network at the antenna base. On air testing suggests a deficit of -6 to -7 db compared to the 160 meter shunt fed tower.

The big shunt-fed tower, which is an excellent top band antenna, is not available year round because I must roll up the radials for several months in spring and summer while the hay is growing and then harvested. Burying the radials is an option that I am unlikely to attempt this year, if at all. It isn't an easy project.

Alternatives that I considered long before I built the 80 meter array were impractical when it came time to build them. Before going further, consider skimming that article since it introduces several issues discussed in this one.

I am planning changes to the 80 meter array that will make it mechanically easier to improve its 160 meter performance, and prepare for 80 meter yagi performance improvements. Both depend on replacing the tower with a taller one. That will allow me to get rid of the long and problematic stinger.

A new stinger at the top of the taller tower would only be used for 160 meters. The tower itself, and probably with the help of a short "tuning" pipe at the top, will be a resonant ¼λ on 80 meters. Modest loading at the base of the stinger will resonate it on 160 meters. The base coil will be removed and the L-network redesigned for the measured impedance.

With an acre of radials (almost 2000 meters of wire) under the 5 vertical elements of the 80 meter yagi, the 160 mode of the array will be far more effective with negligible impact on the 80 meter array. Because it will be shorter than a ¼λ on 160 the load cannot be loss free, the new design will likely be 2 db less effective than the shunt-fed tower, but that's a notable improvement over the present design. That will let me be competitive on top band during the summer when the shunt fed tower is unavailable.

The drawings at right show just the essential aspects of the existing (left) and proposed (right) construction. Since the new stinger cannot extend 20 meters above the tower, loading will still be required on 160 meters. Support ropes for the parasitic wire elements would attach to the top of the taller tower rather than the top of the stinger, as in currently the case. That allows the stinger to be lighter duty. If it is 8 to 10 meters long, the 160 meter vertical will be about ⅜λ, so the loading coil can be small and low loss.

The stinger will have to be switched by relay. Use of a trap or parallel (fan) vertical have been discarded due their negative effects on 80 meter yagi performance and narrow bandwidth on both 80 and 160 meters. A switched stinger is far superior in this regard. The primary challenge is with the switching between 80 and 160 meters. 80 meter impacts must also be quantified. 

The design can be done in the comfort of my shack by computer modelling. My tool of choice is EZNEC and its version of the NEC2 engine. I began by building a simple model to investigate switching methods, and refining that until I had a workable design. 

This article focusses on the simple model to investigate switching behaviour. After the tower is replaced and I can take measurements of the new array, I will refine the design.

Modelling the relay

Relays are not perfect devices. The coil requires wires for power, wires to the contacts have inductance, closed contacts have resistance and there is capacitance between open contacts. When the relay contacts are open, as they are in this application for operation on 80 meters, the voltage across the contacts can be very high, well over 1000 volts for legal limit power. This is a case where my relay phobia may be justified.

The EZNEC model ignores the wires powering the coil, assuming they are suitably routed and choked to isolate them from the high adjacent RF field. We'll return to these challenges later.

You may have to expand the picture to read the tables. There are wires for the 80 meter vertical, the 160 meter stinger and a short connecting wire containing an RLC load. The load is modelled as a pure capacitance. A small value for open relay contacts (80 meters) and a short for closed contacts (160 meters). 

Relay spec sheets may or may not show the capacitance for open contacts as measured at the terminals. The capacitance depends on relay construction, comprised of that between the contacts and the wires to the relay terminals, and the housing and other conductors if they are significant. The reactance decreases with frequency so that "leakage" is greater on the higher bands. In this instance the capacitance is only relevant on 80 meters where the contacts are open.

The SWR curves were drawn with a load capacitance of 0.01 pF. Up to 2 pf the R and X components change by no more than 2 to 3 Ω. That is negligible. With a good radial system (which I have) the impedance is low enough that a matching network may be helpful. For a poor radial system the series ground loss would improve the match at 50 Ω without need for a matching network.

I adjusted the wire lengths (all have a 40 mm diameter for simplicity in the initial models) to resonate the 80 and 160 modes where I want them. Relay lead inductance is ignored but they are almost negligible for these long wavelengths and are easy to compensate with length adjustments in the built antenna.

