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.

Driving Arbitrarily Spaced Verticals on 160 Meters

This time of year most of my activity is on 6 meters. It is the peak of sporadic E season, and that means DX and fun times. That is not my sole activity, and there are multiple projects to be worked on. One that is suited to the summer weather while I wait for the DX paths to open on 6 meters is computer modelling of antennas. It is an activity that is easy to fit in anytime since all it requires is a computer.

One of my challenges is to improve my 160 meter signal, and that involves gain over a single vertical such as my current antenna: a shunt-fed 40 meter tower. There are several possibilities open to me and a few I've previously discussed for the blog. These have largely involved parasitic arrays of various types. They can be simple to home brew and are friendly to experimentation, which I like.

Ground loss is higher for a parasitic array than a driven array because they are close spaced, resulting in a low radiation resistance, and therefore greater loss for an equivalent radial system. Consider my 80 meter vertical yagi and how it compares to the 4-square. It takes less space but is less directive and tends toward inefficiency due to ground loss. But I had lots of fun experimenting with it, and will continue to do so as I strive for higher efficiency.

My two big towers constantly tempt me to try something similar on 160. The 60 meter separation works well for a 3-element yagi, by shunt feeding the towers to tune them as parasitic elements and a wire driven element hung from a rope catenary between the towers. The towers are big enough that they already affect any 160 meter antenna in the vicinity so it makes sense to put them to use.

Unfortunately there are several difficulties with this antenna:

• The radials have to be removed during the summer during haying season. An efficient radial system for a 3-element 160 meter yagi involves a lot of wire, and that means a lot of work to lay them down each fall and to roll them up again in the spring.
• A yagi has just 2 switchable directions, which are roughly northeast and southwest. Those are the most important ones, so it is tempting despite the limitation. The yagi would need an omni-directional mode by using the driven element alone.

I have therefore started to consider driven arrays. The 60 meter distance between the towers is pretty good for that, although not ideal. My challenge is to design a directive array using the two arbitrarily spaced towers in a driven array that is superior to the vertical yagi option. If it can be made to work well there are several advantages over a yagi:

• The higher radiation resistance and one less element (2 vs. 3) allows equal efficiency with significantly fewer radials.
• Impedance matching and direction switching can be done at one location, not at each tower.
• Gain can be had in 4 directions rather than just two. Omni-directional is also possible.

With that introduction let's dive into the two types of 2-element driven arrays. I will then explore what is possible with my towers, with both shunt-fed and not at an ideal separation as a driven array.

The are two important parameters in a 2-element driven array: separation (D) and relative phase. The latter can be achieved in a variety of ways, the simplest of which is adjusting the lengths of coax from the common source (La - Lb). Since the feed point impedance is unlikely to be 50 Ω, a matching network (N) may be required. 

Elaborate power division isn't necessary in a 2-element array. D is sufficiently large to minimize mutual impedance between elements so that driving the elements in parallel works very well. The minimum D would be ¼λ, which is also the typical element spacing in a 4-square antenna.

Extensive detail about the design of driven arrays can be found in ON4UN's Low-Band DXing, and if these antennas intrigue you, that is where you'll learn all you need. My discussion is narrowly focussed on my particular needs, to best exploit my towers. As we'll see, compromises are necessary when you don't build the driven array from scratch.

End-fire and broadside arrays

2-element driven arrays of these two types. An ideal end-fire is unidirectional to either the left or right per the diagram above; that is, on an extended line between the verticals. In my case, these are roughly northeast and southwest, the two directions of greatest utility for my primary interest: contests.

In ideal end-fire array D is ¼λ and La - Lb = ±90° (left or right). D can be smaller with appropriate choice of La and Lb, however driving the array is more challenging due to increased mutual impedance and the criticality of dimensions. D should not be too much larger or the array will sprout unwanted side lobes.

In an ideal broadside array D is ½λ and La = Lb. The pattern is bidirectional. There are similar concerns regarding smaller and larger values for D, but usually of greater negative impact since, in addition to side lobes, arbitrary choices for D tend to circularize the pattern. However, there will still be gain.

The azimuth plots above show ideal 2-element end-fire and broadside array over perfect ground. The broadside array has more gain since the lobes are narrower, despite there being two of them. At right you can see the sprouting of unwanted lobes for an end-fire with a D of ¾λ and a broadside with a D of 1λ. At other large values of D the arrays are not really end-fire or broadside due to those large side lobes.

For proper operation, D for the end-fire and broadside arrays should be odd multiples of ¼λ and ½λ, respectively. However, there will be additional lobes for multiples greater than 1. Intermediate values of D can be compensated for by adjusting the phasing with suitably calculated values for La and Lb. We'll come to this later since it is the focus of this article.

The gain is around 3 to 4 db better than a vertical, over perfect ground; the gain difference is smaller over real ground. Decreasing D for the end-fire configuration can add up to 1 db of gain over the expected 3 db due to mutual coupling, which lowers the radiation resistance and therefore the field intensity due to the higher current. Of course ground loss will increase over real ground and a radial system, so in practice less of that additional gain can be realized.

