Tuesday, June 23, 2020

Spotlight Propagation on 6 Meters

A long time ago I was driving through the city with one of my siblings. The sky was uniformly gray and a steady rain was falling. Further down the road the rain abruptly stopped within less than one block. Two blocks later the pavement was dry and soon thereafter the clouds parted and sunshine blazed. It had become a beautiful day.

She expressed surprise that the rain could disappear so abruptly, with such a sharp dividing line. I answered that the rain has to end somewhere so why not here. Of course this happens all the time but our perspective is different when we are standing still and the weather moves over us rather than the opposite.

Radio propagation can behave the same. For sporadic E on 6 meters we are almost always riding the MUF, the frequency above which the signals don't reach the ground. The E-layer regions of unusually intense ionization are small and typically only support propagation between relatively small areas, except in exceptional cases. We call it spotlight propagation and like a spotlight the edges are often sharply defined.

A shift of a few kilometers can be the difference between the opening of a lifetime and an empty log. I experienced this phenomenon twice in the past week. With the arrival of the summer solstice the sporadic E season is at its annual peak 6 meters is consuming all the time I have available for operating and makes me all too aware of the joys and frustrations of DXing on the magic band.

I will give you two examples that are illustrative of the phenomenon of a bright line between the haves and have nots. These may help to explain why you are in a DX desert or the land o' plenty.

About a week ago 6 meters was wide open almost everywhere at the same time. Within minutes signals could be heard here from Europe, Africa, South America, the Pacific and the Far East. I didn't know which way to turn the yagi. The rotator had a real workout that afternoon.

I worked a lot of amazing DX, including my first KH6 via sporadic E. The spotlight roamed in the Pacific, eventually striking the sweet spot for working Hawaii. In other places the spotlight stood still, such as in South America where I only heard Ecuador and proceeded to work what may be every HC active on 6 meters. What I didn't work was the Far East.

It was aggravating. Just 300 km to the west many hams I know in FN03, EN93 and nearby grids (and eventually extending west right across the country) were working a steady stream of JA, HL, DU and BY. Here is FN24 I decoded a total of 3 FT8 messages, and that was from a single JA. That's it. While it was going on I was exchanging messages with a couple of friends in FN14, about 100 km to the west, and they didn't hear anything at all (my antenna is better).

In the map below you can see that the beginning of the peak probability for the path to Japan and the vicinity is 2200Z when the sun is midway between here and there, and therefore the greatest insolation on the northern edge of the path. Sporadic E is sporadic but it needs energy input.

This was my second near miss with JA this year. It's a difficult path and the rain has to end somewhere. That somewhere was near Toronto. The good fortune did extend further south, along the same path from the northwest that favoured southern Ontario. You just had to be in the right place; FN24 was in the rain and FN03 was in the sunshine.

I shouldn't complain too much. I had my day with working Japan last year, and there is reason to hope for more this year. To the east, further from the line of propagation, the big gun 6 meter stations heard nothing. It's a difficult path since a short move east brings the path from the Far East closer to the north pole. The benefit of 24 hour daylight in the Arctic at the solstice is rarely sufficient.

The next example of spotlight propagation is one that favoured me and few others. During a marginal opening to Europe I saw 6W1TA calling CQ on FT8. I turned the beam east and his signal came up nicely. Since I worked him earlier this year I let him be. I tend not to make duplicate contacts since you never know if the person will be annoyed or glad to know he's getting through. However I am always happy to respond to duplicate callers.

This went on for some time. Was this a spotlight opening and only I could hear him? There could have been other factors like yagis pointed to Europe or the Caribbean which would significantly attenuate west Africa signals. Perhaps I should have spotted him although that would have required starting other software and I rarely spot FT8 stations since anyone can see them themselves if they have propagation.

It isn't every day you are called by a moderately rare station and especially on 6 meters. I suppose he heard little other than me since the CQing and I didn't notice him call anyone else. Those in rare locales are hams just like everyone else and they want to communicate. I was happy to make the duplicate contact.

It's funny that not long ago I chased him hard to earn the new 6 meter country. Last year I was unlucky and missed him because although the opening was superb it was also superb for everyone else in North America.

Eventually he did work at least one or two other stations. I was amused to later discover that he had spotted me. You need propagation to benefit from being spotted and 6 meter sporadic E long DX paths and this was no different. No one scrambled to call me or they did and heard nothing.

I was going to end the article here when the very same thing happened again the next day.

