Saturday, April 24, 2021

Station Improvements for 6 Meters

After the 2020 sporadic E season I planned improvements to my 6 meter capabilities in 2021. Although the antenna is unchanged there are several items recently completed that should make the upcoming sporadic E season a successful one.

  • Adjust the yagi
  • New low loss feed line
  • Power

The redesigned Cushcraft A50-6 continues to perform well. However when it was raised a few years ago the carefully adjusted gamma match did not escape unscathed. As a result the SWR at the shack end of the coax ranged between 1.5 and 1.8 over the important band segment of 50.0 to 50.5 MHz. The rig's ATU handles this well, though at a cost.

First, there is additional transmission line loss due to the mismatch. That -0.3 db is in addition to the -1.3 db matched loss of the 40+ meter run of LMR400, RG213 and some LDF5. The calculated total loss is therefore about -1.6 db. 

The ATU is not loss free. An estimated ATU loss of -0.2 to -0.5 db is as bad as the line loss due to the mismatch. HF rig ATUs tend to have high loss at 50 MHz so my estimate is within reason. Let's guess that it's -0.3 db, thus bringing the total mismatch loss to -0.6 db. The total system loss is -1.9 db. That is more than enough to hurt when conditions are marginal. The loss needs to be lower.

With the help of a friend the antenna was dropped to the ground for service. I discovered more problems than just the gamma match. Several of the hose clamps holding the boom sections were loose and allowed rotation of the tubes by hand. These were tightened. Weatherproofing of the coax connector was not the best so I upgraded that as well. Fasteners on my original A50-6 are not stainless so those required cleaning and other maintenance.

The gamma capacitor showed weathering effects. It is similar to RG213 with the shield removed but with a plastic that does not age so well. It is a known problem with the Cushcraft "Reddi-Match". After the antenna was adjusted I covered and sealed the exposed section of the capacitor with two layers of Scotch 33+ tape.

Adjusting the match was a puzzle at first. Dimensions of the elements and the gamma match are not at all according to the original A50-6 or the revised and optimized A50-6S. It's a good thing I keep this blog since I documented my redesign of the antenna. Finding those notes in my paper records is not easy.

Tuning was done at λ/2, which on 6 meters is a comfortable 3 meters (10 feet) above ground. This is more than high enough for reliable adjustment. When I last adjusted the antenna I had it closer to the ground but pointing straight up. It is easier to set it up horizontally than vertically, although both methods work well. A step ladder was all I needed to reach the elements and gamma match. I had to move away from the antenna to make each measurement since our bodies alter antenna behaviour as these shorter wavelengths.

In light of the dominance of FT8 on 6 meters I centred the SWR at 50.3 MHz. It is below 1.2 from 50.0 to 50.5 MHz, rising to 1.45 at 51 MHz. This is perfect for my operating preferences of CW, SSB and digital modes. The SWR is slightly better in the shack due the small loss of the improved transmission line (see below).

The antenna was raised with a tag line this time so that it didn't bump into the tower. The gamma match did not fall out of adjustment. With a step on the mast I placed it almost a little higher that it was before, which should further reduce the already minimal interaction with the XM240 below it.

Before winter moved in last year the main run of LMR400 was replaced by LDF5-50 Heliax. The matched line loss is improved by 0.5 db over LMR400. With negligible mismatched line loss the total loss is now calculated as -0.8 db. Therefor the total loss improvement -- matched loss, mismatched loss and ATU loss -- is 0.8 db. That's total system improvement of 1.1 db compared to before. I consider this to have been well worth the effort.

Actually, coax loss was not only calculated. I measured the new sections with a VNA to confirm insertion loss. The VNWA plot is of the 9' RG213 rotation loop. Although the coax is old it was properly stored and has the loss predicted for new RG213 (S21 calibration is off by -0.01 db). 

The impedance variation is small and not unusual. I get similar results for the LMR400 sections of the coax. In my experience, Heliax is more consistently close to 50 ohms, probably because it is hardline and less likely to exhibit small deformations. Then again, maybe I really should do that recalibration!

The final area of improvement is power. I now have an amplifier that will do 1 kilowatt on 6 meter FT8. It is the ACOM A1500. I will have more to say about this amplifier and why I chose it in a future article. Sporadic E openings on long DX paths are brief and weak. I need every advantage to improve my results. 

Now that I have over 90 DXCC countries worked on 6 meter FT8 (81 confirmed) new ones don't come easy. Over the past few seasons I have missed opportunities to work many countries because they ran QRO (I could copy them) and I did not (they could not copy me). Despite having the amplifier available, I will not operate QRO full time, but I will not hesitate to turn it on when the DX beckons. My objective is to pass 100 countries in 2021.

The new coax for the 6 meter yagi is not switched: it runs directly into the shack to one of the antenna ports on the A1500. The A1500 conveniently allows port switching when the amplifier is off. This is handy because sporadic E openings are sporadic (of course!) and I will be able to operate on HF via the second rig and amplifier while monitoring 6 meters with WSJT-X and even working stations on HF and 6 meters concurrently. No longer will I be restricted to one or the other. 

This flexibility is nice to have although I don't know if I'll actually do it. Actually, the more important reason is to avoid consuming a port on the 2×8 antenna switch. Until now I used one of the ports for 160 meters in the winter and 6 meters in the summer. That is no longer necessary and it gives me full time access to the 6 meter yagi year round. Perhaps its availability will encourage me to take advantage of winter sporadic E propagation.

All I need to do right now is wait and monitor for the start of the sporadic E DX season. Openings will become increasingly frequent during May. I hope to see you on the Magic Band.

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