Mitigating Audio Hum

My station wiring is extensive and complex. I try to keep it reasonably well documented and the mass of cables reasonably tidy. However that's difficult to do in practice. The antennas and control systems are constantly evolving, and a lot of my home brew devices are not as pretty as I'd like. In short, it's a mess.

While I do have plans to gradually make improvements there are issues that I must either live with or troubleshoot and solve. One common problem that plagues many stations is largely absent in mine: RFI. This is because most of the towers and antennas are far from the shack. Some, though not all of the control lines have RF chokes and bypass capacitors to shunt RF to ground.

One of my challenges is grounding. I have ground rods at the base of all the towers, the coax and cables are almost all underground until they are near the shack, and I try to follow best practices for grounding where it is possible and practical. It is good enough that a couple of lightning strikes this year did minimal damage. They may have been secondary strikes, but I was either not home or not looking out the windows at the time to say for sure.

The grounding issues have more to do with hum than safety. The connections among RF, AF, control line, PC and equipment grounds are haphazard. For example, operating one of the rotators induces hum in the receive audio of one of the rigs. It is enough to impair reception of weak signals on FT8. Unfortunately it doesn't stop there: hum and related noise can appear on my transmitted audio, whether digital modes or SSB.

It came to a head during this year's sporadic E season. As the season progressed I noticed odd behaviour of the transmitted signal. The audio level drifted up and down and there were momentary spikes on my FT8 signal. I didn't know what it was for sure until a fellow ham sent me a spectrogram of my signal. There were weak spurs across the audio spectrum, and there was little doubt these were coming from my transmitter. In an email exchange we eliminated other possibilities.

Using the rig monitor function I confirmed there were hum and crackling sounds. The levels were well below the FT8 audio so that only stations receiving me strongly would notice. I inspected the spectrogram and noticed that the spurs, though not actual harmonics of 60 Hz, were separated by multiple of 120 Hz. It was time to dig deeper.

The first task was to reproduce the problem. There is no easy way to monitor your own transmit audio and I needed to do that. The rig's monitor function can give you a false sense of security since it monitors the input to the RF transmitter stages and not the output of the transmitter, and certainly not that of an external amplifier.

Sampling the RF requires a separate receiver. Since I am configured for SO2R in contests this was not too difficult to set up. I made use of the PC's two sound cards to transmit using WSJT-X on one rig and to receive on the other using JTDX. A wire stuck in the antenna jack provided a sufficiently attenuated signal that didn't overload the receiver.

I performed two tests at different audio frequencies (DF). One was positioned close to that in the original report. The spectrograms confirm that there is a problem and that it is as reported. There are a few unrelated signals that happen to be within the pass band that are from other sources in the house. For example, the heat pump controller is a little noisy when it is in air conditioning mode. Since there are no spurs at 50.313 and 50.323 MHz it is not worth spending time on it.

Notice that the spurious signals are mostly at intervals of 60 Hz and its harmonics, and especially 120 Hz. They shift along with the DF since I use WSJT-X in "split" mode, which offsets the transmit frequency to keep the FT8 audio within the SSB pass band sweet spot.

It was not only my transmit signal that suffered. There have been several hum issues on receive that have annoyed me for some time. I left those unfixed since they were not serious and I was too busy with station building. Here are a couple of examples:

In the upper pane are hum harmonics that appear when I release the brake on a Tailtwister rotator. Notice that the audio frequencies are odd multiples of 60 Hz: 300, 420, 540, 660, 780, 900, etc. These line up with the hum harmonics on the lower pane. These persistent lines wax and wane with no discernible pattern. They are too weak to interfere with FT8 decoding. The rotator hum does interfere with received signals and also appears on the transmit audio. I was careful not to turn the 6 meter antenna during transmit periods.

Finding and resolving the root cause of the hum is daunting. As already mentioned, my station is extensive and complicated. There are endless possibilities to explore. However, there are several simple things to try before diving deeper. I tried all of the following common cures, without positive effect:

• Checking and cleaning audio connectors, and then other connectors to the PC and rig
• Moving the rotator controller and other equipment, in case transformer proximity is a cause
• Moving power cords to different power bars and outlets
• Testing outlet polarity to ensure that neutral and ground pins are correctly wired
• Disconnecting and isolating equipment, including microphones, SM5000 monitor, headphones and PTT lines
  
• Adding and improving ground busses between equipment, and the service ground
  

Despite those failed measure, there were indicators of where the problem might reside. Disconnecting the transmit audio cable from the PC the problem could be replicated by "shorting" with cable with my fingers. This suggests that the hum source could be the rig itself (FTdx5000) since my skin resistance coupled the ground to the signal pin. 

Another indicator was that it was possible to alter the amplitude of the received hum by jiggling the audio connections to the PC. But, frustratingly, not always or not by a lot. This suggests that the PC power supply or internal wiring has degraded, possibly oxidation at a ground connection or a leaky capacitor. The effects are inconsistent and not persistent.

I disconnected the many interconnection cables and hauled the the FTdx5000 onto the workbench and opened it up. I poked around the connectors and audio stages to look for anything amiss. All the connections seemed secure and checks with an ohmmeter uncovered no obvious defects. I put it back together and reconnected the cable. It was no surprise that the hum was still there.

The next candidate was the computer. Modern PCs are notorious for poor and defective power supplies and I've had past trouble with them in my commercial signal processing work. Here I balked since time was running and the sporadic E season was at its peak on 6 meters. Perhaps I'll pursue this during winter when I have more time for indoor jobs. I instead chose to pursue mitigation rather than a cure.

I rooted through my junk box and located a miniature audio transformer. I mounted it and connectors on a small breadboard. I put it in series with the transmit audio line (PC headphone jack and rig audio in jack). It was a struggle to fit it under the fan-out PTT connector without wires getting pulled out of the breadboard.

It worked. My transmit audio was free of hum and all the snaps, crackles and pops. I had two friends check my signal, to be absolutely certain. They gave me a clean bill of health. 

I operated in this fashion for the remainder of the sporadic E season, confident that my signal was finally clean despite my failure to locate the root cause. Hum on the received signal continued, but I could live with that.

The transformer works because the direct ground connection between the PC and rig is broken. Common mode hum cannot cross the transformer. It is for the same reason we use common mode chokes on transmission lines. For small signal levels, such as for audio signals and for my Beverage receive antennas, two-winding transformers are very effective. However, if the hum is differential mode -- on the audio signal and not just conducted over the ground connection -- the isolation transformer won't help.

I needed a something better to replace the flimsy breadboard, and for both the receive and transmit audio lines. Less than confident that I could find the root cause of the hum without a great deal of effort I opted for a permanent mitigation measure. I chose to build a transformer-based isolator for both audio lines. Since I had only the one audio transformer I went online to shop for parts.

My original intent was to order more of these miniature audio transformers, which are very inexpensive. Sites like Amazon have good deals on packs of 10. It doesn't hurt to have a few extras in the junk box. Flea markets are also good sources but those have been few and far between during the pandemic. I already had an enclosure and connectors, and I chopped a piece from a small prototype board to fit inside. The enclosure should be plastic unless the audio connector grounds can be isolated from a metal enclosure. My remote Beverage switch shows one way in which this can be done.

In the list of items from the Amazon search results there were many commercial audio isolators. One or two of those (they're almost all for stereo lines) will work if wired for mono operation. Although they cost more than building an isolator from discrete components many might prefer those.

Further down the list there was a fully populated PCB with transformers (for stereo). Since it would save me the trouble of soldering small parts I decided that for $11 it was worth taking a chance. When it eventually it arrived from China I installed it in my enclosure with the connectors.