Geometry of the end-fire array requires adjustment of the phase and spacing to optimize gain and F/B for a specific elevation angle. The reason is that as the elevation angle increases the effective value of D decreases. Rather than go into detail I'll again refer you to ON4UN's book, where this and more is described in detail. 

It matters since gain of a ground-mounted ¼λ vertical at 0° elevation is always very poor due to the phase reversal at grazing angles over real ground. It is therefore worthwhile to optimize the antenna for a higher elevation angle, typically 20° for DX performance on 160 meters.

The 3D plot at right is for an end-fire array optimized for cancellation at 0° elevation over perfect ground. You can see how F/.B decreases as the angle increases. The same is true off the sides of the broadside array.

Arbitrary D: Gain or directivity

Although quite simple, a 2-element driven array competes well in gain versus more elaborate parasitic and multi-element driven arrays such as 4-squares. What you give up is directivity. There is only so much that can be accomplished with 2 elements. For those like me with a set of directional receive antennas, that is not a problem. My priority in a transmit antenna is gain.

The ideal element spacings for end-fire and broadside arrays optimize both gain and directivity. Since D is ¼λ for end-fire and ½λ for broadside it is not possible to optimize a 2-element driven array for both. However, we can do pretty well with a non-optimal D. 

I am fortunate that my two big tower are 60 meters apart, since that is approximately ⅜λ, or midway between the two optimums. For those with verticals with lesser or greater D, the techniques I will discuss are applicable, although the results may not always be as good.

With a non-optimal D we can optimize gain or directivity, but not both. The plot at right illustrates this for the end-fire (unidirectional, cardioid pattern) by varying the feed point phase shift from 60° through 130° (90° is the primary pattern).

Ground is EZNEC medium, using MININEC ground to simulate a fairly poor radial system with an equivalent resistance of 10Ω. The radial system may be better than this, but this affects efficiency and feed point impedance, and not the antenna pattern.

The larger D requires a phase shift greater than 90° for maximum gain and a smaller phase shift for maximum F/B. This is at an elevation angle of 22° for medium ground. Notice that the "ideal" 90° phase shift is not suitable for medium ground and a D of 60 meters.

For real ground the peak of the main lobe is always substantially above the horizon so it makes good sense to phase the elements so that peak gain occurs above the horizon. In this case, 22°. Targetting a lower elevation angle should only be attempted with taller verticals or better ground, otherwise performance will suffer.

Notice that among this range of element phase differences the gain varies by 2 db and the F/B variation is profound. As expected for the non-optimal spacing the gain is poor when F/B is excellent and vice versa. One curiosity is that for the high F/B cases the main lobe is "two-headed", with twin gain peaks ±40° off the main axis. 

As the phase difference is further reduced the pattern gradually transforms into that of a broadside array. Again, this is what we expect and want. The next plot with smaller phase shifts shows our options for the broadside configuration of the array.

Broadside gain is barely affected with a phase difference of up to 30°. The side nulls are greatly softened with the non-optimal spacing, with little to distinguish them. There is no compelling reason to use anything other than in-phase (0°) feeds for broadside.

Gain is substantially lower than a broadside array with an optimal D of ½λ. It is lowered enough that th 1 db gain difference that favoured the broadside configuration for ideally spaced verticals now favours the end-fire array by about the same amount. Radiation that is "wasted" off the sides is not available to contribute to gain in the broadside directions.

For reference, the 22° elevation angle gain for a single, omni-directional vertical is almost exactly 1 dbi. Thus, for our D of 60 meters the broadside gain is 2.2 db (bidirectional) and the end-fire gain is 3.3 db. Although this may not seem like a lot, on 160 meters it will provide a substantial competitive edge. It is especially attractive due to its simplicity in comparison to a K3LR 3-element wire yagi, like my 80 meter array (about 4 dbi gain) and a 4-square (about 5 dbi gain). Radial requirements for similar efficiency are lower than for either of those larger and more complex antennas.

Feeding the array

To simplify the model, I directly specified the element phasing with sources for each of the two verticals. This is easy to do in EZNEC as follows:

For a real antenna the verticals are connected to a common feed point by coax. Coax has loss that is low enough at 1.8 MHz that it can be ignored. However, coax has another important effect that cannot be ignored since the mutual impedance between the two verticals changes the 50 Ω feed point impedance of each vertical. The complex impedance at the feed point varies with the length of the coax.

The individual vertical impedances are not 50 Ω so their parallel impedance is not 25 Ω. It is possible to adjust the gamma matches of each shunt-fed tower (or other vertical of your choice) to be 50 Ω or some other impedance that will present an impedance at the feed point that is "easy" to work with. That is not as easy as it seems since the feed point impedance is different when the phase is changed, as it must be to support both end-fire and broadside configurations. 

Out of curiosity I played with the model and successfully achieved an excellent match for the end-fire configurations. This is only useful if you only use the array as a reversible end-fire array, with no broadside or omni-directional modes. However, it's a cumbersome process that I do not recommend. The more sensible method is a switchable impedance transformation network at the feed point.

With the elements 60 meters apart the minimum coax to each vertical is 30 meters. I happen to have two ~40 meter lengths of RG213 and two ~40 meter lengths of LDF4 available, and I will likely choose from those should I build this antenna. I would run the coax over ground in the fall and roll them up in the spring for haying season, just as I do for the radials.

