Sure looks like the reflector is really resonating above 6940 once the
antenna is in place. I would set the driven element to 7060 first with the
reflector open, then lengthen the reflector until minimum VSWR is achieved.
You can measure results easily by raising to about 20 feet on the tram.
This may be a PITA but is a sure fire way to get best performance. I would
also take a tape measure to both elements to ensure they are equal length.
[mailto:email@example.com] On Behalf Of Pete Smith
Sent: Wednesday, June 23, 2004 1:21 PM
Subject: [TowerTalk] EF-240S woes
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
The World HF Contest Station Database
was updated on June 5, 2004
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list