All the connectors are stereo, as they typically must be for the PC and rig jacks. I bridged the two channels on all the jacks for mono operation. Thus one side of the isolator is for receive and the other for transmit. Grounds on each side of the PCB are common so one side is for the PC and the other for the rig. Otherwise the unit is symmetric and there is no need to differentiate which side is which. The wires support the board inside the enclosure.

Initial tests with the front headphone jack were promising other than signal attenuation. The 8 Ω impedance is likely a poor match for a transformer designed for high impedance circuits. Signal levels were good with the FTdx5000's high impedance back panel jacks. PC sound cards are even higher impedance, typically 47 kΩ. Frequency response appeared to be reasonably flat from 100 Hz to 3000 Hz. I did not test the unit beyond that range.

One point worth noting is that the isolator is DC coupled. That is, there are no capacitors in series with the 4 transformer windings. Sound card microphone ports have a permanent or software configurable DC bias for electret mics. Depending on the transformer this may pose a problem. These miniature transformers typically have a DC resistance in the range of 100 to 200 Ω. It is unlikely that more than 10 ma of DC will flow due to the electret mic bias and that is unlikely to be a problem. I recommend checking the winding resistances to be sure.

I made a 2" (5 cm) cable from twisted Cat5 wire pair to temporarily connect isolator to the phono jack on the rig. The other jack was wired with a commercial 3.5 mm stereo cable. I plan to make short shielded cables for both rig ports to keep the installation tidy and hum free. Short cables are better than long ones.

FT8 testing was successful. Levels were good and there was no evidence of hum on transmit. There is still hum was on receive when I operate the rotator brake, but it is much weaker than before. It may be due to the short unshielded cable or the plastic enclosure.

With that job done I declared the problem solved. Some cleaning up is required and I will do that in the not too distant future. It is unlikely that I'll use FT8 until the winter 160 meter season. 

Many modern rigs reduce the cable count by combining CAT, audio and other functions on a single USB cable. That reduces the opportunity for noise and hum to sneak into the audio paths. I hope that will be the case when I eventually upgrade the shack equipment, and I can set aside the isolator. If the problem source is the PC that may not work. On the other hand, since PCs age faster than rigs it is probable that by then I'll have a new computer running the shack.

Finally, I strongly recommend that you monitor your RF or ask a friend to do so. Use the buddy system to help each other to keep your signals clean. Most hams are too shy or polite to tell you, so don't rely on unsolicited reports. Not only that, even if you are don't know I can guarantee that most hams in the vicinity know that your signal is dirty. They are likely miffed and silently cursing you for your negligence. Should someone reach out with a negative report, thank them and move quickly to confirm and correct the problem.

Return to the Flea Market

During the pandemic almost all hamfests and flea markets were cancelled. Recently we've seen a gradual return to normalcy. This past weekend I attended my very first flea market since 2019. That it was a 75 km drive did not deter me. It is a small local event hosted in the village of Mayberly by the Lanark North Leeds ARES.

The only reason I've been in that area before was while driving through to go elsewhere or to enjoy cycling the back roads with groups of friends in years past. It's beautiful country but off the beaten track.

I doubt there were more than 50 hams present and the vendors numbered no more than 15. It was done open air, from car trunks and tables at the back of the sod parking area of the local community centre. The weather was perfect for it.

The picture shows the sum total of my purchases. A few connectors and a handful of Bourns multi-turn 10 kΩ linear pots that I use for prop pitch motor direction indicators. The prices were good although they hardly justified burning 150 km of fuel. I did better earlier this month while dumpster diving at a tower service company. I scored numerous LDF4 and LDF5 Heliax connectors and short lengths of Heliax and LMR400. And it was free.

The most important reason for the drive was not the flea market: it was to see people again. That was by far the most enjoyable part of the trip. Bumping into friends I hadn't seen for a couple of years was great. There were good conversations with a couple of fellow contesters who also made the drive. 

With everyone wearing masks I was slow to recognize some people that I knew well. One fellow contester was unrecognizable with his large mask, glasses and a baseball cap pulled low over his forehead. I was only able to identify him by his voice. 

Since larger events in the region have already been cancelled I may not get another opportunity to meet up with many hams until 2022. It is unlikely that I'll be inviting anyone to my station to participate in a multi-op this fall or winter, or that they'd be willing if I did. Caution reigns for the time being.

6 Meter E Season Wrap-up for 2021

For all practical purposes my 6 meter sporadic E season is over. Yes, there will be more openings from time to time. However, the likelihood of DX is low, and that is my primary motivation. For me this is as good a time as any to look back at the season that was while memories are fresh. Look back in the blog and you'll see that I do this every year.

One important observation is that activity is higher. Interest in 6 meters is increasing and that's a good thing. There is more QRM of course but that's a fair trade off for the increased opportunities that the activity engenders. There is a lot of spectrum available that, as a community, we have yet to exploit. Digital modes, especially FT8, tend to encourage the majority to crowd into one or two SSB 3 kHz watering holes. Eventually that will have to change.

With respect to the objectives I set at the beginning of the season this has been a very successful year. I surpassed by one my DXCC stretch objective of 110 countries on 6 meter digital modes. Of these, 103 are confirmed on LOTW. I do not intend to apply for the DXCC award since awards are unimportant to me. It is enough to know that I've done it.

Since early May my log has accumulated around 1000 DX contacts. The station improvements I made earlier this year have paid dividends.

Two of my close 6 meter buddies also passed the 100 confirmation mark this season. We notify each other of openings and happenings throughout the E season, and cheer each other's successes and commiserate when we are not so fortunate. With pandemic restrictions removed we are planning a get together to celebrate our achievements. 

Power

As I've said numerous times on this blog, FT8 is not a low power mode. More precisely, it is no more a low power mode than any other. For example, CW is a low power mode yet QRO is a strong advantage whether you are DXing, contesting or simply having a conversation. 

During the first major opening to Europe on May 19 my country count jumped from 90 to 96. These were countries I've heard and called in the past but could not work. With power I did. Some would prefer to keep it challenging with low power, but for my tastes this is a welcome development. Sporadic E openings are mostly brief and marginal, and I see no reason to continue struggling.

One thing I find myself doing more often is to call CQ for long periods. The reason is that with a bigger signal I have the opportunity to "open" the band. That is, those monitoring a quiet band may hear me and start transmitting. We all like to passively monitor a dead or quiet band for those elusive DX openings, waiting for the "big guns" to break through. With QRO I can now make a contribution.

Yet there are many stations that still cannot hear me. Some have mentioned on chat groups and elsewhere that the local man-made noise is strong so signals must be very strong. One Middle East station said he has to shut down at dusk because the noise of LED lighting wipes out the band. This is a global problem that will only get worse. 

QRO on my end helps to be heard by those living with the plague of noise. Since I hear very well at my quiet rural QTH many stations I can hear don't hear me. It's frustrating, though a little less so now that I can run more power. The downside is that sporadic E occurs when the weather is warm and heat from the amp can make the shack uncomfortable.

Europe and Caribbean

During the peak weeks of the season it is rare to not decode at least one station in Europe or the Caribbean. That's why I've lumped them together. The hops to these areas are geographically favourable for transitory E-layer clouds almost daily. That does not mean that there is a workable opening. These are merely "teasers" that keep hams in the shack when they could be doing something more worthwhile.

That said, there are frequent openings to these areas. Activity is high enough that if there is a reflection there will be a station at either end of the path to decode the digital message. First you hear the big guns and then you wait (or you probe with your own CQs) for the next layer of stations to appear. In the best case the opening brings in the smallest stations. I have had more than a few stations in Europe tell me that I'm their first Canada or first North America on 6 meters.

While I do love these openings my priority is working new countries. Both of these areas are filled with DXCC entities that I need on 6, but where the activity is absent or the stations are small and therefore relatively rare. So I work the openings and keep an eye on all the decodes for that elusive new one. I interrupt the pile up to call them, sometimes successfully and sometimes not. 