The patterns of the 3 combinations of end-fire (2 directions) and broadside (bidirectional) provide good compass coverage without bothering with feeding one of the verticals as an omni-directional antenna. I will therefore proceed to design matching for just those. To use one of the verticals alone is only a 50 Ω match if the other is detuned. I prefer to avoid the additional complexity.

In the model the 40 meter lengths of RG213 are joined in parallel at the feed point. An extra length of RG213 delays the signal (negative phase) of one end-fire element. For the 3 modes -- end-fire left, end-fire right and broadside -- the phasing (delay) line is switched into the right element, into the left element, or out, respectively. A separate control cable can be used for switching or two different DC signals (+12 and -12 VDC, for example) can be placed on the feed line using Bias-T circuits at both ends.

The length of the delay line is easy to calculate. To simplify for repeated experimentation I calculated the length of RG213 for a -10° phase shift. This is 1/36 of λ, which is 163.83 meters at 1.830 MHz. Adjusting for the 0.66 velocity factor the length is almost exactly 3 meters. It is then very easy to mentally multiply by 9 to find that a -90° phasing shift is 27 meters of RG213.

The plot at right is for a -120° phase shift which, as we saw earlier, is close to maximizing gain for verticals separated by 60 meters. For these electrically tall shunt-fed towers the gain is about 3 db versus a single tower. The gain is about 2 db for the broadside configuration.

Although there were no surprises replacing ordinary λ/4 verticals with my tall shunt-fed towers, the validation is necessary so that we can proceed to inspect the impedances.

Due to the small magnitude of the mutual impedance, the parallel impedance in both end-fire and broadside is not too far from half the impedance of each shunt-fed tower alone. However, since it is far from 50 Ω it must be dealt with.

The R component of the impedance is roughly half, which itself is a concern since this is a fairly high Q antenna where the 2:1 SWR bandwidth of each vertical is about 75 kHz. In end-fire and broadside configurations the residual X is mostly negative and positive, respectively, but not large in comparison to R. A relatively small X is a hint that matching will not be difficult.

Below are the model's SWR curves for end-fire and broadside, followed by an L-network for the end-fire case that optimizes the match at 1.830 MHz.

It is interesting that the bandwidth of the driven array is better than that of a single vertical. With an L-network tuned for broadside the result is similar. I experimented with the L-network design using TLW and came up with one that is easy to switch by only needing to switch taps on the series coil and using a fixed shunt capacitor midway between 1500 pf and 1800 pf calculated for each configuration.

I will not describe the network since its design is very sensitive to the design of the verticals and the lengths of the coax from the feed point to each vertical. Using TLW or a similar tool it is not difficult to design a switchable matching network based on field measurements after the array is built. For coax which can withstand the voltage associated with the uncorrected SWR, the matching could be done in the shack with an ATU or other device. The mismatched loss of any suitable coax is very low at 1.8 MHz.

Where I go from here

Although summer is not prime top band season it is a good time to play with antenna models in air conditioned comfort. I do not know whether I will build this antenna, and it is not a high priority for this year. It is good to have several options in hand when the time does come. 

Hopefully this design will spur some readers to explore similar possibilities for a high-performance 160 meter antennas. I like its good -- but not great -- performance and its simplicity. Most hams don't have the land or the resources to put up multiple towers and recruit them for use on top band, so I consider myself fortunate. Of course I moved here several years ago to make antennas such as this possible.

Digital Operating in the Near Future (speculative)

Let's imagine a fictional scenario for the near future. It certainly doesn't exist today, may never exist as described, and yet something like it is probable.

You turn on your HF-VHF transceiver and computer, ready for some contacts using your favourite digital modes. It could be FT8, FT4, Q65 or another one yet to be invented. You point your yagi in the appropriate direction and monitoring begins. 

It's all software decoding, so all you do is select the band and channel, and digital mode(s). Even that may not be necessary with an SDR rig. To check propagation you can monitor or you can select the "ping" option and your station rattles off a series a brief transmissions on all selected bands, the software automatically collects reception data from other stations around the world. Of course this will favour the direction the yagi is pointing and the bands it supports. Alternatively, without transmitting at all, you can display a map of the same for stations near you. There is no need to add to the QRM.

Assume a fairly wide digital sub-band. This is to be expected since digital modes have become the most popular modes on HF, and the trend is likely to continue. The rig and computer software are tightly integrated, more so than today. The rig will be SDR, with all signals in the digital sub-band available all at once, possibly for more than one band at a time. Channelization due to the predominant current use of SSB modes will erode, and spectrum usage may come to resemble traditional modes, bound only by widely accepted conventions, and regulations where applicable.

Since you are effectively monitoring all digital activity on the band, and possibly more than one band, there is too much information for one operator to digest. It must be filtered. The software itself will be comfortable with decoding hundreds of signals, which is already possible with existing computer hardware capacity. It is truly impressive how easily today's processors can handle multiple concurrent FFT, digital filters and data base operations.