It is difficult to believe that only I heard him or that everyone in this region has worked him. I monitored 30 minutes of CQ with no apparent callers. His signal was weak but steady all that time. There are a few others active in Africa so hopefully one day I'll work them. But I worked TT8SN already and elected not to bother him.

I will stop now. Should you be wondering what happened to all the technical articles, fear not. I have several projects underway and I won't write about them until they're done. Pictures and words are accumulating and the articles will eventually be published.

While that work continues in the background and with sporadic E in full swing these little articles almost write themselves. I suspect my slow summer blogging pace won't return this year.

Wednesday, June 17, 2020

Don't Be a Curmudgeon

"I've come up with a set of rules that describe our reactions to technologies:
  1. Anything that is in the world when you’re born is normal and ordinary and is just a natural part of the way the world works.
  2. Anything that's invented between when you’re 15 and 35 is new and exciting and revolutionary and you can probably get a career in it.
  3. Anything invented after you're thirty-five is against the natural order of things." 
The quote is by the British comedy writer Douglas Adams. For brevity we'll call the last point the curmudgeon's credo. It applies to pretty much everything. Yes, that includes amateur radio. The demographics of our hobby are such that over 90% fall into the third category. I am and it's very likely that you are, too.

As we grow older not only do we because suspicious of new technology and ways of doing things we are more likely to say it. The social governor we grew in adolescence begins to malfunction. We say what we want when we want to everyone and anyone. We ignore or fail to see the reactions of others. Physical presence is unnecessary: we say it on air or by pounding on a keyboard.

When I was a new ham in the 1970s quite a few of the older generations remained dissatisfied with SSB and transistors -- "real radios glow in the dark" -- and made sure you knew it. Contests and DXpeditions were frequent targets. For my operating interests I endured many derisory comments from the gray haired crowd. I quickly lost my respect for these elder hams. My teenager friends and I sneered and laughed at them and finally ignored the curmudgeons altogether.

Yesteryear's adolescent hams are today's curmudgeons. The passage of time does that. Not all of us succumb though far too many do. They're everywhere. Indeed, you may be one and not realize it.

Do you believe that the quality of hams declined when CW was removed from the license exams? Is FT8 not real ham radio because it's a machine talking to another machine? Do you demean those who use modelling software to develop and optimize their antennas because you know that any old wire tossed into a tree works just fine? Do you lament the endless cries of "599 04" filling the bands during contest weekends despite your not having turned on a rig other than a 2 meter handheld for the past 3 months?

If you answered "yes" to any of those questions you may be a curmudgeon. Even if you thought to yourself, "no, that's not me," there is likely another similar question out there that will tempt you to answer "yes." I consider myself pretty adaptable to the progress of technology and operating practices and yet I occasionally get caught. The certainty of the old is insidious.

There is an antidote. There is a cardinal rule to remember to test yourself for curmudgeonly behaviour. I don't know the original source though variations are widely quoted. It can be a tough pill to swallow.
Never mistake a personal preference for a universal truth.
You see this play out daily in the news, infecting politics, religion, our families and our jobs. The tide against this style of thinking currently fills the streets with protests. Hams are not immune to the same deep misunderstanding. Escaping the trap of "universal truth" requires a leap of perspective.

No one is beneath you because they make different choices. Time changes everything. What was once common is now rare and what was once impossible is now routine. Technological progress changes our culture and especially the interests of the younger generations of hams. There is no right and wrong about it, just that we are most comfortable with what we know and believe.

Remember that and you will avoid becoming a curmudgeon. If you discover that you have become a curmudgeon it is never too late to change. Kick yourself out of your comfort zone and try something new. You have nothing to lose and you may find a new way to enjoy amateur radio and gain new friends.

Sunday, June 7, 2020

L-network for Stacked Yagis

An important component of switching stacks of yagis is an impedance matching network. When yagis are connected together their impedances are in parallel. Two 50 Ω yagis present a 25 Ω load to the transmission line. The parallel impedance should be transformed to 50 Ω to lower transmission line loss and to keep the transmitter happy. The network is switched out of circuit when only one of the yagis in the stack is selected.

The network is 2:1 for a two yagi stack and higher ratios should be used for 3 or more yagis. More than one network may be needed if a variable number of yagis can be selected in stacks with 3 or more antennas. The 15 and 20 meter stacks I am building have two yagis each so that is the antenna system I will primarily address in this article.