The rare ones can accumulate quite the pile up. HV0A moved their pile ups from 50.313 MHz to 50.303 MHz to make it more managable. Other shift to 50.323, 50.333 or elsewhere. You must pay attention to the spotting networks or QSY frequently or you'll miss when this happens. When I worked OH0Z they chose an audio frequency (DF) of less than 200 Hz. It was necessary to shift the VFO lower to get a reliable decode. Digital modes are not as simple as many imagine! There are tactics to be learned to successfully work the DX that are different than for traditional modes.

The more northerly path to Scandinavia, the Baltic countries, Ukraine and Russia opened numerous times but were so marginal that contacts were difficult most of the time. New ones on that path included OH0, ES, YL and UA.

Far East

To my surprise there were many openings to Japan this year: 6 that I can recall. Some were so feeble that few stations could be worked. One remarkable case was discovered by accident. Around 2330Z one evening -- an hour later than the typical peak for a JA opening -- I checked PSK Reporter while I was working the Caribbean. I had a flag in JA8 (Hokkaido), off the back of the beam. 

I turned the yagi, half convinced it was a mistake of some kind, heard nothing and called CQ. I was immediately answered by JA8EPO. After our QSO we both CQ'd for quite some time and worked nothing. I gathered several flags in JA8 over the next 20 minutes but heard and worked no more stations. No one else nearby appeared to hear anything at all. That was quite the spotlight opening.

The path to the far east was otherwise disappointing. I heard one HL briefly, and that was it. The only success on that path was finally working KL7HBK for a new one on digital. I heard several more Alaskans that were not coming in for long enough to work. Stations to the south and west had more success, working BA, BV, HL and DU. Maybe next year.

Middle East and Central Asia

I had success though not as much as I had expected. Two 4X stations were worked and then I had a drought. There were many openings into the Arabian peninsula, with many enticing new countries, yet little resulted. While calling an A7 station I was called by two in 9K. That was nice but the others went unworked. Heard and not worked countries included: TA, OD, A7, 7Z, A9. 

As mentioned earlier, it could have been noise on their end. Others had similar difficulty getting through. Another problem is that we have to penetrate the European "wall" on this tenuous path and there is no good way to know whether our chosen frequency is clear. Calling on their frequency, which ought to have been fairly clear of QRM, was also unsuccessful. Of course there may have been other callers on that frequency, and due to the peculiarities of sporadic E propagation I wouldn't hear them. 

Those a short distance to the east in W1 had far better luck, as did those in VE1/VO1/VY2. The same goes for the openings to central Asia. Others were heard working UN and EY, which were not heard here at all. Last year I did hear a few of these stations but it was impossible to be heard without an amp. Hopefully my luck will improve next year.

Pacific and West Coast

There were numerous openings to W6/W7/VE7 this year. A handful were remarkable. One was so good that I heard many new stations. In an email, a California station told me he uses an indoor antenna and it is difficult for him to work my side of the continent. Portable operation from several rare grids were there for the taking for those on the hunt for grids. I worked a few of them despite that not being my interest.

Going further to the west, I worked two more stations in Hawaii. During one opening, NH6Y was heard for almost 30 minutes. That's impressive. Unfortunately most of the Pacific is devoid of land and hams so the propagation goes to waste. One exciting event was several in the northeast copying a ZL near 00Z one evening. I was on the great circle route aligned with those stations so I listened and CQ'd for some time. Unfortunately I heard nothing and I raised no flags far away. It would be a stunning achievement and you can be sure I'll try again next year.

Africa

The main difficulty working continental Africa is the low activity level. There are few hams and even fewer are active on 6 meters. Digital helps since it encourages more to make the attempt. Even if there were more activity it would be difficult to reach beyond northwest and west-central Africa with sporadic E. East, central and south Africa are particularly challenging. I have yet to hear any of those stations during my most recent years of activity. Yet I keep listening.

A few stations are well equipped and within reach. Good examples this year are TZ4AM in Mali and 5T5PA in Mauritania. Of these I've only worked the latter since Jeff TZ4AM has a high noise level and I have yet to catch him when signal levels are sufficiently strong. It is admirable that he keeps trying and many in North America got lucky this year. 

Other Africans I notched up this year include XT and 7X. These were difficult because they do not have big stations. I did not hear TT8SN this year, which is unfortunate since my uncertain QSO with him in 2020 did not make it into his log. I regrettably erased TT from my worked list. 6W1TA was worked again this year, while 9G5FI remained out of reach. 

With more activity I am sure I will put more Africans in my log next year. Other than hoping for exceptional openings there is little I can do until the current solar cycle peaks.

South America

Working the north coast of South America is becoming routine. There are a few well equipped stations in north Brazil (PV8) that are also frequently heard, and occasionally a few in HC and ZP. Crossing the equator to the countries further south is far more difficult. Sporadic E is not enough to reach the far south. Concurrent TEP is needed to complete the path. Conversely, this is a relatively easy path via the F-layer during solar maxima.

I heard one LU last year who could not hear my 150 watts. This year there was one great opening to LU and I worked 3 stations, including LU5FF on FT4. I'm glad he changed modes since FT4 is much faster and more stations can be worked during these brief opening. Yet few others do the same. I get few takers when I CQ on FT4 during strong openings.

In that opening and another, CE and CX stations were heard and worked by those not too far away from me. Unfortunately I copied none of them. These are two more countries to try for next year.

The wanderer

It is not only more hams that are showing up on 6 meters. As more and fancier digital technology invades our communities there are more noise sources. I am occasionally affected even though my closest neighbour is 250 meters from my 6 meter antenna.

The latest invader is sometimes called "the wanderer". It is showing up in numerous places. The signal takes the form of a carrier, either alone or with weaker companions, that wanders up and down the band in the vicinity of the digital watering holes above 50.3 MHz. An example spectrogram is shown to the right.

In my case it peaks to the northeast. This is odd since the distance to my nearest neighbour in that direction is 1.5 km. Others see the wanderer far stronger, including some VHFers that I know in the Toronto area. It hasn't affected my operating, yet, although others have been less fortunate. I am sure the responsible devices are known, just not by me.

There are other noises on the band that I can identify. One is my own heat pump that radiates a weak though narrow signal when it is in air conditioner mode. It is not causing me any difficulty so I'll let it be.

Signal quality

There are many hams who have no idea how to adjust their rigs and PCs for digital modes. The result is a lot of heavily distorted signals that cause grief to everyone. The worst offenders are not the newer hams. These more experienced hams ought to know better. We all make mistakes but some persist in their bad behaviour despite having been informed by their neighbours. 

I don't expect any improvement until there is a new generation of equipment that does not rely on the SSB mode of current generation rigs. There are too many ways to mess up our digital transmissions so that even the most careful of hams will run into problems. 

Below is an example of third audio harmonic distortion that may be due to over-driving the transmitter or amp, or forgetting to turn off the SSB compressor. Most distortion case are unintentional so they may need nothing more than a gentle reminder from someone they know. They may be remain unaware of the problem if others are too shy to tell them.

When a ham informed me of a signal quality problem this year I went to work on it immediately. Although the root cause remains elusive I was able to find a workaround to clean up my signal. I'll have more to say about this in a future article since it could prove useful to other digital operators. I am awaiting a few parts to make the solution permanent.

Remote confusion

Numerous 6 meter enthusiasts in locations less favourable for select DX paths are opting to operate remote. I see just the US side of this phenomenon, and I know it is also done elsewhere. The motivation is to improve results for DXCC, VUCC and other awards, or to simply have some fun. With many commercial "for rent" stations in the northeast US they are routinely heard here. 

The rules for DXCC credit remote operation from within the same country. Like Canada, the US is a large country, and with 6 meter propagation being what it is there is a strong temptation to take advantage of remotes thousands of kilometers away. It is not unusual to hear US 6th district call signs sporting W1 and W2 grid squares during openings to Europe, Africa and further afield.

