Pete,
About the same time you bought yours, I bought an EF240S too. I tried to get
the thing tuned. Even rented a 120 foot crane twice and tried to tune it in
place. After phone calls and three efforts, I had no luck. Not to mention
the wires that load the elements broke and one the little metal stand offs
broke. (at different times) It did work, but not anything like I expected.
I move three years ago, so I trashed it and bought the Cushcraft XM240. It
does work as advertised and the measurements in the book are correct.
Please don't take this as an "anti-Force 12". I have two C-3E antennas and
they are outstanding.
Bill, W5VX
>-----Original Message-----
>From: towertalk-bounces@contesting.com [mailto:towertalk-
>bounces@contesting.com] On Behalf Of Pete Smith
>Sent: Wednesday, June 23, 2004 1:21 PM
>To: towertalk@contesting.com
>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
>2728 contest stations at
>www.pvrc.org/WCSD/WCSDsearch.htm
>
>_______________________________________________
>
>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
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
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.
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