|Subject:||[TowerTalk] EF-240S woes|
|From:||Pete Smith <email@example.com>|
|Date:||Wed, 23 Jun 2004 14:20:36 -0400|
I'd like to share a cautionary tale, and ask for advice.|
About 7 years ago, I got a Force 12 EF-240S and put it up at the top of my tower, at 104 feet. As most probably know, this antenna uses two identical elements, with only the shorting bars on the linear loading determining which is the reflector and which the driven element. In those days, I had no means of readily tuning the reflector, so I just set the shorting bar to the "cookbook" dimensions in the antenna manual and left it at that.
The antenna seemed to work reasonably well, but the pattern didn't seem very crisp, and the SWR bandwidth was larger than spec, which I took to mean that the reflector was probably tuned too far below the driven element. This summer I had some other work to do, and so we trammed the antenna down. I measured the resonant frequency of the reflector, with the driven element's center open, all as recommended by Force 12, and sure enough, it was about 6750 KHz, versus the 6940 recommended by F12. So I moved the shorting bar, took it back up to 30 feet, and bingo - 6940. Tram down, replace feedline and hairpin, and tram back up. Everything looked OK, except the resonant frequency of the driven element was now about 7070 KHz, and the SWR curve turned up quite sharply at the low end.
And so I ran some tests with a nearby ham, using the technique of adjusting my transmitter power to produce a reference S meter level as I turned the antenna, and then converting the power ratios to dB. Biig trouble. At 7005, the antenna now has about .5 dB gain in the reverse direction. At 7050, it is essentially bidirectional. At 7150, the F/B ratio, in the proper direction, is about 10 dB. Clearly the reflector is now tuned too close to the operating frequency in the CW end of the band. Nobody's fault but my own -- I did the measuring, and I must have gotten it wrong.
Trouble is, I'm primarily a CW operator. It looks like my options with this antenna are pretty limited -- mainly, I can tram it back down, lengthen the reflector, attempt to measure its resonant frequency, and tram it back up. Maybe it'll be better, and maybe it won't. This is enough to make me think seriously about changing to an antenna that isn't so prone to adjustment effects. In the low-sunspot years, 40 is just too important to let slide. Does anyone have a better idea?
73, Pete N4ZR
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