Thursday, October 5, 2023

Rearranging Yagis

No long ago, I described that during this fall's antenna work several yagis would be moving or removed. You can see a graphic of the plan in that August article. Here it is in words:

  • Remove the TH6 from the rotatable side mount halfway up the 150' tower
  • Move the XM240 from the Trylon to the position vacated by the TH6
  • Raise the recently acquired Bencher Skyhawk onto the Trylon

Later in the article I'll discuss why I'm doing this. But first I'll describe what was done.

Not including preparation and cleanup, the latter two items were accomplished in one day with the help of Alan VE3KAE and Dave VE3KG. I had hoped for one more helper but we made for it with mechanical muscle.

Since making my annual plan, I had a change of heart and the TH6 was moved rather than removed. I spent a couple of hours in the workshop to make a fixed bracket for the big tower. I installed it a little above the rotatable side mount. With Alan's help a week earlier we lifted the TH6 the short distance to its new perch. It is fixed to the south (170°). It's been here before. That freed the rotatable side mount for the XM240.

I have written so much in this blog about the various methods for raising yagis that I won't repeat myself. Do a search of the blog and you can read all about tram lines, tag lines, pulleys, vehicle power and much more. One picture will suffice for this article, of Alan posing with the XM240 at the tram line's launch point.

The only significant difficulty was on the smaller tower, the 70' Trylon. Being close to the house rather than in the middle of a hay field like most of my towers, there are obstacles. The yard area of my large property is very pretty with many trees, but they pose a challenge for raising and lowering antennas. For one thing, they grow! The challenge increases every time I lift or lower a yagi.

The XM240 and Skyhawk are quite large. Each weighs about 75 lb; the original XM240 is lighter but this one has been strengthened per W6NL. The antennas must land or be lifted a distance from the tower so that the elements and boom clear the overhead cables and several large and growing larger trees. Steering with tag lines is critically important. Finesse is needed more than brute strength, however the latter is needed when the yagi is close to the ground and far from the tower base.

I will emphasize one very important lesson about tag lines. Steering a yagi requires two tag lines, one on either side of boom centre. They are either attached outside an element clamp or prevented from sliding with a clamp or other means, and be easy to remove once the yagi is attached to the mast. 

Two tag lines sounds like a job for two hams. Wrong! No matter how many times I have tried it over the years with two people, it is almost always impossible for them to perfectly coordinate their actions and intentions. If you don't believe me, try it. But try it with your yagi and not mine! Element tips are fragile.

Instead we divided the work. One person drove the vehicle for lifting and lowering power, one controlled both tag lines and the third person relayed communications between the driver and the handler of the tag lines. 

The latter job went to me since I have the most experience with the delicate matter of steering yagis. I was also highly motivated since it would be my own yagis at risk! Communications was key since the vehicle was in the driveway and out of sight of us, and the driver could not see the tower or the yagi (trees and house blocked the view). 

It was entertaining and tense at times but we got it done without damaging the antennas. For added excitement, we had to avoid bumping the wireless internet radio mounted near the top of the tower. Knocking it out of position would involve a costly service call. 

I climbed the tower once to uncouple the XM240 from the mast and again to attach the Skyhawk to the mast, all while the vehicle was holding the yagi's weight. I won't get into the rigging details since, even with pictures, it is likely to be more confusing than enlightening. The rigging allowed the vehicle (my lawn tractor to lower the XM240 and a car to raise the Skyhawk) to move in a wide compass of directions while the line going up the tower stayed in a fixed position where it was clear of trees, tower and internet radio.

The pictures show the successful results. The antennas checked out and I reconfigured my software to reflect the changes. To my chagrin, that uncovered a couple of bugs that I tracked down and successfully fixed that same evening. 

Over the next several days I finished the job of boom trusses, dressing cables and weatherproofing. Late September and the first days of October were unusually warm and sunny. Temperatures soared to 30° C. That's record breaking for our northern climate at this time of year. 

As I write this, the one outstanding task is hooking up the TH6. The Heliax is connected by I still have to reconfigure the antenna switches to free a port for it. Other than that, the XM240 and Skyhawk are fully operational and being used on the air. It's auspicious that my first QSO with the Skyhawk was VK9LAA.