Conducted 80 meter current in the 160 meter stinger peaks in the centre of its length. For 1 pf of relay capacitance the peak current is 6% of that at the base of the vertical. It rises to 7% for 2 pf. That's comfortably small but may impact F/B when the array is operated in its directional modes on 80. I am deferring the exploration of that interaction to a later time.

Differences between the SWR curves and gain on both bands are negligible when compared for single-band verticals; that is, without the relay, and the stinger removed from the model on 80 meters. That is what I expected. So far so good.

Relay requirements

You don't often see antennas like this, using a relay to switch bands. Traps are far more common in this application despite their inherent loss and increasing the antenna Q on both bands. A relay has neither of those disadvantages. This is readily apparent in the EZNEC model's load data with the relay open for operation on 80 meters.

On any antenna with an open end -- which is almost all antennas other than closed loops -- the current at the ends of elements falls to almost zero and the voltage is high. Should you attempt to feed the antenna at one of these locations -- such as an EFHW (end fed half wave) -- the impedance is very high. Matching it to 50 Ω can be done, at the expense of transformer loss and difficult to control common mode current. In our case, it is only the voltage that needs to be tamed.

Relays exist that can withstand over 1500 volts of RF but they are not ones you commonly encounter. Contact flash over voltage is misleading since the spec is typically for DC or the low frequency AC found in power systems. The coil, insulators and conductors behave differently with RF flowing across the contacts. When the contacts are open, some RF current will flow due to stray capacitance or due to humid or polluted air. 

The resistance can be higher than expected when the contacts are closed, and the capacitance higher when the contacts are open. Further, the actual voltage could be higher than in the model due to voltage modelled in the wire segment rather than at the wire tip, and various environment and construction details. I would at least double the relay's voltage breakdown spec to be safe. A properly rated relay can still be destroyed by accidental hot switching. Luckily that's unlikely since a relay for changing bands is only operated when the transmitter is idle.

There are two classes of high RF voltage mitigation: use a relay designed for the application, or design the antenna so as to reduce the voltage where the relay is placed.

The preferred choice for applications like this is a vacuum relay. They are available with breakdown voltage ratings starting at 2 kV and going much higher. Unfortunately they are expensive: starting at well over $100. There is a good market for surplus and used vacuum relays to limit the expense, if you can find those with suitable specs.

There are other considerations: they can be fragile, difficult to mount and protect on a tower, detailed specs may be difficult to locate (e.g. capacitance for power and signal relays), and have inconvenient coil voltage (24 to 28 VDC is most common on the used market). Choose carefully.

Mitigation measures

There are ways to reduce the stress on the relay to reduce the risk of failure, and in some cases it may be possible to use a conventional open-frame or sealed relay. These are the ones I modelled, and all work, though not necessarily very well:

• Capacitance hat below the relay
• Leakage capacitor across the relay contacts
• Large diameter wire (tower) below the relay
  

The models I developed to test these methods are solely intended for the purpose of exploration. I will make no recommendations or provide dimensions for a real antenna. NEC2 and pretty well all modelling engines are not highly reliable with respect to voltage, current and impedance at the open ends of wires. Real antennas and relay terminal voltages will never exactly match the models. But they can come close, and that makes the modelling experiment worthwhile.

The capacitance hat option (left diagram) was a disappointment. The voltage across the relay only dropped by 25% with two arms that are 5 meters long. For an antenna that is 20 meters long the effect is severe and must be corrected by shortening the vertical quite a lot. That's unacceptable since the shorter length would degrade performance of the 80 meter array. I did not bother to dig deeper to quantify the effect because a 25% voltage reduction isn't enough to eliminate the vacuum relay.

The reason I expected better from the capacitance hat is that it partially mimics a large diameter wire (as in the right diagram) which is known to reduce corona effects found with sharper antenna tips. Instead it behaved as if the voltage was measured inward of an ordinary element or T-top vertical.

To model the "fat" lower wire (right diagram) I increased the diameter of the 40 mm wire to 200 mm (8"). This is quite close to the top (#1) Delhi DMX tower section I am currently using, and will be again when two larger bottom sections are added to the tower. Tower taper can be ignored for the experimental model since we are interested in the voltage at the top of the tower and not its exact height. The model's wire containing the relay (again, a low-value series capacitance load) was made 2 mm (AWG 12), while the 160 meter stinger diameter remained at 40 mm (1.6").