The simplified schematic shows the topology of a stack switch for two yagis. The options are upper, lower or both in phase (BIP). Switches for both out of phase (BOP) and 3 or more yagis are similar but more complex. I am sticking with the simplest version since it helps with the explanation and is what is needed for the 15 and 20 meter stacks I am building. 'N' is the matching network.

Most stack switches I've looked at typically have all yagis connected and in phase, therefore the network is in line by default. When only one of a two yagi stack is selected the network is bypassed and that yagi is connected to the input port. The basic two yagi circuit requires 4 SPDT relays rated for the RF power. The diodes isolate the control lines from each other since both must power K1 and K2.

The unused yagi(s) can be left connected to the network under the assumption that the yagis are far enough apart that their mutual impedance is low. This is a fair assumption since otherwise the network would require customization to accommodate impedances that are neither 50 Ω individually nor 25 Ω for two together.

With DC injectors, reverse polarity for one the selections and several diodes the transmission line can be used in lieu of separate control lines (3 wires for the above circuit). For the design of my station it is more convenient to use a dedicated control cable. With a common DC ground a total of 5 wires are required for the 15 and 20 meter stack switches (they are on the same tower).

There are a variety of networks that can be used for the impedance transformation. The most popular for HF are:
  • Broadband transformer: Typically a transmission line transformer with trifilar windings on a ferrite toroid.
  • Transmission line section: ¼λ transmission line to transform the 50 Ω of each yagi to a higher value so that the parallel impedance is 50 Ω or, alternatively, to transform the parallel impedance to 50 Ω.
  • LC network: An L-network or similar network comprise of discrete inductors and capacitors.
Commercial products are almost all of the first type (example). These can be made to work on all bands from 80 through 10 meters with high efficiency and so a single product can suit many applications. Many home brew networks for mono-band stacks use the second type with 70 Ω coax (e.g. RG11) switched into both yagi ports. In all cases the electrical lengths of 50 Ω coax from the switch to each yagi must be equal to phase the yagi feed points for maximum gain.

Although they look simple enough building your own broadband transformer is not without its challenges. These are not conventional transformers but transmission line transformers. These are variations of the excellent design to be found in Sevick's Transmission Line Transformers book. Modern versions use Fair-Rite 61 mix 2.4" OD ferrite toroids.

The number and arrangement of the trifilar windings has a significant effect on efficiency, optimum port impedance and impedance ratio. Get it wrong and the heat generated at maximum legal limit, especially on 10 meters, can destroy the transformer. Compensation for stray capacitance at the highest bands may be required. When properly designed and built they perform very well, with a loss better than -0.1 db (20 watts dissipation for a 1000 watt transmitter) from 80 through 10 meters.

The transformer, and any matching network for that matter, can exhibit different behaviour with high Q yagis (e.g. most tri-band yagis) at the band edges where the SWR is high. Deviations of the impedance ratio and efficiency in these situations can become a serious problem with high power. Regardless of the matching network and SWR it is important that the yagi impedances are near equal at all frequencies of operation to achieve equal power division. Special design considerations for stacking dissimilar antennas are beyond the scope of this article.

For a multi-band yagi a broadband transformer is the best choice since the others have a narrower bandwidth and are only suitable for a mono-band stack, with narrow exceptions as we'll see. Since most stacks are mono-band the narrow band choices deserve a close look. I plan to build my own stack switching systems since they are not complex and I can put the money saved into other projects. The learning experience is another benefit. But I would like to keep it simple, hence the motivation for this article.

Since transmission line sections require more extensive switching systems and I don't have a ready supply of RG11 I pivoted to L-networks. Both can be very efficient for the broadband yagis I've built since the impedance is close to the ideal 50 + j0 Ω across the band. TLW produced the following design (with a low pass network topology) of a 15 meter L-network for a 2-yagi stack.

The L and C values are easily attainable. For stability the capacitor should be one that is not temperature sensitive and must have a low ESR (equivalent series resistance) for high efficiency and large enough to safely dissipate the heat. The coil will be physically small and with an easily attainable Q of 400 will only shed 3 watts at a power level of 1000 watts. Efficiency is worse for large deviations from 50 + j0 Ω so design the network accordingly.