To limit the information deluge of wide bandwidth reception and decoding the operator needs filtering tools or features. Some already exist. These include inclusive or exclusive filtering by one or more of: region (continent, country, grid field, etc.), frequency range, call sign or prefix, signal strength, award requirements (country, grid, state/RDA, etc.), contest participants (or not) and certainly many more you can image. Custom filters will be supported. Labels (colour, flashing, etc.) are an alternative to exclusionary filters for those who want to see all stations without constantly changing the filters.

Imagine spinning a VFO knob and adjusting a bandwidth knob or software control and watching the list of decoded stations within that spectrum immediately appear on the screen. Since all signals are being decoded all the time there is no need to wait. Digital modes will be auto-detected. Indeed, you can do searches to see all the messages associated with a station, sorted by time and frequency. The data is already there due to the mass decoding performed by the software.

There will be alerting apps that allows an operator to be notified when a wanted station is heard. The alert can be local or remote, such as to a phone. This already exists, and we can expect it to improve.

Transmitting will also have supporting features, whether to CQ or to answer a particular station. The software can find a free frequency for your digital signal that is wide enough and that hasn't been used for a specified time. Time slots can be automatically selected when you set the target region or station you are calling, in accord with operating conventions. Mode will be set to that of the station being called.

Conventions may determine mapping between frequency ranges and digital modes, or you can override when necessary. Select sub-band or mode, and the other will be set accordingly.

Coexistence with non-digital modes will be easier than today. Features can be used to strictly enforce mode use by frequency. Restrictions can be loosened or overridden for contests, DXpeditions and other events when one mode is temporarily more popular and needs expanded bandwidth to fit the activity. Indeed, CW at least may be just one more mode supported by the rig and software since it is compatible with digital modes in most cases, including decoding and automated message transmission.

Although we can continue with this speculative exercise, let's end it here. The reality will eventually go much farther than I've described.

If you dislike or even despise digital modes, you may be aghast at what I've described. It isn't the amateur radio you and I grew up with. Technology makes it possible and the generational tide will bring it to fruition, no matter our personal opinions and operating tastes. It will happen, if not quite as I've described, and we need to think about it to coax the technology and operating behaviour in the most beneficial direction.

Radio will remain the foundation. There will always be a need for the RF, including transceivers, transmission lines and antennas. Of course, there will be remote operation, and it will increase from what currently exists. The better the RF components of your station the better you will do, just as for conventional modes.

Operating skills will also be needed, however some of those skills will be different. Anyone operating both digital and conventional modes will understand. There will also be lids who flout convention or never learn how to properly operate their equipment and software. QRM, DQRM and all the rest will also continue, but so will our tools to deal with them.

I love CW and I will continue to use it, probably for the rest of my ham radio career. After initial trepidation I am now also comfortable operating digital modes. Many hams I know have gone through the same evolution, and many are still standing at the threshold, not yet ready to take the next step. The novelty and potential of the technology are draws to hams, so many of whom are technophiles.

I currently limit my digital operating to VHF and 160 meters. The former due to the advantages for DXing and the fact that almost all activity is digital. Outside of contests and DXpeditions on 160 there is little CW activity, and I do like top band. In time it is likely that my digital activity will increase, since that is where everyone else can be found.

As the transition to digital continues I will look for equipment that supports those modes with innovative and useful features. Most of the restrictions today are limitations of our rigs, forcing us into using SSB and 3 kHz channels, and 20 kHz sound card interfaces to computer software. Those limitations will disappear as SDR technology rises to dominance and the floodgates between rigs and software are thrown wide open.

My speculations about possible and probable digital features is the tip of the iceberg. Hams are inventive and we can be certain that far more interesting and sophisticated features are on the way. With a baseline of SDR rigs and computer intefaces, the features will be determined by software applications and not by the rig manufacturers. In my opinion, that is how it ought to be.

Are you horrified? Don't be. Embrace the change, learn something new and increase your enjoyment of amateur radio. The boundaries you see are self imposed. They really don't exist.

Amplifier Technology: Tube vs. Transistor

Choice of active device technology in a kilowatt class amplifier continues to be a relevant question in 2021. Be wary of those who will not allow that there is a valid choice to be made based on their merits and demerits. Rigid adherence to one technology or the other are rarely founded on the evidence. Never mistake a preference for a universal truth!

There is no absolute best when it comes to tube versus transistor technology for kilowatt class amplifiers. However, it is certainly true that tubes own the past and transistors own the future. Although we are currently well along the transition to solid state there is a strong case to be made, today, for vacuum tube technology. Since I am making a choice for today, I considered both.

This spring I sprung for a new amplifier. I've talked about it for a while yet did nothing. There was no immediate rush since it is primarily for contest work, and since there is a pandemic -- no multi-ops here -- I delayed doubling up. The Drake L7 was good enough for the interim. But with sporadic E season approaching and my lofty objectives for 6 meters this year, it was time to act.

After considering many alternatives I settled on the Acom A1500 kilowatt amplifier which covers 160 to 6 meters. It is not in any sense the "best" amplifier. It suffices that it fits well with my current and near future operating objectives. 

I will use this article to explain my decision process. Everyone's needs are different and my choice may not be your choice. When I eventually come to replace the L7 it will not be an A1500, and indeed it is likely to be a solid state amplifier. Having different types of amplifiers has its advantages. That said, over time it is almost certain that my shack will become all solid state.

