Tuesday, July 9, 2013

Summer Antenna - TH1

Since my previous posting I have decided to move ahead and do something productive with my small tower. As I mentioned at that time I first hoped for a wind storm as a test of the guys, and especially of how the guys would respond to tree movement. This has not happened despite favourable conditions for stormy weather.

The first step I took was to add tension to the steel (top) guys. Before they were only as tight as I could achieve by hand. This left ample room for tree trunk movement. It was this motion I wanted to test in the wind. Sadly, the strongest winds in the past week barely hit 50 km/h. The guys are now more suitably tensioned. This was done quite simply with rope and steel bars to draw the guys in towards their anchors. The tower now moves less but has a little less freedom to follow the tree trunks should they move much in a high wind (>80 km/h). A test is called for even though I am comfortable with the present configuration.

In the picture you can see the temporary antenna I've put up. I'm calling it a TH1 since it is the driven element of the TH6DXX I have stored in the garage. By itself it is a trap dipole for 20, 15 and 10 meters. As pictured it is about 10.5 meters above ground, which is λ/2 on 20 meters and 1λ on 10 meters. Its orientation (broadside) favours Europe, Africa and the South Pacific. As with any dipole it does radiate reasonably well off the "ends" although only at higher angles.

The good news is that it is works. Testing it on 20 meters CW I quickly logged 5T0JL and 3B8CF, both of which eluded me despite much calling with my eavestrough antenna. I also worked a number of European stations, most of them having no difficulty hearing my 10 watts. After 6 months of struggling to be heard -- albeit having surpassed 100 countries -- the change is very pleasant. I can see how QRP can be so attractive to many operators, assuming of course that the antenna system is effective.

Although the antenna works it did require tuning. The Hygain TH6DXX uses a beta match to adjust the yagi's low radiation resistance up to 50Ω. There a couple of good articles in QST earlier this year that go into the detail of how beta matches work. The important point for this antenna is that the driven element in a beta match must be shortened to add capacitive reactance. Thus when employed by itself the driven element must be lengthened so that it resonates at the design frequency. No impedance transformation is needed. The question is by how much longer the antenna must be?

That isn't easy to answer since the absolute amount of element shortening was designed by the manufacturer to simultaneously match the antenna on 3 bands, each with its own requirement for capacitive reactance while also allowing a single hairpin setting (inductive reactance) to suit all 3 bands. In this situation I decided that experimentation would be superior to calculation.

I started by propping the antenna at the top of the tower and running 12" clip leads to each element half. Checking the SWR it was quickly obvious that this was too much added length. That was intentional since it is always easier to cut than to add! For my next test I soldered a PL-259 with lengths of wire that, though unequal in length, added up to about 12". There is no need for the antenna halves to be of equal length.

This worked quite well. In fact it worked so well that I stopped right there. Although the SWR curves favour the SSB sections of the bands, at a height of 9 meters it was still well under 2:1 at the bottom of each band.

The construction of the feed and mounting mechanics can be seen in the picture above, looking up at it from just underneath. The transmission line is RG-213 of which I have lots, already connectorized from its use in my station from the 1980s and 1990s. While it's nice to have the lower loss compared to RG-58 I don't actually have any RG-58 other than short patch cords. My old station was designed for high power.

The air-core coax choke has 6 turns at 7" diameter. It isn't easy to make a common mode choke from RG-213 that works well from 14 to 30 MHz. Since the antenna is temporary I decided this is good enough. It is possible to do better with a smaller diameter, however this is not safe with RG-213. You need to keep in mind the minimum bending radius. The generic RG-213 rating is ~3.8", and Belden (8267) recommends 5".

Since the beta match requires a dipole feed (element cut into two halves) I needed to use the TH6DXX mounting hardware. I hacksawed the rusted bolts then adapted an ABS pipe to mate the element clamp to the 1.5" OD mast.

I did get a surprise when, after using the antenna for one evening, I raised the antenna to its final height of 10.5 meters. Even that small 15% change in height affected the SWR. It is still fine on 20 and 10 meters, however the SWR on 15 meters jumped to 3:1. At first I suspected that I shook something loose, but that made little sense since it works fine on the other bands. I then noticed that the horizontal run of RG-213 from the tower to the house is now almost exactly λ/2 below the antenna (on 15 meters), and parallel to it. There may be an interaction.

Now on to operating, which is what all the work up to now is all about. I will have more to say on antenna performance, probably in my next article.

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