Tuesday, June 20, 2017

Antenna Swap: How They Play

I write this article with a light heart since at long last there is significant progress on my 150' tower. That story I will leave for another time since it is an interesting and lengthy tale. With that progress note I will return to my most recent antenna project. It has now been close to a week with the new antennas on the Trylon tower, enough time to speak about how they are performing and my immediate plans.

6 meters with the redesigned A50-6

On the very first DX opening I worked 9 new countries, and another the next day. This brings my total to 30 DXCC countries on 6 meters. This does not include the many more countries I worked back in the 1980s. I chose to reset my DX counters to zero when I returned to the hobby in 2013 after a 20+ year hiatus.

From my experience in the pile ups and marginal openings that the A50-6 up almost 24 meters is competitive with respect to big guns in this area of the continent. While there are better equipped stations and sporadic E propagation is fickle enough to confound easy comparisons I feel confident in stating that I have a very good signal on 6 meters.

Looking north from the top of the tower
The pattern is sharp, the F/B excellent and the SWR is not perfect but reasonable. Compared to testing on the ground the frequency of minimum SWR moved upward from 50.3 MHz to 50.9 MHz. Either the gamma match was disturbed while lifting it or there is interaction with the TH7 antenna lying 2.5 meters below it. I may yet push the A50-6 a little higher on the mast.

This success is despite using 40 meters of ancient RG213 that has more loss than new coax. Compared to the AVA7 Heliax I intend to use on VHF I estimate at least -2.5 db loss. That hurts a little, especially on transmit. In addition to the main run there is ~6 meters of LMR400 equivalent Ultraflex for the rotation loop and run up the mast and down the boom.

The old RG213 is contributing to the impedance problem since it measures ~47 Ω. For now I am using the rig's ATU to allow full power at the low end of the band by taming the ~1.7 SWR.

To alleviate common mode I used two snap-on ferrite cylinders at the feed point plus a 1-turn coax loop. It isn't perfect but the best I could do with what I had on hand. The clean pattern I am experiencing is a good sign.

I am eagerly anticipating the remainder of the sporadic E season. Unfortunately it is unlikely I'll have the AVA7 installed before band fades away. That requires a trench, burying it and other cables that are not yet in hand, and figuring out how I will terminate the ground end of the Heliax. I am not sure I want to bring this 1-⅝" coax through the wall and into the house! I would like to do all coax terminations and switching outside the house.

TH7 performance

This long boom tri-band yagi is performing about as well as expected. The difference between it and the short boom Explorer 14 it replaced is not something that is evident without a carefully done A-B test. This is impossible since I cannot have them both up at once. Suffice it to say that it certainly seems to perform at least as well. That's good.

What isn't good is that the match on 15 and 10 meters is not as it ought to be. On 15 meters the SWR minimum is at 21.375 MHz and on 10 meters it is 28.550 MHz. Both are too high and are not what they should be based on the measurements from the manual. On the other hand 20 meters resonates exactly as it should.

It is possible that there is a trap problem or I have somehow made an assembly error. Another area of concern is the rotation loops for both it and the 6 meter antenna since both run near the phasing lines between the two driven elements. This may be creating some imbalance and thus mismatch, and it would tend to be more noticable at higher frequencies. But I really don't know.

What I will do is raise the TH7 several inches to provide more clearance from the tower top. This will allow routing of the rotation loops in a way that would reduce interaction with the phasing harness. It will also reduce the likelihood of the rotation loops snagging the tower legs.

It is conceivable though unlikely that the balun I installed at the feed point is part of the problem. I replaced the Hy-Gain BN86 voltage balun with the more appropriate current balun from Balun Designs. It is wider than the BN86 so the arms of the feed point had to be spread a little.

Aside from that it fits surprisingly well and is a superior solution to common mode than a scramble wound coax choke. Although I do notice less computer and other local noise on receive compared to the scramble wound coax I had on the Explorer 14 there are likely too many variables to confidently say that the balun has significantly improved common mode rejection.

Loss of 40 meters for the summer

Taking down the XM240 certainly reduces my capability on 40 meters. This is acceptable for the remainder of the summer months when low band activity is lower than other times of the year. The only significant impact that I see is during next month's IARU Radiosport contest. Since I haven't decided whether to enter that contest I am not too worried. In a pinch I can put up an inverted vee.

The yagi has been disassembled and will remain in storage until I decide what to do with it. For expediency I may put it up at the top of the 150' tower. There is also the possibility that I will convert it to a W6NL Moxon. I'll wait and let circumstances dictate what I'll do when the time to act arrives.

An antenna for 40 meters at the height it was on the Trylon -- 21 meters, or λ/2 -- is needed for contests since it is a good height for short haul paths such as the US, Caribbean and even Europe. I intend to address that lack with a side mount antenna (to be determined) on the big tower.

Mast and tower load

With the swap of antennas on the Trylon I am now more comfortably set for storm force winds. The high wind load of the TH7 (approximately 12 ft²) is right at the top of the tower, which is far better than the 10 ft² of the Explorer 14 that had been 2 meters up the mast.

The wind load of the A50-6 is 3 ft² and it is only slightly higher than the Explorer 14 former height. Better yet, the maximum projected areas of the antennas are orthogonal to each other: on the elements for the TH7 and on the boom for the A50-6. Therefore they are never both present at one time.

In advance of doing a detailed calculation it appears to the best to turn the antennas orthogonal to high winds so that the booms face the wind. Back in the 1980s I had a similar arrangement with a TH6 at the top of the tower and the A50-6 10' (3 meters) above the tower, on the very same mast. That tower was lighter duty than the Trylon yet survived some fierce wind and ice conditions.

However that is no guarantee of future success. All is takes is one strong gust of wind to take it all done. If you wait long enough there will always arrive weather conditions that will take down any tower and antenna combination. The more conservative your approach the longer you will wait. All anyone can do is make that wait longer than your lifetime, and do regular maintenance.

One point about the current arrangement bothers me: the TH7 boom. The XM240 it replaced has a larger diameter boom that has also been strengthened per W6NL's prescription. Although thick walled the TH7 boom flexes quite a lot in the wind. When combined with the brake play of the Tailtwister rotator there is a substantial movement of the antennas and mast when the wind rises. Although not an immediate concern this sets up the possibility of oscillatory load adding to ordinary wind load. I plan to watch this closely.

Part of the larger plan

With the completion of the change from temporary yagis to a more permanent arrangement I am one more step along the road to realizing my antenna farm plans. For the summer my attention will be focussed on the big tower.

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