In a forthcoming article I will talk about the Skyhawk in depth since I found the antenna design to be quite intriguing. It is well worth a close look. The XM240 I've owned for quite some time, from before I moved to this QTH. I bought it when it became available during the planning stage while I continued to live in the city. After the move to this QTH, it has been on a variety of towers:

  • Onto the Trylon when it first went up the first winter
  • Moved from the Trylon to the top of the 150' tower when it was built the next year
  • Removed from the big tower and put back on the Trylon
  • Now it has moved from the Trylon back to the big tower, but side mounted

This is all fine, but by this point many readers must be thinking: why? Why keep moving these large yagis from one tower to another? Surely it's better to decide what I want or need, put them up and keep them there. 

There is a method to my madness. I am not intimidated by big antennas and towers, and I have the experience (and time) to undertake these frequent changes. I choose where I want an antenna based on the state of station construction and my operating objectives. The latter is primarily contests. For daily operating there is no great need for so many towers and antennas. Of course many non-contesters do it anyway, just because they can and it can be a lot of fun. 

I'll take you through my thinking so you can better understand why I've taken the trouble to do all this work. While you read, keep in mind that my primary operating activity is contests. Several of these points were documented in the 2023 station plan that I published in January.

Traps: Traps in yagi elements have loss and narrow the SWR bandwidth. I have long yearned to free myself of them but the reality was that I had them and they filled a need. I've reached the point where I can mostly eliminate them. The Explorer 14 was sold last year and the TH7 sold this year. As I explain below, I decided to keep the TH6 to fill an ongoing need.

Solar maximum requires more high bands agility: 10 and 15 meters conditions are great and they're going to get even better. With only so many hams and hours in a day, it is no surprise to find increased activity on 10 and 15 meters, and less on 20 and below. For the next few years it is valuable to have more antenna options on 10, 15 and 20 meters. For the coming contests there are now 3 antenna choices on those bands, and the ability to split the stacks.

Rapid access to W4, Caribbean, South America and Central American: Several years ago, before building the rotatable side mount, I fixed the TH6 south. It's back but a few feet higher. Skip is shorter when the solar flux climbs and there are more southern US stations available to be worked on the high bands. It isn't a high traffic path so it typically isn't desirable to point the better antennas in that direction. With a click of the mouse, the TH6 provides instant access to the south for DX multipliers and the southeast US. Once the targetted station is worked, another click returns the operator to where they were.

Prop pitch motors are slow: The upper yagis of the 20, 15 and 10 meter stacks and the 3-element 40 meter yagi are turned by prop pitch rotators. They are poor choices for rapidly working multipliers. It is more profitable to use them for running and long DX openings. I turn them to a direction and leave them there for a long time. The smaller and lower yagis are used for less productive paths, shorter paths (e.g. US) and multiplier hunting.

Unfortunately, the changes I've made are not all positive. Trade offs were unavoidable. These are the most notable cons of the new arrangement:

  • Reduced capability on 40: Pre-sunset and post-sunrise the DX elevation angles tend to be higher than during the night due to D-layer absorption. The XM240 at half the height of the 3-element yagi often outperforms its big brother at those times. Since the side mount only allows 130° coverage from southeast through west, I may suffer a deficit to, for example, Europe in the late afternoon. It can still be used effectively in the morning towards the Pacific and South America. I plan to replace the XM240 next year with an antenna that has more complete compass coverage and that has no loading coils. I can live with the XM240 for one more year.
  • Potential interference on 20: With so many yagis there is the ever present risk of destructive interactions. The 3-element 40 meter yagi was expressly designed to avoid pattern degradation of the 15 meter stack. I have not modelled the interaction between the tri-banders and the stacks, however I know the TH6 and Skyhawk won't interact because the TH6 points south and the Skyhawk is to the west of it. Due to the respective heights of all the yagis, I suspect the only significant issue may be on 20 meters with the lower yagi of the stack. Modelling the scenarios will have to wait for inclement winter weather when I'll have more free time.

I believe the negatives are far outweighed by the positives. At least for my style of operating. The 40 meter issue is short term since I plan to replace the XM240 with a better antenna, hopefully in 2024. Coil loaded elements, like traps, increase loss and decrease SWR bandwidth. Yagis suffer more than single element antenna because they are inherently high Q antennas and the radiation resistance is low.

The agility I've gained on the high bands is worth the risk of yagi interactions. Pattern degradation depends on where the yagis are pointed so it will occur only some of the time. In any case, I don't require ultimate performance from the tri-band yagis since they will be mostly used for rapid multiplier hunting.

With this big job out of the way I can focus on several other antenna projects. I'll write about each of them as those projects come to fruition.

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