This article is not a review of the A1500. Perhaps I will do one later. My purpose here is to review the pros and cons of alternatives relative to my various operating interests. I believe it is more important to understand that rather than simply going on a quest to find the best amplifier, which doesn't really exist.

Operating objectives

One of my most important applications of amplifiers is HF contests. For that I have several requirements:

• Reliable
• Rapid tune or no tune
• Tolerant of antenna mismatches
• Protection from operator errors 
• Low distortion: modulation and harmonics
  
• Ability to switch bands under software or hardware control
• Quiet

For daily DXing my requirements are a little different:

• Rapid on
• Rapid tune
• Tolerant of antenna mismatches
• Protection from operator errors
• 6 meters
  

Contests are scheduled and typically run for 12 to 48 hours. For general DXing time can be critical; that is, you want to use the amp as soon as you hear the DX or see the spot, or you risk competing with the descending horde. Leaving an amp running all the time is not too much of a problem for solid state devices. In contrast, tubes will experience gentle wear and burn electricity that will heat up the shack. That can be cozy in winter but not when the weather is hot. However, repeated power cycling can significantly reduce tube life.

That is one important difference for choosing an amplifier and device technology. What follows is my perspective of how my requirements map onto amplifier alternatives. While your interests will differ the overlap is likely to be large and therefore of potential interest.

Tubes

Transmitting tubes in the 1 kilowatt category are becoming an endangered species. They were never primarily marketed to hams and with the near 100% transition to solid state of commercial transmitters in this power category most are no longer manufactured. There still exist NOS (new old stock) tubes, if you can find them, and there are clone manufacturers that eke out a living producing a select set of discontinued tubes marketed to hams and others.

Caution about clones is understandable, although there are products that have proven to be reliable and a good match to the original. Two that are particularly important to me are Taylor and Setec. The first makes the 3-500Z clones used in my Drake L7, and the second makes the Eimac 4CX1000A clone used in my Acom A1500. Their prices are reasonable though not cheap. Manufacturing high quality products to a rigid spec entails significant cost, and deviations can quickly sink the business when unhappy customers and amplifier builders look elsewhere.

Glass envelope tubes are typically less expensive, although you'll almost always need 2 or 3 to make a kilowatt. They are physically large and kilowatt tubes like the 4-1000 are larger yet; it takes a lot of glass surface to dissipate the generated heat. Ceramic tubes are compact, with more critical air flow needs. Their small dimensions make more practical an amplifier that covers 160 through 6 meters. Tetrodes operated class AB typically have excellent distortion specs.

Well made tubes in a well built amplifier can be remarkably trouble free for a long, long time. The best tubes in a poorly designed amp, or operator abuse, can rapidly destroy the best tubes. Ground grid triodes are more tolerant of operator errors than tetrodes. It is recommended that these amps have built-in protection to prevent damage (more on this below). Vintage tube amps rarely do.

All tubes require warm up before use. Most glass envelope tubes have filaments and cathodes that are reasonably tolerant of short warm ups, although waiting a minute or two is recommended. Most ceramic envelope tubes have strict warm up requirements, typically a 3 minute minimum. This is fine for contesting but not when you need instant power to work a rare DX station.

Due to the high voltage and large output transformation ratios, it is rare for tube amps to have broadband output networks. It just isn't economical or practical.

Transistors

In the early days, transistors RF amplifiers were low power bipolar devices that required power combiners and other circuitry to put up to 8 of these devices in parallel. They were technological marvels that were rarely up to the high demands of active hams. In the decades since then the improvement of RF power transistors has been remarkable. There are now single LDMOS devices that will deliver a kilowatt of power from 160 through 6 meters.

As with any new technology, there is limited data on device lifetime. Performance is often not on par with the very best tube amplifiers. However, there are developments such as feedback between a well integrated transmitter and amplifier that deliver exceptionally reliable performance and low distortion. There is little doubt that the future of RF power amplifiers belongs to solid state.

Due to their small physical dimensions the great bugaboo of transistor amplifiers is heat. Inventing junction structures for efficient heat transport and a high frequency cutoff, while ensuring efficiency and linearity, have been challenging. Low tolerance to poor operating conditions and practices has required strict electronic protection measures. That has had implications for operating flexibility and amplifier reliability.

Progress continues apace. Increasing numbers of kilowatt class transistor amplifiers are reliable and tolerant of mismatches for an SWR of 2, and sometimes higher. The critical challenge is good design and construction since mistakes can be costly. Protection circuits are mandatory.

Mismatch tolerance

In a fantasy station every HF antenna is exactly 50 Ω at every frequency in its range. While every ham would benefit from this it is the contester that is most attracted to the ideal. Low SWR means no operating time is lost when changing band or frequency, thus (ideally) enabling a higher score by allowing time for more contacts and to catch elusive multipliers.

Real antennas are quite different. Multi-band antennas have narrow SWR bandwidths. This is also often the case with any antenna for 80 and 160 meters. For example, at right is the measured 20 meter SWR of my Hy-Gain TH7.