This option is promising. The voltage across the simulated relay contacts dropped from 1500 to 850 volts, which is more than 40%. The resonant frequency on 80 meters barely changed. The voltage reduction is enough to consider using an inexpensive relay with contact and wiring isolation voltage of 2000 volts or more. The model is not definitive since there are factors to be considered in a physical antenna. For example, the relay is likely not close to the tower top plate because a tuning stinger may be required for height adjustment. There are also the effects of humidity, pollution (dirt particles in the air) and precipitation.

Despite the concerns, it may be worth the experiment when the antenna is rebuilt. Flea market open-frame relays are inexpensive enough to risk destroying a few! Since I am designing for high power it is highly recommended to do the experiment using an amplifier with fast-acting fault protection. Luckily I have one of those.

A capacitor across the relay contacts (middle diagram, above) may be an unusual option since until now we've been trying to minimize stray capacitance due to the relay. The trick is to increase the capacitance to pass enough current to cause the voltage to fall to a value where a conventional relay can be used. Reactance decreases with increasing capacitance.

The 80 meter resonant frequency drops since the series capacitor electrically lengthens the anteanna, and that can be a problem. There is no effect on 160 meters because the capacitor is shorted by the closed relay contacts. We need to know how much capacitance is needed to substantially lower the voltage across the relay while avoiding excess lowering of the 80 meter resonant frequency, as we saw for the capacitance hat.

The result is not good. It took 30 pf to reduce the voltage by 25%, and the resonant frequency dropped ~10% to 3.2 MHz. This is very similar to the 5 meter long capacitance hat described above. I suppose that should be expected since a capacitor and a capacitance hat are close relatives. Current in the 160 meter stinger peak at just under 25% of that in the 80 meter wire (tower). Gain of the vertical on 80 meters dropped by -0.1 db, which is negligible. However, the stinger current could be a problem in the array's yagi modes, but I have not run the model as yet.

The only promising mitigation measure is the "fat tower" option; the others have too many deficits. I may play with the model further to see if I can improve it beyond what I did for this article. It would only be for curiosity since I now know enough to proceed.

Further considerations

Whether a vacuum relay or the ordinary kind is placed atop the top, wiring it to the switching system at the base of the tower must be done with care or the wiring itself will modify the antenna's behaviour. The usual way of doing this is to run the cable inside the tower. Skin effect is our friend in since the antenna currents primarily run on the outside of the conductor. It is more complicated with a lattice tower than a solid cylinder (wire or tube) but the current flowing along the inside of the tower should be much lower than on the outside.

Spacers must be used to keep the wires several inches from the tower surfaces. The tower is not grounded so two wires are needed, one being DC ground. RF chokes should be placed on both wires at both ends of the cable and another set midway. That will detune the relay wiring on 80 and 160 meters and keep RF out of the switching electronics. The cable will have to be carefully routed around the top tower plate. Check the relay specs to ensure that the minimum voltage breakdown between the contact wires and the coil wires is at least as good as we need between the relay contacts and wires.

If the stray capacitance of the relay is very low and the loading elements on the stinger keep it far from resonance on 80 meters, there should be little enough current on the stinger to keep it from disturbing the 80 meter yagi modes. This was modelled and discussed earlier, but I have yet to model the full 80 meter yagi to confirm that the stinger current does not degrade the yagi pattern. It probably won't but it would be foolish not to check.

That said, there is reason to add a capacitor across across the relay contacts. We want a small value that does not appreciably load the 80 meter vertical. Perhaps no more than 5 pf. Its purpose is to damp corona effects at the relay that can amplify the voltage across the relay contacts when the humidity is low. A high value resistor should be added in parallel to bleed static charge on the tower and stinger when it rains or snows. Vacuum relays can't do the impossible so we should do what we can to reduce the stress on this valuable device.

The top of the tower can be made wider with a wire cage between the top plate and the base of the relay to lower the voltage across the relay's open contacts. A capacitance hat won't do that, as we've seen, because it is too thin. The cage complicates construction and may not be worth the trouble just to avoid a vacuum relay. The structure would also make working at the top of the tower awkward and possibly dangerous. Out of curiosity I may model it regardless.