None of this bothers me. I don't often bother working the US and Canada on 6 meters so it is easy to ignore. Besides, with US call sign regulations it is common to hear mismatched grids and call signs. However, there are some that are less observant of the regulations. A few Europeans have been heard on these remotes using their home grid squares and not signing portable. I can only shake my head at this absurd behaviour. A stable and strong signal with an east European call sign on 6 meters is suspicious when other signals from that area are weak or absent.

I know that the commercial station owners tend to take a dim view of this use of their facilities. How well they police their customers is unknown to me. I am not really affected other than to shake my head at the absence of operating ethics of a (hopefully) small number of hams.

Wrap up

I was so focussed on 6 meters that, on reviewing my log, I found that I made only 4 QSOs on HF in July. That is, other than 800 during the IARU contest. My HF activity continues to be low in August, so far, while I am busy with antenna construction and non-ham summer pursuits. That will change. It takes time for me to switch gears and get back into the HF mindset.

I continue to monitor the spotting nets for signs of 6 meter activity despite my absence from the band. Anything can happen at any time of year. I am not terribly interested in meteor work, such as the ongoing Perseids meteor shower, since it does not mesh well with my DX ambitions. Who knows, perhaps I'll try EME one day.

Overall, my 2021 season was a good one. I had fun and it kept me active during the months when my attention would otherwise wander to other things. Soon I will begin to consider my plan for the 2022 sporadic E season. From here on the difficulty of working new countries will increase quite a lot. Either I will have to get lucky or countries less distant will have to be active. There are quite a few in the Caribbean I have yet to work. I anticipate no changes to my 6 meter capabilities in 2022.

The coming winter E season is less compelling since my geomagnetic latitude is not favourable to generate DXing opportunities. Unlike last winter I will have my 6 meter antenna connected, so we'll see.

Skewed Vertical Arrays

Not long after my article on arbitrarily spaced driven verticals a reader posed an interesting question, one that was only briefly touched on in the article: can the array be steered by adjusting the element phases? The actual question wasn't exactly that so I am paraphrasing based on our exchange. Unfortunately the answer is no, not really.

With 2 elements in a driven array you can steer pattern nulls but not the main lobes. To steer a null you adjust the phasing so that at a selected direction (azimuth and elevation) the phase difference is 180°. In the general case there will be more than one null, but no more than two for the optimum spacing of ¼λ. This can be useful for attenuating local noise sources, but it is not useful for a transmit antenna.

With 3 or more elements there are improved steering options. The design can be complex since power splitting and phasing must be adjusted for all the elements, and the vertical elements cannot be collinear (in a line). Examples include triangle arrays and 4-squares. General steering isn't worthwhile since the patterns are broad enough that several fixed, switchable directions are sufficient.

Since there are ample designs and commercial product for 4-squares and other multi-element driven vertical arrays, I will focus this article on steering of 2-element end-fire driven arrays and 3-element reversible vertical yagis. 

The steering will be done by skewing the elements hung from a common catenary (rope) between non-conductive or non-resonant supports. The supports are thus excluded from the models. Skewing can be useful when the positions of the supports do not allow the catenary to be run in the desired direction. That is, we have a means to put arbitrarily positioned supports to good use.

I previously showed how to skew a horizontal wire yagi for the same catenary challenge. The design ideas in this article can be seen as complementary to that one. The examples are for 160 meters since the low bands are where these designs are likely to be attempted. It is easy to scale these antennas to other bands.

Modest skewing

I adapted this antenna from a 2-element 160 meter end-fire with optimal ¼λ element spacing and 90° phase shift. The base of each element has an orthogonal offset of 10 meters so that the angle to the catenary of the line connecting the element bases is about 26°.

For convenience, the model was done with perfect ground. This is acceptable since the pattern skew is not affected by ground quality. For a real antenna the elevation angle for optimum F/B is dependent on ground quality and element phasing.

The skew of the azimuth pattern is 20°. This is close to the value of the physical skew of the element bases. That's a good result. The bases are separated by 0.28λ because of how the skew was achieved. Despite that change the phase shift to preserve the F/B performance remains at 90°. As mentioned above, for real ground the phase shift should be adjusted to optimize F/B at a more realistic elevation angle.

Greater skewing

As you increase the skew angle the array performance deteriorates. Even so, you can push quite far and get a good result. In the example below the bases of the 2-element end-fire are skewed 45°. Unlike the previous example, the bases are rotated on a virtual circle so that they retain their ¼λ separation. The apex of the elements is lower due to the tilt.

Pattern skew is, again, almost the same as the physical skew: 40°. The phase shift was increased to 130° to get the best F/B. F/B is better with a smaller skew. 

I included the elevation plot since there is a significant change. The horizontally polarized radiation due to the greater element tilt fills the pattern at high angles. Depending on your operating interests this can be good or bad. For a DXer the high angle radiation is not useful in most cases. For a contester it can be beneficial to work closer stations.

Vertical yagi skew

For this antenna I took an existing 160 meter model with 3 wire elements and shifted the base positions of the director and reflector by 15 meters orthogonal to the catenary. Element spacing is equal (30 meters) since the antenna was designed to be reversible. As with the first end-fire the angle to the catenary of the line connecting the element bases is 26°.

Pattern skew of 20° is only slightly less than the base skew. That's better than what I achieved with the skewed horizontal yagi referenced earlier. It is interesting that the pattern skew is the same for the 2-element end-fire that also has a 26° physical skew.

Adjustment of the elements to optimize for the new configuration is recommended to get the best from this antenna. I skipped that step since it does not affect the skew angle.

Geometry of skewing

The reason why a skewed vertical array works is that the field strength of an antenna (or antenna element) is in proportion to the current. Since the current on a ¼λ vertical is greatest at the base and the height of the average current is low, the virtual position of the vertical is not far inboard of the base. Thus the pattern skew is only a little less than the physical skew angle.

While the horizontal polarization is increased with element tilt the impact is modest for low tilt angles. As discussed above this can be bring an operating advantage for some and an annoyance for others. Extreme skew is not recommended. It is possible to compensate by raising the catenary height and tying the tops of the wire elements to the catenary with ropes. But if the catenary must be lower the tilt and horizontally polarization radiation are increased and the elements will likely need to be loaded in some fashion.

The direction of the element base offset only slightly affects performance of a driven array since the elements are far enough apart that the mutual impedance is low. However, element phasing must be changed to accommodate the geometry. That is why keeping the element bases at the ideal separation for an end-fire by skewing them on a virtual circle is not critical. Yagis are different and the adjustment for large skews will require model experimentation.

Element tilt and spacing has an impact on the feed point impedance. Modelling is helpful but you should measure the impedance after building one of these arrays. Design the matching network for the measured impedance.

Ideas, not designs

Details for the models in this article are deliberately avoided. I have not delved deep into these skewed vertical arrays and I am reluctant to provide detail that might be read as recommendations. The ideas presented in this article are just that: ideas. Should you wish to further explore these antennas, I suggest that you model alternatives and choose ones that suits your operating objectives and circumstances.

A few ideas for more interesting skew verticals that come to mind include:

• Pair of skewed end-fire arrays for 4 switchable directions
• 4-square with skewed vertical wires
• 3-element skewed vertical yagi, with 5 elements for 4 switchable directions
  

I have no intention of building a skewed vertical array so I have no real life experience to share. I can achieve my most urgent needs on 160 meters by shunt-feeding and driving both of my big towers.

Perils of Software Applications

The modern amateur radio station is filled with software. It has been a long time indeed since this was a hardware hobby. Not only is there a lot of software, many of these applications communicate with each other and are connected to the public internet for our convenience and to access external services and to allow updates and maintenance. 