Even if you could achieve the ideal, weather will throw a wrench into the works. Rain, snow, ice and sometimes wind, alter antenna impedance, and in severe cases can greatly distort the pattern or make the antenna unusable. 

We simply do the best we can. Our transmitters and amplifiers need to cope with the real antennas out in the weather and their effects, predictable or not. But if your broadband amp cannot be tuned and the SWR climbs out of control at a critical moment, you are stuck.

ATU 

Adding a built-in ATU to a kilowatt amplifier typically increases the price from 30% to 60%. This or an external manual or automatic tuner may be needed for an SWR as low as 1.5 on some bands, and almost always when the SWR exceeds 2. The heat dissipation and heat density associated with high power devices demands less tolerance for impedance mismatches compared to low power equipment.

Manual tune tube amps are usually happy up to an SWR of at least 3, depending on the complex impedance at the antenna port. Since this is difficult to dynamically measure, most protection circuits keep it simple and measure either or both of the SWR and absolute reflected power, and make decisions based on that data.

There are a few tube amps that feature a built-in ATU, such as the larger version of my A1500: the Acom A2000a. ATUs are more common on modern solid state amps.

Broadband outputs or an automatic ATU would seem to be ideal for the active ham or contester to deal with our various antenna impedances. The reality is not so simple. Even if you are able to have a low SWR on all of your antennas there will be problems. Consider the following:

• Rain, ice and snow will alter the impedance of an antenna, often more than you expect
• Low bandwidth antennas -- trapped tri-band yagis and almost any low band antenna -- have high SWR at the band edges, and often over a large fraction of the bands they cover
  
• Switching among antennas on a single band -- something contesters do a lot -- will require retuning the ATU, switching the ATU in and out, or manual adjustment
  

The point is, whether you have a broadband output or an instant switched ATU, you will have to adjust tuning. It isn't a once-and-done deal. The tuning may be manual or automatic but it must be done. Choose antennas that reduce the need for tuning adjustments.

Protection

The A1500 is not my first tetrode amplifier. In 1985 I purchased a Collins 30S1 that used the Eimac 4CX1000A. As will all of these vintage amateur radio amplifiers there was no protection circuitry. You had to watch those meters and tune carefully! I was very nervous in contests since rapid tuning adjustments when switching bands and antennas was risky.

The seller included a 4CX1500B, which is plug compatible to the 4CX1000A and is a little more tolerant of poor tuning, though not by much. From what I've seen of the specs, the Setec 4CX1000A has grid specs similar to that of the more robust 4CX1500B, but don't quote me on that.

Compact tetrodes are very sensitive to secondary emissions that can quickly degrade and ruin them. It would be foolhardy to have a modern tetrode amplifier without a full range of electronic protection. DXers and contesters must rapidly tune, change bands and frequencies, and change antennas that require adjustment of the amp. Doing so without protection can be very expensive. We all make mistakes.

Kilowatt class solid state amplifiers have their own extensive protection requirements. Here the problem is excess heat, voltage and current in the active devices and transformers due to load mismatch and faults. Without protection, failure in a kilowatt class solid state amp can happen faster than you can react to meter displays. 

In contrast, a grounded grid triode amp is far more tolerant of abuse and some protections are optional rather than must have features. The smaller set of protections on late model triode amplifiers is not necessarily evidence of poor quality.

Price

Neither tube nor transistor kilowatt amplifiers are cheap, and are similarly priced. All require a hefty power supply, expensive active devices, heat removal systems, and RF circuits and switches that can deal with high heat, voltage and current. Some amps are more economical by using several less expensive, lower power tubes or transistors. There are a few manufacturers that cut the price, and corners, by reducing protections or providing little headroom in their published capabilities. Avoid the latter unless you enjoy gambling.

Before you opt for the cheaper amp you need to question whether you are pursuing false economy. Many vintage amps are available at attractive prices. Just be sure you know what you're getting into since you will have fewer protections built in and you will likely need to service the equipment soon after acquisition. This most often involves the tubes or power supply. Worse is a failing high voltage power transformer. They have a long though limited lifetime and are expensive to replace, if you can find one.

As mentioned earlier, if you need or want a built in ATU you should expect to pay a hefty premium. An external high power ATU can be purchased at a lower price, but it may not integrate well and will therefore require manual band selection. Alternatively, you can forego the ATU by making your antennas as broadband as possible, at the risk of not being able to use the amps when an ice storm hits right during a contest or DXpedition.

Service

Getting service for amateur radio equipment is worse in Canada than in the US and Europe. All the manufacturers of transceivers, amplifiers and other pricey equipment are foreign, and domestic retailers have largely exited the service market, even for equipment under warranty. When you need service under warranty the equipment must be shipped across borders, in both directions, with all the expense, paperwork and risk that involves. Amplifiers are heavy and fragile, so shipping to a professional service centre is to be avoided if possible.

The ideal is equipment that is reliable and that is easy to repair by a knowledgable ham. These days, amp manufacturers don't particularly like hams who open their equipment, let alone replace the tubes, and will void warranties when it is done. Tube equipment, especially vintage tube amplifiers, are easier to work on simply because their size is larger and you can get in there to inspect, test and replace components more easily than in tightly packed solid state gear. Unfortunately that temptation to dive in has led to many regrets, so never underestimate the size of the problem, or your abilities.