While this is a simple and efficient way to match the stack parallel impedance there are a few issues to be considered:
  • Tuning: Unlike a broadband transformer the L and C values must be carefully adjusted. A small variable capacitor with a rating of at least 1000 volts in parallel with a similar fixed capacitor is a good choice. The coil can be tapped and once the correct value is found the tap can be permanently bonded.
  • Bandwidth: The design is for a single frequency near band centre. The network must work across the entire band and behave well when the yagi SWR is high. Yagis optimized for gain can have a high Q and therefore high SWR at the band edges.
  • Field management: Within a metal box the value of a coil is different due to the field intersecting the enclosure, either increasing (steel) or decreasing (aluminum). Toroidal coils are mostly immune to this effect. Variable capacitors, the coil and wiring will exhibit stray capacitance with each other and the enclosure walls.
On the positive side the network is cheap and efficient and easy to adapt to bigger stacks by switching in one or more capacitors. For a 3-stack C is 210 pf and 260 pf for a 4-stack, while L decreases very little and can be left alone. An intermediate capacitor value can give a good match to 2 or 3 yagis in the stack without the need for switching.

The tuning process is not too demanding. To deal with enclosure effects simply cover the box after each adjustment. Broadband transformers and transmission line sections have similar issues that, although smaller, can be more difficult to compensate.

For me the critical issue is whether the L-network is broadband enough to use across the typical amateur band without additional tuning elements that must be dynamically switched. To test the concept I used EZNEC to model the L-network. As a first step I simulated the 2-stack 25 Ω parallel impedance with a long lossy transmission line. I have found that this is a good technique to emulate a resistive load in EZNEC. Although only virtual wires are needed the model requires a real wire so I specified one but didn't use it.

That's excellent. The match is even broader than the 15 meter band. I developed similar L-networks for 20 and 40 meters and achieved the same result. L-networks for a 3-stack and 4-stack were equally good across the entire band despite the higher transformation ratio.

Real antennas do not have a perfect 50 Ω impedance across the entire band. I did not explore the L-network's performance with high SWRs since my 15 and 20 meter yagis have low SWR (below 1.5) across each band. Besides, the other impedance matching alternatives would fare no better. With EZNEC I stacked the 5-element 15 meter yagis at 100' and 150' (close to the actual heights of my antennas), including the gamma matches previously modelled.

The match is perfect, barely deviating from the SWR curve for an individual yagi. The lengths of the phasing harnesses are nominal and close to the actuality but would only have a significant effect on the match were the SWR high, which it isn't in this case. To be fair the match is also very good for the broadband transformer despite its 22.25 Ω antenna port impedance (shown below). It is common for the same transformer to be used in a 3-stack, with 2 or 3 yagis selected, since the SWR is moderately good for a 17 Ω parallel impedance.

Having reached this point there is one important question to be explored: does a real L-network live up to the promise of the theory and model? Happily enough the answer is yes. I bread boarded the 15 meter L-network with little regard to good layout. The many sources of stray L and C tune out during adjustment of the network. The yagis are simulated by two parallel 51 Ω carbon composition resistors.

I measured the same excellent result with the L-network tuned for 20 meters. The only difficulty with the tuning was moving the coil tap with this fragile setup. Squeezing and spreading the coil turns does not allow a wide enough adjustment range.

To give an idea of how far the network can be pushed I modelled a L-network centred midway between our two closest (by percent) HF contest bands: 10 and 15 meters.

Although it does reasonably well it is marginal. It certainly cannot be used for a stack of tri-band yagis or for any other pair of adjacent bands. To test the model I measure the SWR of the 15 meter L-network prototype from 14 to 30 MHz. It does better than I expected for 20 meters but is unacceptable on 10. There may be unexplored loss in the network at the frequency extremes that damp the measured SWR and therefore overstate its actual performance.

Building it

The upper 15 meter yagi was raised last week. I'll have more to say about it in a forthcoming article. Once the yagi is in position at the top of the mast and the phasing harness installed I will build a stack switch using the L-network discussed in this article. If all goes well I will do the same for the 20 meter stack. The upper 20 meter yagi will not be raised before late summer.

Sometimes it feels like progress on the station is glacial. That is unavoidable since I do most of the work myself and rely on friends to help out with the big jobs. But I would not experience the same sense of accomplishment by hiring out the work and only using commercial products.

If all goes well I am going to have a lot of fun during the upcoming winter contest season.

Tuesday, June 2, 2020

6 Meter Blues

This is my fourth full season of 6 meter operation from my new QTH. It is also my third using FT8 for sporadic E DXing. Compared to my previous intensive 6 meter activity in the 1980s the experience is very different. With activity and propagation ramping up for this season I am doing some reflecting on where I am and where I'm going with respect to 6 meters. It's still fun although I find that my attitude is changing.