Amateur radio software is no less susceptible to privacy and security flaws and risks other devices in our homes and pockets. Hams can be very trustworthy, however that trust comes with risks other than the assumed good intentions of the software authors. Danger lurks in the margins. What tweaked my current interest was an article about a backdoor in a popular SDR application.

This article is not specifically about that compromised software. I have nothing to say that isn't covered better elsewhere. This is an example of the software author, supposedly with good intentions, leaving a gaping security hole in its users' shacks. Exploiting the vulnerability is straight-forward, if you know that it's there. Some did know.

With that introduction let's look at a small sampling of the potential dangers of software. This is meant to make you wary and not to scare you away from ham software. I know hams that avoid all software they absolutely cannot live without. That's extreme and it's an attitude that will diminish your enjoyment of the hobby.

Signing authority

Signed software is safer than unsigned software. An application signed with a digital key issued by a reputable CA (certificate authority) provides a degree of assurance that the software came from the expected source. If in a subsequent download (e.g. update) the key is missing or different, or from a different CA, it should raise alarm bells. Don't install the software until you can verify that it is legitimate. In a minority of cases there may be a good reason for the change.

I have previously held the signing keys for two companies. They were closely guarded. An attacker that gains access to the keys can cloak malware as legitimate software. Losing the keys causing grief for the author and for the application's users. When you install digitally signed software you are trusting that the author has kept the keys secure.

I have a product that uses a root CA that is unrecognized by Windows. This is risky and Windows will understandably pester you about it. This CA issues the keys for the drivers used by the VNWA3 from SDR-Kits. I accepted the root CA and all is well. Users of products like this need to be diligent. Check user forums and other sources to validate as well as you can that it is safe before you proceed with installation.

Of course, quite a lot of ham and hobbyist software is unsigned. There is a cost involved to acquire and maintain the key, which hobbyist software authors may want to avoid. The CA is a business which has expenses and it must maintain high standards. 

You can never easily know the trustworthiness of unsigned software. If it's open source you can inspect it for potential malware or flaws and then compile it yourself. However that is a tedious and difficult process that few of us would willingly undertake. It is easy to miss exploits, even by software professionals, since they can be disguised in many ways. You can hope that another user will spot the malware and make it public, yet the one mentioned earlier either went unremarked or unreported to the user base for years. Software updates must be similarly validated.

Desktop sharing

Desktop sharing apps have been around for a long time. Original uses included remote PC maintenance by IT personnel, and as a brute force means to share presentations and other live communications between individuals and groups. These applications are used for station remote control by allowing the station owner or another operator to use the station PC as if they were in the shack.

It should be obvious that protection of credentials is critical since if they are acquired by a bad actor (and it could be another ham!) the PC is compromised. After sharing credentials they should be revoked. New credentials can be created as required. 

You probably want a free desktop sharing app, but don't be so cheap that you ignore who produces it and where it comes from. Commercial software is often more secure and that may be worth the price.

Phone apps

I have several radio related apps on my phone, and many hams have more. There are apps to monitor DX spotting networks, ham chat groups, radio remote control, and much more. The major apps stores -- Google and Apple -- are quite good at preventing malware from being distributed but cannot catch everything. For example, user privacy is often disrespected. 

The app may ask for security permissions it does not require and the user may not be able or be interested in scrutinizing the app before installing it. Some of those permissions can be quite risky. But if you disagree you cannot use the app, and that will reduce the value of the associated hardware and software products: you are damned if you do and damned if you don't. Exploits can persist for a long time before being discovered. By then it could be too late. 

It is safer to stick with reputable vendors but that is not a guarantee. Some risk taking is unavoidable. One thing to consider, particularly for Android, is to avoid side loading of apps. Side loaded apps are not scrutinized by the name brand app stores. The only apps I have side loaded are those I developed myself.

Don't click!

There are endless sites on the internet that promise free software and manuals for everything you can imagine, or that you can't imagine or wish you hadn't. Many of the popular search engines filter or flag suspect sites and those known to host malware. Not all do. Even the best may not have identified the latest rogue site.

In the pursuit of free too many people are tempted to click first and ask questions later. This is one of the highest risk things you can do on the internet. There are many zero-day exploits not yet patched by the browser and OS vendors, or your PC may not be up to date with patches for those vulnerabilities. Anti-virus software offers little protection against zero-day exploits. You must rely on your own good sense.

A single mouse click can ruin your day. There may be no recourse by the time you realize what has happened. Free can have an outrageously high price. It is better to pay for software and manuals. The companies and individuals deserve recompense. You will receive little sympathy for falling prey to the attackers that rely on your desire for a "good deal".

There's more...

This article touched on only a few potential security risks associated with ham software. There are too many for me to list, and I certainly couldn't make a complete list. The only difference with ham software compared is that the authors are usually individuals or small companies that may be unwilling or lack the expertise to keep their software secure.

My intent is to make you stop and think when you next download and use software for your station. Even software that comes from reputable manufacturers can pose risks since they may be cavalier about your privacy and security, or their internal security so lax that a third party or disgruntled employee can surreptitiously embed malware.

Consider this article food for thought. Think about risk the next time you download and install software onto your PC or phone, or update the firmware on your equipment. Guarantees are impossible but you can reduce the probability of becoming a victim by being sensible, careful and a responsible consumer of software and software products.

High Power SO2R Novice

I am not very good at SO2R contesting. It takes a lot of skill and practice, and I have little of each. I am working at improvement, but slowly. The IARU contest last weekend was particularly challenging for me since I was not at my best (bad year for allergies). It was my first SO2R experience with high power. My band pass filters are built and working and the two amps are wired up and ready to go. 

I've come a long way since I first tried SO2R. The filters were an interesting project that will in the future (but not the near future) be a topic for the blog. They are prototypes, built from kits, that the designer plans to develop into a product.

Since everything in my SO2R setup is manually switched, all the equipment has to be on the operating desk where I can access the controls. The desk is becoming very crowded (and my photography skills have not improved).

When I change bands on one of the two operating positions the actions I must perform are:

• Change the band on the rig
• Switch the band pass filter band
  
• Switch the antenna
  
• Switch the amplifier band
  
• Tune the amplifier

Over time my intention is to make all but the first automatic. That will take a lot of work and, yes, money. Some are not so difficult (filters follow the rig or software band data) while others require home brewing (antenna selection) and the purchase of auto-tune broadband amplifiers.

The following notes describe my SO2R experience with high power in the IARU contest. There are no great revelations. I document these items to help me decide what work to do on the station and which operating skills to improve. Other contesters might benefit from these notes as well.

The attraction of power

Big signals attract big crowds. This is great in a contest, if you deal with the pile-ups and weak callers. It takes skill and talent to make the most of it, and even more to do it SO2R. Having your concentration and focus rapidly alternating rapidly increases the challenge. With two radios the challenge is more than doubled. The progression is not linear. With power you run (and must run) more frequently and that imposes a discipline that is difficult to perform well without years of experience and dedicated practice.

I admire those who can reliably isolate one caller among many. It is more difficult when (on CW) they have almost identical audio tones, stations call out of turn, all amidst the QRM of a popular contest. Mortals like me often resort to sending partial calls (hoping the rest standby) and narrow filters. When the two QSOs slip out of sync (it's inevitable and frequent) you end up having to copy on both receivers at the same time. It is difficult but it can be done.

Weak callers are a different and in many respects a more difficult problem. Pulling out a weak signal may require temporary adjustment of receive audio level, RIT and selection of antenna or receive antenna (low bands). Many of those weak stations aren't QRP but stations in a direction not favoured by your antenna. Remember, you have a big signal so hams everywhere will hear and call you, not just those you're pointing at.

The solution is practice, practice and more practice. At the moment I am struggling to deal with the many callers a big signal attracts. It can be a lot of fun but, oh, what a mess!