Attaching the 160 meter stinger to the tower has its own challenges. It will be quite tall with substantial bending stress at the bottom. There are a couple of ways it can be done: A) inline at the top of the 80 meter vertical's short tuning pipe, or B) bracketed to the tower. The loading coil and relay should be close to the bottom of the stinger. High quality and mechanically strong insulators (red) are needed since, as we know, the voltage can be very high. 

I believe the best approach is to bracket the stinger to the tower. The mechanical demands on the insulation are far less than placing it inline, especially for a long stinger. For example, one or two layers of PVC pipe can be placed over the stinger where the tower bracket clamps to it. One layer may be enough if excess voltage due corona and static are managed as see above.

Next steps

While this article is about my particular antenna, it is applicable to other vertical multi-band antennas of the same design. The technical challenge to incorporate a relay is modest and may be well worth the effort to achieve maximum performance. This can be particularly welcome on the low HF bands where a higher Q method such as traps significantly reduces the SWR bandwidth.

I am undecided whether to begin working on the 80 meter array this year. I am behind with other projects and I am trying to take a rest from major projects this year. The earliest it will happen is this autumn after the insects die off. The project will proceed in stages to avoid finding myself without a good 80 meter antenna when contest season begins in earnest. 

The 160 meter change described in this article is more likely to be undertaken in 2024. Replacing the tower and guy anchors must be done first, and that requires taking the tower down and doing a lot of digging and concrete work. After the tower is replaced I can attach the wire elements to the top of the new tower and not have to change any of the existing switching electronics and matching networks. 

Unlike the models in this article, the physical stinger height for 160 meters will not be full size (20 meters). That will lower the SWR bandwidth on 160 but not on 80 meters. Loading is irrelevant to the characteristics of the relay and mitigation methods.

One item I'd like to revisit for the blog is a survey of the alternative methods for multi-banding vertical antennas that I listed but did not delve into at the start of the article. It may be worthwhile to compare and contrast them in more detail. I've done all this work in the past but my perspective has changed. Perhaps this winter.

New Zealand on 6 Meters

The first half of the summer sporadic E season has not been the best in this part of the world. DX openings have been weak and brief. I haven't seen it so poor for several years. We're hoping for improved conditions in the second half of the season.

Despite the lack of fireworks there has indeed been DX to work. It just takes more work. You have to monitor the band, watch what others are working and spotting, and have the antenna pointed in the right direction at the right time when that rare DX signal rises out of the noise. I started the year with 122 DXCC worked and I now have 130. That's pretty good but I am never satisfied. I want more.

Let's roll back to the evening of June 23 when I worked #130, since that was a good one. There was periods of very good propagation to W6 and XE during the day, often quite strong even with the yagi towards Europe. That made me hopeful for propagation over the Pacific Ocean in the evening. It's a not uncommon pattern. But with so few hams over that vast expanse I could only monitor, occasionally CQ and hope for actual activity. Surprises happen.

When I saw US stations working New Zealand I paid close attention. The path slowly crept my way, moving from W0 to W9 and W8, and then in the Toronto area, which is about 300 km to the southwest. Soon enough I decoded weak FT8 messages from a couple of ZL1 stations. 

This was my first time hearing New Zealand on 6 since March. That opening was very brief and I missed out. I flipped on the amp, starting its 3 minute warmup. It was a warm day and I didn't want to heat up the shack unnecessarily by turning it on sooner.

By the time the amp warmed up there were no ZL decodes. So I called CQ and was quickly answered by ZL1RS on the North Island, north of Auckland. On my one visit to New Zealand over 30 years ago I was in that area so I could picture the scenery. Signals were painfully weak but we quickly completed the QSO. I was very happy to log him.

I notified my 6 meter buddies (we're a foursome now) and hunted for more, and CQ'd when there were no signals from across the Pacific. I heard only one other ZL that evening, albeit too briefly to attempt a QSO.

You have to anticipate the propagation to maximize your chances. I had pointed the yagi to ZL well in advance because there was strong propagation to W6 earlier in the day, and then XE came rolling in. As you can see from the PSK Reporter map below that these bracket the great circle path to New Zealand.

After the contact I received a nice email from Bob ZL1RS. It must be quite the challenge to operate 6 meters from that corner of the world due to the distance from there to the most active corners of the globe. On the other hand, almost every QSO is DX!