Changing the transistors in a amplifier has risks beyond the mechanical and electrical skills required. For example, heat sink compounds are almost all toxic and require expert handling to avoid health risks. Broadband matching networks are a black art for most hams and they would not (or should not) attempt to repair or rebuild them.

Power supplies are easier. Many hams can repair these when the need is parts replacement. Care is required when working on high voltage supplies (tube amps) and high current supplies (transistor amps). When in doubt, seek a professional.

There is a false meme that says the more expensive a product the less you need to worry about pricey repairs. The underlying assumption is that you are buying peace of mind for the higher price. This is as untrue of amplifiers as it is for appliances, vehicles, entertainment equipment and other products. The truth is that lifetime service cost tends to track the purchase price. Parts are more expensive and service rates are charged at a premium for premium products. Of course, cheap, poorly built equipment will fail sooner and require more frequent repair or replacement, but these repairs are typically less costly, and you may be able to do it yourself.

Station automation

An amplifier that matches the transceiver looks very pretty on the operating desk. That is true whether it is an SB220 paired with an SB102 or K4 paired with a KPA1500. In my opinion, appearance is at best a secondary reason to consider a matched pair. Although the interconnection between any transceiver and amp is straight forward, the benefits of well integrated products can pay big dividends in station automation and therefore operator effectiveness.

Some of these benefits include:

• Automatic band changes, following the transceiver, or both following a software app
• Low distortion, by managing gain by negative feedback (ALC) and, now emerging, compensation for amplifier generated distortion (e.g. PureSignal)
• Software control of the ATU, antenna port, metering, etc.

Perhaps surprisingly, there are a number of modern tube amplifiers that do these things well. After all, the electronics and software control surrounding the core of the amp can be the same for both tubes and transistors. However, most of the design and product energy is directed at the latest generation of solid state amplifiers. This is not surprising since, whatever you think of tubes, they are not the future.

Obsolescence

Would you purchase an amplifier that uses obsolete tubes or transistors? What will you do if and when they need replacement? Take advantage of current suppliers of NOS components to stock up immediately. If you wait until they're needed they may well have become unobtainium. The transistors in older solid state amps are becoming hard to find, new or NOS. Not only tubes become obsolete.

It is not just these headline parts to be considered in an old amplifier. Consider meters, transformers, relays and all the rest. I have had problems locating obsolete digital ICs to repair old equipment. Looked at individually, these are reliable components. Put hundreds of reliable components into an amplifier and there will be inevitable failures among them. Know what you're getting into when you buy an old amplifier.

How my amps measure up

The 40 year old Drake L7 is a well-built product. It is pretty easy to adjust on the fly due to its grounded grid design and 3-500ZG tubes. But it's old and parts fail. So far these have included the high voltage filter, T/R relay and variable capacitors. The lack of 6 meters is not a concern for those only interested in the HF bands.

The A1500 is a well-built modern tube amplifier that makes the most of the Setec 4CX1000A tube. It also covers 6 meters, which I had to have. During contests it requires more attention than the L7 because the tetrode is more sensitive to impedance changes when I change frequency and switch among antennas. It takes only a moment to tweak the load control to compensate, and you must do it to avoid tripping the protection. Protection circuits trip more easily at the high end of the amplifier's power range.

Despite the extra attention it requires, it is nice to have an amp that will comfortably and cleanly drive my diversity of antennas. As I evolve away from tri-band and other high Q antennas it may be possible to replace or complement it with a broadband transistor amp. When that happens the L7 will likely be kept on the shelf as a spare.

My problem now with the two amplifiers is that the operating desk is crowded! What you see in the picture is messier than usual due to the parts and test equipment for the various projects I am working on. Stuff gets shoved aside or I do a proper cleaning before major contests. 

One nice feature of fully automated amplifiers is that they can be taken off the operating desk. Should  you do that, keep them readily accessible for service or replacement during a contest. Manually tuned amps must be on the operating desk. They are typically laid out horizontally due to their weight (the A1500 is almost 30 kg). The L7 is light enough to stack on the transceiver because the hefty power supply is a separate unit.

Until the band pass filters are ready I am not using both amplifiers. All should be ready for the fall contest season. I will not be revisiting the amplifier choices in my station until the antennas and station automation are significantly improved.

6 Meters and the Mode Rut

Don't be surprised to see another article about 6 meters. It is my current focus and therefore it's what I am most likely to want to talk about. Actually, the topic of this article is not solely applicable to 6 meters, as you will see. Human nature being what it is the lessons are universal.

May 19 delivered an extraordinary opening between North America and Europe. With my improved station I exceeded my best ever day working Europe. Not a simple percentage increase, but 4× more QSOs. I also worked 5 new countries, bringing ever so close to 6 meter FT8 DXCC.

It was a busy day and I enjoyed every minute. From the few emails received afterwards there are quite a few Europeans who have yet to work Canada, or North America for that matter. One thanked me for being the "big gun" that made it possible for him. I was pleased to make it happen.