Signal quality

SO2R is one of those rare occasions when you have to eat your own dog food. Does your transmitter have key clicks, phase noise, excess harmonics, IMD, hum and other signal quality problems? You have to put up with it on the other radio. The majority of hams never really know how their signal sounds to others on the band, and most hams are too polite to say. The transceiver monitor feature is of no use to assess RF signal quality attributes that impact others on the band.

For low power BPF the transmitter harmonics are strongly attenuated. This is not the case for amplifier generated harmonics. No amplifier is perfectly linear so there will be harmonics; the BPF on the other radio does not filter in-band harmonics. You remove harmonics with band-switched notch filters (coax stubs) or more expensive high power BPF.

This is more frequently a CW problem since the harmonic relationship of the HF bands permits more opportunity to encounter harmonics your own harmonics. One avoidance technique is to operate near the lower end of the higher band and the higher end of the lower band. Compare the potential interference of the combinations 7020 and 14040 versus 7040 and 14020.

Harmonics are broader than the fundamental signal, being two and three times broader at the second and third harmonics, respectively. The harmonics of a dirty signal can be brutal. These include key clicks, phase noise and IMD. The second 40 meter harmonic of my FTdx5000 can wipe out 15 to 20 kHz of usable spectrum on 20 meters. This is with the latest firmware and options set to minimize key clicks. High power worsens the problem by 10 db, which is a lot.

I am increasingly disillusioned with the quality of Yaesu equipment. Signal quality flaws have been baked into their designs for decades. When the shack equipment is updated the Yaesu transceivers will be replaced.

Managing the amplifiers

Both amplifiers -- Drake L7 and Acom A1500 -- are tube amplifiers that must be manually tuned. Correct tuning of the latter is more difficult due to the power tetrode design. The grounded grid triode design of the L7 is more forgiving.

For every band change, large frequency shift or antenna selection the load and plate controls require adjustment. A quick antenna swap to work a multiplier is easier with the L7 since you can get by with a mistuning for a QSO or two. The A1500 is less forgiving. For that reason I strove to have the left radio with the A1500 always running. In the long run it saved time.

One particularly annoying aspect of tuning the amps is the transmitter power setting. Not all transceivers have a front panel control for power: the FTdx5000 does and the FT950 does not. For the latter it's a menu item. Before the contest I set the menu to the entry for power for rapid access. Rigs with small physical size are more likely to hide this item in a menu to save panel space. The power level must be higher when using BPF to compensate for filter losses -- in my case, from -0.22 to -0.55 db.

Eventually I hope to have all low SWR antennas and broadband amps, perhaps with an ATU. When that happens there will be no need to repeatedly fine tune the amps. Band and frequency changes would require no operator action in most circumstances. Not always, of course, since not all antennas are so perfect and weather can play havoc with antenna impedances. There will always be a need for manual tuning or reprogramming of the ATU

Time spent tuning is time not spent working stations. It is also a task that accelerates operator fatigue. SO2R is particularly difficult since you are working stations on the other radio while tuning the amp. I had to be quick or be interrupted by the QSO in progress.

Hot as heck

The typical efficiency of a linear amplifier running in class AB is no better than 60%. For a power output of 1000 watts there will be 700 watts or more of heat dissipation, plus that of the filaments (tube amps) and heat due to the inefficiencies in the power supply. The same is true of the transceivers. Luckily the duty cycle is not 100%. To be more precise, it isn't 100% on each radio but close to 100% overall with SO2R.

Since the objective is to transmit at all times, by alternating the two radios there is always a transmission occurring. With CW or SSB the effective duty cycle is about 50%. Put it all together and the continuous heat dissipation in a high power SO2R station is around 600 watts. It will be different in each country for those operating at the legal limit (e.g. 1500 watts in the US).

Our summers are short, hot and humid, and the peak is July. A mid-July contest guarantees that the shack will be uncomfortably warm. The air conditioner is not a perfect solution since the thermostat is not in the shack. Dress accordingly and have liquid refreshment within reach. In our cold winters the amps keep the shack warm and cozy.

Software reliance

Computer hardware and software are integral components of the modern contest station. You can resort to the 3 P's in an emergency -- pen, paper and paddles -- at the expense of time, rate and a horrible mess to deal with after the contest. Suffice it to say that the machinery should be kept in good working order.

I had computer and software issues for the first hours of the contest. SO2R was very difficult. From diagnosing the problem I discovered that Windows 10 was waging a forever war against a privacy protection app running in the background. The 2 keyboard feature of N1MM Logger+ is very sensitive to latency and I had severed latency. Key presses frequently went to the wrong radio's window. Once I corrected the problem the software preformed as it should. Microsoft won this round.

There is more to be done to reduce the severity or recurrence of problems like this. The PC is dedicated to the radios and I typically have no other apps running during contests. 

I have modest RFI issues with the PC which are exacerbated by high power. This is primarily a 40 meter problem since that is the only HF contest band for which I have an antenna nearby. The RFI has not yet caused a computer or interface failure. More ferrite chokes on cables must be installed to reduce the risk. I've been lucky so far.

Physical condition

SO2R is intense. It demands a lot from your mind and your body. You must remain alert for long periods and have the physical endurance to manage the equipment, listen, log and make frequent choices about bands and directions to target. With 2 keyboards the physical demand is greater due to swivelling back and forth between radios, and some awkwardness using the paddles and mouse which may be difficult to reach and use amid the desk clutter.

High power is more demanding because the rate is higher. QRP SO2R can be quite relaxed since there is often just one QSO happening at a time. Low power boosts the rate enough that it is almost never relaxing. High power SO2R is frenetic.

As we age our concentration and physical endurance deteriorate. On the positive side, SO2R helps fight the decline: use it or lose it. Although I am fit for my age it still becomes a grind as the hours accumulate. Although I practice and do the best that I can, I will never be an great SO2R operator. Oh, to be young again! Actually, I doubt that youth would help me. The inherent talent is weak in me. We can't be good at everything, so we do the best with what nature gifted us.

Next steps

For the next while my focus will be on antennas and not station automation. I'll live with the shortcomings of my SO2R setup until then. I could move faster to get everything completed sooner but, hey, it's a hobby.

Aircraft Scatter on 6 Meter FT8

Aircraft scatter has long been used by microwave enthusiasts. Enthusiasts monitor aircraft data and operate remote to get the best from it. Although the propagation mode is artificial in a way, it is very useful for getting over mountains and other obstacles, and to make the contact when more natural propagation is uncooperative. On HF aircraft scatter is of little value since ionospheric propagation is very reliable, and in any case the scatter is poor because aircraft are too small to be good mirrors at the longer wavelengths of the HF bands.

On 6 meters, aircraft scatter is not terribly useful since there are other and better propagation alternatives. It is of no benefit for my focus on DXing, and can be a nuisance. Aircraft scatter is easily seen on the spectrogram window of digital communication apps, when you know what to look for.

For our summertime amusement let's look at aircraft scatter on 6 meters FT8. For those within about 100 kilometers of a major airport it is common.

The example below right shows aircraft scatter for a signal source less than 100 km from me, as the crow or aircraft flies -- the intervals are compacted because I was transmitting during the odd/second periods. 

The distance matters since you will only see this for stations not too far away. This will be explained later. 

Those images look like and are often incorrectly identified as distortion. Sometimes they are decoded and sometimes not, depending on the coherence of the scattered signals. Above is a report I received from a ham in the Montreal area (about 175 to 200 km away), when we were both new to FT8. He wondered what was going on, wondering whether there was something amiss with my transmitter or his receiver. So did I at first. It is interesting that the primary signal at 314 Hz was not always decoded when the scattered image was.