None of my friends in the area had the same luck as me. However the scales soon tip the other way, and the next day one friend worked 3B9FR while I came away empty handed. That day there were other long haul "almost" QSOs in Asia, and I did manage to work 9K, but that's it. There are enough new and promising signs of DX propagation to suggest that the second half of sporadic E season will be better than the first. Hope springs eternal.

Win or lose the chase is exciting and I'm having fun. I wanted to share that experience by deferring completion of a long delay technical article so that I could publish this one while the propagation continues to be good. 6 meters is truly the magic band.

Perils of PVC

PVC products are plentiful and inexpensive in hardware stores. There is pipe, conduit, weather-tight electrical boxes and much more. It is very tempting to this ham. I have used it in endless places throughout my station. The following images taken from my blog will give you an idea just how pervasive it is.

You can see enclosures for baluns, matching networks and switching systems, coil forms, open-wire spacers, wire antenna spreaders, gamma match supports, supports for Beverages and other antennas, and there are more that I haven't included. Flexible PVC is abundant in our stations as wire insulation and coax jackets. I would be surprised if you could not find many examples in your own station.

Despite its utility, there are important considerations before choosing to use PVC. There are many types of PVC and you can't easily know the characteristics of the material you're looking at. There are questions of rigidity (flexibility), dielectric constant, insulation qualities, UV resistance, thermal stability and fatigue life.

Flexible PVC conduit pipe can support my Beverages only because of the wire tension. The pipes would bend if they had to support all the wire weight. The pipes hold the wires in their correct positions and harmlessly bob back and forth in the wind. When the pipe is under load, as it is, for example, at the top of the stinger for my 80 meter yagi driven element (which must not be conductive), a wood dowel is inserted to give it the strength to withstand vertical and horizontal forces.

Depending on additives, PVC can be a very poor dielectric at RF. I use it for coil forms, but only for 80 and 160 meter antennas. As the frequency rises, loss in the PVC coil form can be substantial. 

UV resistance can be difficult to predict for PVC material bought at your local hardware store. I have seen PVC that has weakened in the Sun and I have seen PVC that endured many years outdoors. For example, the PVC jacket of RG213 has additives that give it excellent UV protection. 

Rigid PVC electrical boxes may be less UV resistant but can last a long time. It depends on the manufacturer, of which there are many and I couldn't tell you which are the best. I have seen discolouration and breakage that are due to a combination of UV and thermal cycling in our extreme seasonal cycle. They are also not immune to animals since some like chewing on them, much to my dismay.

With so many variables and uncertainties, many hams eschew PVC entirely while others believe that it's a miracle plastic. The truth is more nuanced. You can avoid PVC, and the convenience and economy it brings, in favour of less available and more expensivematerials. It is difficult to provide good guidance, so I won't.

All of this brings me to the point of this article: PVC structural failure. While doing work at the top of the 150' tower supporting the 3-element 40 meter yagi, I passed the side mount TH6 about halfway up the tower. I wasn't paying it any attention other than to climb past its tower support struts on my way up and then down. It was only on about the third trip down that I noticed something was amiss.

For those of you who don't recognize it, that is a ferrite balun made by Balun Designs. I used these to replace the ineffective Hy-Gain BN86 balun on both my TH6 and TH7. The TH7 (and balun) have been sold so I have two of these products left in my station -- the other is on the 80 meter inverted vee.

I can't say for certain that the discolouration is due to UV damage, but it is suspicious. The other two baluns have a similar appearance -- check the pictures in the link above for the 80 meter antenna -- as does an outdoor electrical junction box. My assessment of what transpired is that the 4 tabs of the standard PVC electrical box snapped off, and later the enclosure shattered at the coax connector due to the cable tension. I have had the mounting tabs snap on other PVC boxes mounted outdoors, and those were of different manufacture. The PVC must have been weakened by UV or thermal stress to break so easily.

I am amused that the reason I hadn't noticed the problem until then is that the antenna continues to work perfectly well. The wire windings on the ferrite core that are connected to the UHF jack and wire studs are holding it together. A corner of the ferrite toroid is visible through the crack in the enclosure. 

