From: jmccombi@testament.bbn.com (Jon McCombie)
Newsgroups: rec.radio.amateur.misc
Subject: 2m/440 homebrew antenna summary
Date: 30 Jul 92 21:57:10 GMT
Reply-To: jmccombi@bbn.com. (Jon McCombie)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
NNTP-Posting-Host: bbn.com

Thank-you to the many folks who sent e-mail with advice on homebrew
2m/440 antennas.  Several people asked for a summary; this is it.

THE SUGGESTIONS:
By far the most popular solution was the 2m J-pole which is reported to
work well on 440 MHz.  Plans for the "Copper Cactus" J-pole are in a
recent edition of 73, and for the "Electrician's Delight" J-pole in a
recent edition of CQ.  [E-mail to me if you need exact citations.] The
J-pole is also discussed in the ARRL Antenna Book and/or the Handbook.
Finally, I received two (slightly different) plans for a 2m J-pole made
from 300-ohm TV twinlead.

Two folks suggested I buy (e.g., from Radio Shack) or build (plans in
Antenna Book(?)) a discone antenna.  This antenna will present unity gain
at best, but should present < 2:1 SWR across all ham bands 2m thru
(way-the-heck-up-there).

One person noted that fanned dipoles work fine at HF, and suggested that
I modify my existing 2m 1/4-wave groundplane by adding elements for 440.

THE RESULTS:
I already had--and routinely use for public service events--a copper-pipe
J-pole built from plans I got off the net, so I tried it out using a
dual-band HT and an SWR meter designed for 140-450 MHz.  SWR curves were
(still) OK on 2m but never below 2.5:1 on 440.

I built a Copper-Cactus J-pole from the plans in 73 (the single J-pole,
not the "double cactus).  It presents better SWR curves on 2m, dipping to
1:1 at 146 MHz and about 1.3:1 at band edges.  SWR performance on 440 was
better than the other J-pole, at about 1.5-2.0:1 across 440-449 MHz.

Next I built a TV twin-lead J-pole.  After some trimming, I got excellent
SWR curves on 2m but awful performance on 440, never below about 2.5:1.

Finally I attacked the multi-element ground-plane idea.  Not wanting to
climb on the roof and un-mount my existing 2m 1/4-wave groundplane, I
built a new one from plans in the Antenna Book.  After trimming, SWR
curves on 2m were excellent.  For curiosity's sake, I tried running this
unmodified 2m 1/4-wave ground-plane on 440.  To my surprise, it performed
well, with SWR under 1.3:1 across the band.

Next I added an additional element to the vertical element and each of
the the radials, each element cut for 440 MHz.  There was no noticeable
difference in 2m SWR curves, and the 440 SWR curves actually got worse!

Thinking that perhaps I had happened upon the right length of coax to
make an impedance transformer, I tried using different lengths of patch
cords (3 ft. vs.  5 ft. vs. 7 ft.) and of different types (RG-58 vs.
RG-213).  My results didn't change.  I also tried the 2m 1/4-wave on my
roof with 146 and 440 MHz.  SWR curves were similar to the unmodified 2m
groundplane in the shack.

I do not have antenna modelling software nor have I done any tests of the
radiation patterns of any of the antennas I played with.  [Pointers to
something like NEC for a Macintosh would be appreciated...] Follow-ups
are encouraged, either via private e-mail or on the net.  (See also my
much shorter posting following this one, subject "2m qtr-wave
ground-plane on 440?")

Jon   N1ILZ
Newsgroups: rec.radio.amateur.misc