The spectrogram at right is from a station in Ottawa (80 km away). There are two of these shadows, due to at least two airplanes. We live underneath the busy air corridor between Montreal and Toronto, and there are medium size airports in Ottawa to the north and Syracuse to the south. In this airspace there can be many aircraft scattered signals. The multitude of local airfields contribute little to the aircraft scatter we see because their traffic volume is low and the aircraft are small with a small reflection surface.

If the station is farther than a few hundred kilometers and you see these images, and they're not decoded as coming from other stations, it might indeed be distortion. For comparison with aircraft scatter the spectrogram below shows actual distortion on an FT8 transmission. There are similarities and obvious differences.

 The diagram below is a simplified view of aircraft scatter. It is not to scale and it is restricted to just two dimensions. That is enough for us to get started.

There are similarities and differences with CW (continuous wave) Doppler radar systems. The receiver (A) and transmitter (B) are widely separated and the two signals are not brought together for processing. I have commercial experience with products incorporating low-power CW radar so investigating aircraft scatter recalls memories from years ago.

Because A and B are far apart we have a large area (2) that is atypical of conventional radar systems. It is the most common scenario for aircraft scatter because for (1) and (2) the path loss is substantially greater and tropospheric refraction is required when the aircraft is below the radio horizon for one or both of A and B. Diffraction may be needed to get around tall buildings and hills when the elevation angle is small.

Since the aircraft velocity is high and the fuselage has a complex shape, the specular reflection is not coherent and varies with time. Also, the projected cross section and orientation of the aircraft affect the amplitude and coherence of the reflection. We are not concerned with calculating a precise reflection signal so we need only be aware that several factors impact signal amplitude, variability, Doppler shift and FT8 signal integrity.

Now we come to the radar equations. The first impacts path loss. The inverse square law tells us that signal strength decreases 6 db when the distance between the transmitter and receiver is doubled. For radar the path loss is 12 db because the path loss is 6 db to and from the target. For aircraft scatter, the radar equation for path loss is a limiting value as the aircraft distance goes to infinity. In most cases the path loss is intermediate and is close to the inverse square path loss when the aircraft is between A and B: case (2) above.

Aircraft scatter is therefore more likely when the aircraft is not far from both A and B, and is unlikely for cases (1) and (3) when the aircraft is more distant. The projected surface area of the fuselage is at a minimum in the latter cases cases, which contributes to a scatter signal too weak to be detected. 

The second radar equation is for the Doppler shift. It is double the value of conventional Doppler shift because it applies for the incident radiation and for the reflected radiation. For (2) the Doppler shift is lower than for conventional radar because it is positive in one direction and negative in the other, and the sum can be a small Doppler shift. Despite the reflection not being visible on the spectrogram it can interfere enough with the direct signal to reduce the probability of decoding success. The decoding algorithms are fantastic but they don't handle overlapping synchronized signals very well.

Let's do a calculation. The maximum air speed of a commercial aircraft is very roughly 1000 kph. Yes, it can be higher when at cruising altitude and travelling east within the stratospheric winds, but those are uncommon for the scatter scenarios we are likely to encounter. We do not require high accuracy for this exercise, so 1000 kph is useful as an appropriate maximum velocity.

The Doppler radar frequency shift is easy to calculate. A speed of 1000 kph is 278 m/s, or 0.00000093c, where c is the speed of electromagnetic radiation. At a frequency of 50 MHz the Doppler shift is therefore 92 Hz. At higher VHF and UHF bands the shift will be greater, in proportion to the frequency. 

It is a coincidence that the aircraft scatter Doppler shift and the FT8 42 Hz bandwidth are of the same order on 6 meters. Since the 92 Hz maximum is uncommonly seen, the majority of shifts are closer to that of the FT8 bandwidth. When the shift is less than the bandwidth of an FT8 signal the direct and scatter signals will overlap.

When the two signals overlap the effect is more likely to be mistaken for distortion. The effect reduces decoding probability but it is not the fault of the transmitting station. Be very certain of your signal analysis before criticizing the operator!

Perhaps the most obvious example of aircraft scatter is the one at right. Do you see what is happening? The aircraft is most likely turning to a direction that increases the relative velocity with respect to A or B. The aircraft rolls during the turn, orienting its belly to present a greater projected surface area. When the rolls ends the scatter surface is reduced. Doppler shift continues to increase as the aircraft gains speed.

This is most likely occurring soon after take off with the usual turn from the runway direction to the desired heading. Certainly there are other explanations consistent with that spectrogram, however, even if my interpretation is incorrect the depicted characteristics of aircraft scatter are illustrative of the general phenomenon.

We are not done. Our antennas play an important role in another characteristic of aircraft scatter on FT8 signals. The amplitude of the scatter signal is not constant, as is easily seen across the long 15 seconds on-off intervals of FT8 transmissions. That alone is highly suggestive of the effect not being due to distortion.

At right is an example. Both the scatter and direct signals vary in strength, however they do not vary in tandem as they would for distortion. The scatter signal strength also oscillates over time. Again, although there is more than one possible explanation for that specific spectrogram I want to focus on one in particular and probable explanation.

VHF antennas develop numerous elevation lobes due their great height relative to wavelength, and far filed interference with ground reflections. The greater the height, the more lobes there are. These multiple lobes come into play when an aircraft enters the field, and they do so in a manner that is far more evident than far slower propagation path changes involving the troposphere and ionosphere. 

The elevation angle between the aircraft and both the receiver and transmitter changes quickly as the aircraft races past. It traverses those lobes and nulls as illustrated below. As before, the diagram is not to scale.

The elevation angle from a fixed point changes with the aircraft's position. The elevation plot is for my own 6 meter yagi that is up 24 meters (4λ). With all those lobes the signal strength to and from the aircraft will oscillate in a regular pattern for several minutes. The more rapid oscillation at VHF and UHF bands above 6 meters will see the oscillation occur within a single FT8 interval. At lower HF frequencies the oscillation may be mistaken for ordinary propagation effects.

Most 6 meter operators will observe a lesser effect because their antennas are lower and will therefore have fewer elevation lobes. There is a similar effect in the azimuth pattern for aircraft moving across the field of view (transverse motion) which is less observable because the Doppler shift is small and there are fewer lobes in the azimuth pattern of a yagi.

So much for fun with 6 meter FT8 aircraft scatter. Now it's back to working the DX on 6 meters. It's hard to believe that the sporadic E season is beginning its decline and soon my attention will turn back to HF. I'll have more to say about this year's sporadic E season in mid to late August when it is done. All I will say right now is that it has been interesting, challenging and rewarding.

Towers and Surface Rust

The title is not really serious: there is no such thing as surface rust. Rust is rust. It starts at the surface and then penetrates deeper. There is a quip that says surface rust is a term usually only heard when someone is trying to sell you a tower. 

The question to ask is: how bad is the rust? Over the decades I have come to almost always dismiss a used tower that has "a little surface rust" or that "just needs a coat of paint." Buyer beware! You may not want to overlook a bargain but know the danger signs. In particular, beware of used towers exposed to saltwater since rusting is accelerated and it is more likely the tower is unsafe. A fresh coat of paint can hide many defects.

There is another saying: rust never sleeps. This one is true. Rust never goes away on its own; you must deal with it. If you don't, you end up with a tower that looks like this:

This article is about tower rust, judging the severity and dealing with it.

Hopefully your tower will never suffer rust like that pictured above! It is a Trylon tower section leg that I replaced with a new one. The previous owner stored the tower for years on the ground -- which you should never do -- allowing moisture to erode the hot-dip galvanized coating and then aggressively attack the base metal. Tower sections stored outside must be kept off the ground with lumber, bricks or another sturdy support.

In this case the price was right so I took it and repaired the damage by removing the rust and replacing parts that were beyond repair. The manufacturer advised me against doing the repair, which was very understandable. Confident about the extent of the damage and my ability to deal with it, I acquired the parts and went to work. The tower has been standing for almost 5 years and has largely been trouble free, despite supporting large antennas and all the wind and ice to be expected in this climate.