Here are my mode statistics for May 19. Please note that these are all on 6 meters, and all the QSOs are with Europe:

• FT8: 202
  
• FT4: 13
  
• CW: 23
  
• SSB: 0

I did not attempt to work Europe on SSB, which explains the zero. Signals at times were so strong I could have done so despite the 10:1 SNR challenge versus CW. This is important since those numbers are a rough indicator of what could be done and not what was possible. Momentum, activity level, QSO duration and SNR all figure into the statistics. 

By momentum I mean that while the pile up is ongoing it is difficult to pull back and switch modes, abandoning all those callers, or to switch modes when the spectrogram is enticingly lit up by countless DX signals.

Momentum can be temporary or it can be a regular characteristic of our on air preferences or beliefs. Why are there so few FT4 and CW QSOs listed? The signals were not there to be worked. By running and by calling others I soon ran out of stations to work on those modes. I would go back later to work a few more, who because of their own activity or propagation shifts were not there before.

When momentum becomes a habitual feature of our operating we are in danger of living in a rut of our own making. We dig the rut, jump in, like it and stay there. Hate digital? You stay in the SSB rut. You have a small station and can't work DX on SSB, and you don't know or like CW? You operate FT8. Repetition and time dig the rut, and soon our choices become habit. In time we forget that it was a choice and we rationalize our preference as the "best" mode.

Ruts are not only for modes. Far too many will not stray from 50.313 MHz. The 50.323 MHz intercontinental window is there for the taking. When the QRM is fierce on 313 it makes sense for the little pistols to try 323 so that their signals are not buried under others. The likelihood of success is higher on 323. However, most don't QSY, or they won't QSY. Humans are curious creatures.

I made a point of moving around during the opening. To me there was a price to sitting on 313: I might miss a new country. Let's say I'm having a good run on 313. While I'm doing that I worry that there might be a new country on 323, FT4 or CW. I tried not to deny any callers so I would delay moving until I had no callers. It doesn't take long to check the activity elsewhere and to call CQ a few times. There were a few pleasant surprises by doing so.

Since there is less activity outside the primary FT8 watering hole I was never away for long. I doubt I missed anything on 313 due to my frequent QSYs and mode changes. From the paucity of activity on FT4 in particular, and 323 to a lesser degree, too few are moving around. Why is it that so many find it easy to spin the VFO on HF and few will do the same on 6 meters? From the many recognizable call signs I know that they, like me, are active on both digital and traditional modes and know the value of a VFO.

FT8 is slow. It is so slow I can do other things while running a pile up. At least one ham I know has operated 6 meter FT8 while putting in a serious effort in an HF contest! Above is a picture of me working FT8 on 6 meters while adjusting my almost-complete switchable BPF (band pass filters) for HF contests. Being fleet of foot I have successfully operated FT8 while cooking meals, answering email, and doing a variety of tasks. You don't have time to do that on CW, SSB or FT4.

That's another thing that amazes me: why don't more use FT4? It's twice the speed of FT8 and only a little less sensitive. FT4 is fast enough that it almost feels like running stations on CW. So why don't more try it? During big 6 meter openings I can repeatedly call CQ on 50.318 MHz and have not one reply. Why not? It's those ruts we allow ourselves to fall into.

Can our tendency to fall into ruts be changed? Should it be changed? If hams are comfortable sitting full time on 50.313 MHz operating FT8, who am I to suggest they do otherwise? All I can do is point out the advantages of FT4 and jumping among the channels to improve the chance of success. 

For me, one forceful fact is the rapid QSB of sporadic E. I hate it when I fail to complete an FT8 QSO with a needed DX station because the opening is shorter than the time needed to complete the contact. By halving QSO duration with FT4 the 3 db sensitivity difference is near irrelevant. Compared to QSB of 10, 20 or more db it is clear that time is of the essence.

It is easy to QSY to 318: click FT4 in WSJT-X and there I am. I do so periodically in the hope that others will do the same. So far I've had few takers. Give it a try and we'll both have one more station to work. What was an amazing 200 Europe QSOs in the big opening could have been 300 or more using the much faster FT4. 

Typical FT4 activity level on 6 meters

Had there not been enough stations to make that possible (there were!) there would have been more time to monitor and react quickly when a rare one appeared. Instead I was more often locked into a slow FT8 QSO. It would have been rude to call some one else midstream. FT4 would allow me to both run the pile up and react quickly to call the rare ones between QSOs.

I'll end the article here. It seems a little curmudgeonly to complain when activity on the magic band is higher than ever. Now that sporadic E season is well underway there are signals to be found any time of day. Mostly these are tiny bursts of propagation -- whether by meteor reflection or E-layer scatter -- that are not workable on FT8 or FT4. Nevertheless it is comforting to see so many watching, waiting and calling.

Between openings I am keeping busy with various projects that I will be talking about in the coming months. One you can see in the picture above. You can also expect more 6 meter articles, inevitably.

Off air, I am waiting for local outlets to reopen from the latest lock down to order aluminum for antennas, doing tower maintenance and dragging my winter projects into the spring season. I might operate in the upcoming CQ WPX CW contest, and I've delayed rolling up the radials of my 160 meter antenna just in case I do get on. 

The warm weather is also drawing me outside to partake in non-ham activities. That is, when 6 meters isn't open and keeping me busy in the shack.