It's similar to when I was a new ham. Every DX contact was a thrill. There weren't many but each one counted. As time went on my expectations rose. No longer was I content with the same pace of DX accomplishment. I improved my skills and my station to do better, and when I plateaued I would at occasionally become frustrated.

Propagation on 6 meters this year is not out of the ordinary but my expectations have risen. So I sit here through the early part of the sporadic E season amid growing impatience. Where are the big openings? Why is my log not filling up with DX? Returning to CW and SSB from 50.313 MHz didn't help since almost all the activity is on FT8.

I'll run through a few examples of the categories of near misses I've experienced so far this season.

False dawn

A common occurrence during the summer sporadic E season is for the north Atlantic route to open in the early morning hours, from 2 to 4 hours after sunrise. The majority of these openings are unworkable from VE3 except with high power and big antennas. It's the E layer's big tease or, as I call it: false dawn.

Those to the east in Maine and VE1 fare better in these marginal openings. I can only watch as they work a few stations that barely decode here, if at all. The opening evaporates like dew as the sun rises higher in the sky.

Fortune favours...others

It is typical to hear your neighbours working DX that you cannot hear or is too weak and fleeting to be workable. Sometimes it's an adjacent province or state with the advantage and other times it's the lucky hams in the adjacent grid square. At least that's the way I always remember it. It is all too human to overlook those times when the advantage is mine.

It know my time will come, eventually. Hearing VE7/W7 have a great opening to Europe when I've worked next to nothing this year is especially aggravating. That's a remarkable and rare opening so kudos to them. I could deal with it better were we to get the correspondingly difficult opening to Japan and the Far East that is relatively easy for the west coast.

Another opening had W8/W9 working Hawaii. One more hop and we're right in there. Unfortunately "one more hop" is no easy feat. The only KH6's I've heard on 6 meters are Americans who've relocated to the mainland. My hopes are dashed when I decode the grid squares of those enticing call signs.

Same old, same old

Until yesterday (see below) I worked just 3 Europeans, all EA. Other DX, while welcome, was on well-trodden propagation paths: EA8, Caribbean, northern South America and Central America. Following the philosophy of appreciating what you have I jumped in with a modicum of enthusiasm. Many new stations were worked and a few from previous years were reworked.

There are surprises to be found on these paths. I worked 4 new countries, though none rare. These come about from increasing 6 meter activity from otherwise commonly heard countries. Same old, same old isn't necessarily bad.

Those enticing single decodes

The spotlight nature of sporadic E frequently results in fleeting openings at the highest frequencies. 6 meters, when it opens, if often riding the MUF. What might be long enough for a CW QSO on FT8 these blink-of-an-eye openings can result in a single decode and nothing more. It can be enticing.

Hearing TT8 was especially intriguing. From here the path is the same as for EA which were coming through at the time. Just one hop more is what it takes. But that one decode of TT8SN was all that I heard. A week later 6W1TA was in for a while with a weak signal. This bodes well for the coming weeks of improving propagation. The path to Africa is often open, but just as on HF there is little activity. Were Africans as common as Europeans the band would be sizzling.

Many single decodes were seen on the more common European path. The ones that catch my attention are the farthest stations and the countries I haven't yet worked. Eastern Europe and the Middle East  were the ones that kept popping up through May. This year I hope to work more of them.

Things are looking up

Had I written this article a day earlier I would have nothing more to say, and I would remain frustrated. Today we finally had a good opening into Europe and beyond. Adding 10 European QSOs in the FT8 log was very welcome. Among the enticing (and needed) countries heard were 4X, OD, LX YL, ES and more. I was called by a 4X station but we didn't complete the QSO. It was exciting nonetheless. Later in the evening several stations from TF, OX and LA were worked and I had a partial QSO with OH.

With the many projects underway my plan to upgrade the transmission line to the 6 meter yagi has been delayed. If all goes well it'll be done this month. That will give me a little more power by reducing loss. An amplifier remains in the plan but is not yet at the top of the list. A bigger signal would have netted me several new countries this season alone.

I now await improved sporadic E conditions as the peak approaches at the solstice in less than 20 days. It's a short season and that makes it intense and rewarding. If it were easy 6 meter DXing would be far less interesting.