Above is a less severe case; I've seen far worse in ham installations. The tower is several decades old and has been painted a couple of times after the original galvanizing eroded and rust appeared. How bad is it? There are several increasingly aggressive methods of rust removal to guide us:

• Rust that can be wiped off or comes off with several strokes of sandpaper, steel wool or similar abrasive pad
• Rust that requires a wire brush or moderate use of a rotary power tool, before sanding
• Rust that will only come off with a file or grinder

In all these cases it is possible to achieve a flat, smooth and rust free surface ready for painting. However, other than for the first in the list you need to think carefully about the consequences. Rust is steel that has been chemically altered and removed from the base metal, which unavoidably weakens the structure.

That weakening has nothing whatsoever to do with removal of the rust. We remove the rust to, first, inspect and assess the remaining metal and, second, to prepare the surface for a protective coating to prevent further rusting. More advanced rusting with fissures and pitting requires removal of good steel to achieve a clean surface.

In most cases a judgment call is required, and that is not easy to perform. Engineers and tower manufacturers will press for replacement over repair for ultimate safety. This is sensible and not a ploy to separate you from your money. It is difficult to impossible to certify a rust damaged tower.

To put it another way, without a detailed engineering model you don't know the impact of removing deep rust damage. When you've ground down to base metal and you measure the thickness of the remaining metal with precision calipers, what will you do with the data? This is especially critical with narrow gauge steel where removing 0.75 mm (~1/32") of a 1.6 mm thick (#16 galvanized) brace leaves only 50% of the original. The same rust depth in ¼" steel plate is rarely a concern.

I have purchased towers with rust, including my Trylon tower. However, I tend to advise others against doing the same. The reason is that I can inspect a tower and apply my decades of experience to the assessment. This is still far from foolproof since I am not an engineer and they aren't always right. 

My one inviolate rule is to never purchase a used tower that has been painted due to rusting. The paint makes it very difficult to assess the tower. I have to see the metal to have any hope of judging its quality. I can paint it afterward, if necessary.

Quite a lot of the rust on the above tower falls into the third category. That's a problem for those narrow gauge braces. It's not my tower, so all I did was pass along my assessment. 

Compare that to the rust inside the Trylon leg, at right. This is an area that was cleaned of rust and coated with cold galvanizing paint before the tower was raised. Some of the rust has returned. It came off with a few swipes of sandpaper, ready for painting. This is not rust that imperils the tower.

Rust maintenance on the Trylon

Towers with tubular legs worry me. There is no good way to inspect the inside for rust damage. Sealing the top of the legs isn't helpful since weather-induced condensation will coat the inside walls with water and the interior humidity will be high. For those opting for towers with tubular members it is strongly recommended to buy the very best. 

In the US, Rohn is a good example whose tubular leg towers are popular with hams. They have an admirable service record. But, do not bury the bottom section since the condensed water inside the liegs will collect at the bottom and eat the steel from below. Use a proper concrete base to avoid water pooling and the invisible corrosion it promotes.

Trylon Titan towers are made from formed sheet steel members that are heat treated, hot dip galvanized and bolted together. They are reliable self-supporting towers for medium size HF yagis -- but, please check and follow the specs. They are in wide use by Canadian hams. For the purpose of this discussion, there are no hidden spots for rust to hide. If it's there, you will be able to see it.

Nothing is forever and so it is with rust removal and paint. After being up for more than 4 years there is rust on a few of the repaired areas. This was to be expected, and now it has to be redone before it becomes more than "surface" rust. I had to climb the tower in any case to reposition the mast after a windstorm overpowered the rotator mast clamp, again (hmm, maybe that's a topic for a future article).

Before pushing on, it is worthwhile to discuss rust that does not require treatment. Above is the Hy-Gain Tailtwister rotator on a Trylon tower shelf (in shadow at upper left). The steel shelf is not rusted. It is due to rust being washed by rain and snow from further up, pooling on the horizontal surface and the rust residue remaining after the water evaporates. Typical rust sources are steel masts and non-stainless fasteners.

At right you can see the beginning of rust at a spare hole in a tower leg and on the left edge of the leg. Galvanizing and paint often don't adhere well or form a thinner coat at edges and corners. Often it is better to leave it be since removing the rust and painting may make it less durable than its current condition.

My recommendation is to not touch it until the rust is more prominent and begins to spread onto the flat surfaces of the leg. Loss of steel from rust in these places does not weaken the tower. My complacency ends when I rust repair reduces the thickness of the tower member.

Like an experienced woodsman I blaze a trail while inspecting the tower for rust. As I go up the tower I place markers where rust is found. You don't have to do this if you repair each spot as you locate it. I find that marking first and repairing later is a a more reliable approach since, if you do it on the same climb you will get tired and sloppy, or the weather will change and you have to continue another day, and you will forget where you stopped.

A marker is as simple as a bit of electrical tape. It's cheap and reasonably visible, and you almost certainly have a roll of it handy. Use a colour other than black for improved visibility. Take care to place the tape near the rust patch and not on it.

Finding all the rust can be difficult. Towers have vertical, horizontal and diagonal members, and insides and outsides. For maximum coverage I climb the tower on one face and down another. I look up, down, inside and outside every few feet. I clip on for a closer inspection and to mark the spot. It can be tedious work, so stay alert to minimize the chance of missing rust.

On the next climb I clean the rust on the way up and paint the bare metal on the way down. It is very important to paint on the way down, which should be obvious if you think about it for a moment! All the tools I need are a few scraps of sandpaper and a rag to clean the dust. When it comes time to paint, look closely to ensure you can see all the sanded areas. Galvanizing and bare steel can look remarkably alike and it's easy to miss spots. Missed spots will soon have a new coat of rust.

It is safer and easier to use spray paint on the tower. Calm days above ground are rare so wear old clothes and stay upwind if you can. Don't fret over paint that blows away; paint isn't expensive. I doubt that more than half the paint I use hits the tower even in a light breeze. Return a day or two later to apply a second coat.

Keep your hands, feet and fall arrest equipment away from the fresh paint, or in the path of the spray. As you climb down it is easy to forget the wet spots when you're focussed on your personal safety. The paint won't hurt you but it will make a mess of your clothes and equipment and ruin the tower repair work you've just completed.

I use a cold galvanizing paint. Although the zinc in the paint coating is not nearly as durable as galvanizing it typically endures better, in my experience, than rust protection paint. Rust paint erodes quickly where your feet and hands land during tower work. I use cold galvanizing paint to repair galvanized and non-galvanized tower steel, to protect guying hardware and more.

I hesitate to make a product recommendation since the VOC (volatile organic compounds) and zinc content varies with local environmental and safety regulations, even for the same brand. In Canada, retail cold galvanizing paint has lower VOC and zinc content than what professionals can access. I use a retail product by Rustoleum and it has worked pretty well for me. Talk to a tower pro who can direct you to the best locally available product.

One caution about paint is to be careful that it is the right type for the job. Above you see the rust protection paint on one of my guyed towers has worn off by repeated abrasion from my boots. The original is very good and adheres to the galvanizing below. It can be difficult to find a paint that adheres well to older paint, and to properly prepare the surface. It can be an extraordinary amount of work. 

Where the original paint is gone, a galvanizing primer is usually needed before the top coats are applied. Surface preparation includes removal of curled and bubbling paint. This is important since these create voids where water can pool and promote rusting. Failure to prepare the surface can make the problem worse because the old paint is stiffened and thickened so that it will last longer and more effectively trap water than if left alone.

Aside from rust repair, I have a few more tower maintenance tasks to complete this summer. Once those are out of the way and the hay is harvested the antenna work can begin.