Hi Gang,
Several of you wrote and asked for more details on my 2 el beam that uses
monoband reflectors and a trapped driven element. Here's a short comment on
the C3 and the rest of the info on my beam.
First re the C3 comment: I also experimented with a sleeve driven element
like the C3 but had trouble getting it tuned up and stable. The 20m el in
particular sagged and swayed in the wind and kept throwing off the other
bands. It's something I might try again, but it's seemed to be kind of
tricky with the materials I had.
Back to my beam:
I used a TH2 formed wire matching section (only part I had to buy new from
Hygain aside from manuals) since I could not get the TH3 matching tubes
adjusted for an acceptably low SWR. I figure that's because the TH3 has a
very low Z (being a closed spaced 3 el) compared to a wide spaced 2 el. The
rest of the parts came from 4 beams (TH3, TH4, TH5, and DB24) that I picked
up at hamfests.
The DE was assembled per Hygain's TH3 instructions. I only adjusted it
slightly for my desired center frequencies (14.12, 21.2, and 28.3).
The DE to reflector spacing is a constant .15 wavelgt. The reflectors lgts
were also per Hygain and adjusted slightly for desired bandwidth.
Dimensions in feet are:
band spacing length
20 10.5 37
15 7 21.4
10 5 18.6
The reflectors use tubes that starts at 1 1/4" and taper to 7/16". The .15
spacing gives maximum gain (abt 4.5 dBd) for a 2 el with reflector parasitic
and also pretty large Z (all per YagiMax by K4VX which only models
monobanders so the trapped DE changes the figures somewhat).
The F/B is abt 10 dB measured which is nice for contesting since I can point
at the East Coast and also work the West in stateside 'tests, or point to JA
and work SA off the back in DX 'tests. I still have problems breaking
through the East Coast curtain to run Europe, especially since I usually
compete in the QRP category (to avoid neighbor complaints) when I don't op
Multi/single.
Please note that the dimensions are starting points only. I put the beam on
a push-up mast attached to an eave, raised the beam to about 17 feet,
pointed it up at about 75 degrees and used an MFJ259B to tune the DE for the
desired center frequency and the reflectors for bandwidth. I found very
little change when I put it up at 57 feet on my tower. The swr vs freqs
(measured through 90 feet of RG8 foam at 57' with the 259B) are:
freq
14,0 1.7
14.12 1.1
14.28 2.0
21.0 1.8
21.2 1.1
21.42 2.0
28.0 1.7
28.3 1.0
28.8 2.0
I hope this all helps. Let me know if you have any questions.
73, Ed
Ed Jensen, K5ED, El Paso, TX, k5ed@dzn.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|