Oct 5, 2008

Upgrade vertical polarized yagi for FM repeater reception

Previously I have mentioned that I do like DX'ing on FM repeaters on 145MHz .
Not only both my transceivers have been specially upgraded for this (customized 455Khz filtering), but there is a dedicated vertical yagi antenna for this too. It is fixed at about QTF 160 in which I can constantly work PI3ZLB near the Belgium/German border on 145.725. During ducting propagation I can work, or at least hear other repeaters on that same frequency from Belgium, Germany and Luxembourg:

Repeater Near city Locator QTF QRB
PI3ZLB Geleen JO20WX 153 78km
ON0NR Namur JO20KJ 195 139km
LX0HI Bourscheid JN39AV 166 196km
DB0ZK Koblenz JO30SH 133 209km

The above vertical yagi antenna has been in use for more than 5 years now at different locations. It is a 4el '50 Ohm' direct feed version. The boom has been extended as the tower had a strong negative influence on the SWR. After the extension it worked much better.
Since I am planning an overhaul on my whole antenna system, this one needs some attention too. And while we're there, let's take it to the 'next level' right?

Old Flexa yagi's
At my old QTH I had a stack 2x 9el Flexa yagis's installed on my roof :

These were used for comparison to a single yagi having double boom length at the same height, mounted on a second mast. Total stacking gain was similar to the single long boom yagi, but the performance was slightly less. The wider horizontal angle(better for contesting though) showed more background noise.
My current location does not allow for a second tower/mast unfortunately :(
So both 9el Flexa yagi's have been lying around doing nothing ever since, actually rotting in the rain:

For those of you who wonder if these Flexa yagi's elements ( 2mm steel whip elements on extreme lightweight alumn) corrode on the element to boom contact, see below:

After 8 years of use this looks pretty corroded to me.

Rebuild to 5 el. FM vertical yagi
I decided to cannibalize one of them and rework it into a 5 element vertical yagi with offset mounting.
Time to do some serious cleaining & polishing :).
I just cut the boom and mounted the first 5 elements of this 9el design only. SWR measurements showed right what was expected.
Next, the boom required an extension for both the offset mounting bracket and the required extra distance from the tower.
I planned to insert a piece of the right alumn tube for the connection between the two boom pieces. The first few cm's go easy, but after that it's a tough job. The alumn insertion tube quickly grinds to a final stop. For durability I was planning to mount a parallel piece of boom anyhow, so after a sunday's afternoons job we get the following result:

Flexa yagi's have received a lot of critical remarks in the past about the (too) thin 2mm dia precious steel elements. I don't care about the 0.5dB discussions, all I see is its current merits:
  • Extreme light weight
  • Visual unobtrusive
  • Low wind load
  • Thick, high impedance dipole + balun
Can't wait to get this one it up in the air :)

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