I'll have to replace the box, at the very least. It won't be difficult to transfer the ferrite core and hardware to another PVC electrical box. The question is whether I should do that. Won't it just happen again in a few years, or sooner?

Many hams keep their ferrite toroid baluns open-air since they are not perfectly efficient. With a kilowatt there can be 10 watts or more of heat in a choke in which small diameter coax is wound around the toroid or wound with wire as a transmission line transformer. Weather-tight is also heat-tight. Heat build up is unlikely the cause of PVC failure in this case because the TH6 only sees intermittent use, even in contests -- it's my multiplier antenna, and is rarely used for more than a few QSOs in a row.

I may forgo the PVC entirely and mount the balun directly to the underside of the resin plate on which the PVC box was mounted. That provides weather protection and ventilation of the balun without the risk of using PVC. The back plates of all the Balun Design baluns appear to be in good condition. When I come up with a suitable solution I'll write it up for the blog.

Accessibility for repair

Go ahead and use PVC in your station, but do so with an understanding of its merits and demerits. PVC can be more fragile than you may believe. This is particularly true when the PVC is deployed outdoors where it is difficult to repair. 

An example is my use of PVC conduit pipe in gamma matches and coax chokes on the driven elements of long boom yagis. Repair would require lowering the antennas to the ground. I hope that won't be necessary but it is a possibility.

In contrast, the busted balun enclosure on the TH6 is close to the tower and therefore convenient to remove and repair. May all your PVC repairs be as easy as that.

Outing Robots

As I mentioned in a previous article, I don't like FT8 robots. I don't dislike them on principle or because they're new and less "human", but for practical reasons: they call others when they shouldn't and they consume valuable spectrum. I steer clear of robots when I can positively identify them. I might a robot just to silence it, and then forget to log the QSO.

Identifying a robot is not as easy as you might believe. Human operators can exhibit behaviour almost indistinguishable from robots, robot algorithms are frequently changed, or the operator alternates between live and robot operation. Although I'm not obsessed with robots, identifying them is useful for avoiding them. But how?

For illustrative purposes consider this fictional conversation with what may, or may not be a robot:

Human: Are you a robot?
Other: No!

H: You call CQ for hours on end, and only a machine would do that.
O: I like making lots of contacts and earning awards.

H: When you call stations you annoy them by always transmitting on their frequency.
O: That's because they choose frequencies that are free of QRM at their end.

H: You also answer a CQ from every station you haven't work before.
O: They send CQ because they want others to call them. Nothing's wrong with that.

H: Even when they call CQ DX or CQ JA?
O: That may be what they prefer but often they will answer me.

H: But you call them 20 or more times in a row.
O: A lot of stations don't hear well so I have to keep trying.

This is deliberately written to mimic the most common form of the Turing test, where a interrogator interviews a subject via text messages and has to determine whether the subject is human or an AI (artificial intelligence). It is a difficult challenge and, to be blunt, most people are easily fooled. Many humans already have difficulty correctly identifying text responses from a LLM (large language model), which is not intelligent.

The question remains: how can an FT8 robot be identified? There are three major areas of inquiry:

• Self identification
  
• Behaviour pattern
• Stimulus-response

Self identification

Robots sometimes identify themselves. This might be surprising unless you know where to look. For example, digital stations connected to PSK Reporter will dutifully identify the software application. Bring up a map in PSK Reporter and hover the mouse over the location marker over the suspected robot station (you can narrow the search by noting its grid square).

Unfortunately this no longer works as well as it once did. I tried this for many likely robots while writing this article and I didn't find one. Robot operators have learned the hard way that they need to be less obvious because some hams took to tracking them down and telling them what they thought of what they were doing.

There are two ways to mask the software. One is not to not connect to PSK Reporter. The second is to use a feature of most (all?) of these applications to misidentify as WSJT-X. For this latter method it is possible to root out likely robots in a subset of cases. Look at the software version being reported. Negligent robot operators may not notice that the version being reported raises suspicions.

I located one station connected to PSK Reporter that by its longtime behaviour is almost certainly a robot most of the time its on 6 meters. I redacted the call sign and grid in case I am wrong. Is there anyone still using this ancient version of WSJT-X? A scan of other connected stations on the map will soon convince you that the use of this ancient software is rare.

Behaviour pattern

Since I discussed robot behaviour in the previous article I don't need to say much more. I would only caution robot hunters that behaviour patterns can be quite complex depending on the operator's configuration of the software. In many cases they keep it simple, opting to CQ or respond to CQs using mostly default parameter choices. Those are the easiest to spot, especially when they endlessly call CQ.

Don't be too certain that endless CQs or answering every CQ are sure signs of a robot. Human operators do that too. Cancel the watchdog timer and you can call CQ forever, and that's what some hams do. They use the auto-answer features of WSJT-X and JTDX and glance at the monitor from time to time in case there is a QSO in progress. When it's done they log the contact (or make logging automatic) then manually re-enable the transmitter to resume CQing.

Quite a few human operators pounce on any station they haven't worked before (those CQs have a distinct colour code). When the contact is logged they pounce on another. Most hams are more discriminating about who they call, so it may see robotic to see stations that call anyone and everyone they haven't worked before.

Stimulus-response

If you suspect a station is a robot it is possible to give it a poke and see what happens. This is analogous to the Turing test described earlier. The best way might be to call CQ with an unusual call sign that almost every human operator would react to differently than most robots. 

Before I continue, I must caution you that to do this deliberately can be unethical and even counter to the regulations for your country. I've never done it but I have seen it done inadvertently. When it occurs the results can be quite educational. I will give you an example that I recently encountered.

There was a station in the Caribbean that was very popular for DXers. Many common countries are often not so common or easy to work on 6 meters. The station in this instance was VP2MKP, and it was a new one for me when I worked him. 

Apparently he wanted to work stations faster so he composed a couple of custom messages to announce that he would move to FT4 on 50.318 MHz. This is a good idea and I wish more stations would do it. When conditions are good, doubling the QSO rate is worth a few decibels of sensitivity. But let's put that aside since it is not the point of this article.

There were two free form messages that were each transmitted several times. There is a strict limit to the length of these messages so they are often somewhat cryptic.

• CQ VP2M/FT4
• CQ VP2M/50318
  

The following screenshot captures only part of what ensued. It is enough to help you to see how different reactions of humans and robots. Call signs of suspected robots have been redacted, but not their grid squares.

The robots instantly reacted to the CQ message that appeared to contain a portable indicator for what was interpretted as a call sign. Although not real call signs, WSJT-X (at my station) tagged the messages as valid CQs. The port of the same software used by the robots did the same. Obviously the robots have never worked these call signs and so they went to work.

Would a human have clicked on the messages and answered, and do so in the very next period? That is highly unlikely, and it appears that none did in this instance. I can say that I stared at the screen for several seconds until I understood what those blue labelled messages from VP2MKP signified, and there is no way I would have replied, instantly or later, since that would have been foolish.

Not only did the suspected robots call, and call instantly, most stopped calling at almost exactly the same time! A lot of robot operators clearly stick with the default software parameters. There were variations. For robots that were in QSO at the time there was a 30 to 60 second delay before calling the false CQs.

How would you have reacted at the time. I can tell you how I reacted: I laughed. Then I scrolled back and forth and to confirm what had occurred. I knew I had a great idea for a blog article so I took the above screenshot.

There are many ways to deliberately provoke robots without waiting for chance to deliver what I've shown above. I'm sure most readers can come up with at least one. However, almost all are as unethical as the robots themselves since they require the transmission of deceptive messages. I won't do it and I would discourage you from trying it. Two wrongs don't make a right.

What should you do?

I don't know. What do you think you should do? If you really hate robots, my advice is to proceed with caution. Without walking in to the suspect's shack and inspecting their station, you cannot know. Never assume that you have found a robot with any of the methods discussed here or elsewhere. Being 95% certain is not 100%. Everyone deserves the benefit of the doubt.

In this and the previous article, I hinted at how I typically deal with robots. They are an annoyance but not a capital offense. I will not allow the presumed faults of others to become my obsession. If you are emotionally incapable of that, don't expect help from any regulatory authority, club or operating award sponsor. Avoid confrontation! That also applies even for (what you believe is) a friendly approach.

Most robot operators give it up after a while. Watching a machine make contacts is ultimately pointless and boring. Of course, others will come along and give it a try but they, too, will soon stop. In my opinion it isn't worth worrying about. I'll now give it a rest and not soon talk again about robots on the blog.