On Sun,4/26/2015 9:22 AM, Grant Saviers wrote:
OTOH two 80/40/20 fan dipoles (at right angles) I modeled and built
worked super at 65' high. They worked on 15m with a tuner (<2.5:1
swr) and perhaps on 10m if the coax is cut for a match or there is
enough loss. I separated the ends by 24" using 3/8 sq pvc rod which
makes for mechanical complexity and difficulty winding it up (I use
10" diameter concrete column forms).
I use similar construction, with separation of 8-9 inches between
wires. I've never tried to put more than three resonant dipoles on the
same feedpoint.
Someone stated that fan dipoles are a compromise antenna. That's
fundamentally false -- all elements of the fan produce the same field
strength as a single dipole at the same mounting location. The ONLY
compromise is that the higher frequency bands have reduced SWR bandwidth
as compared to a single antenna. Since the bandwidth of 2M is only 2.5%,
that's not a problem with a 40/30/20 fan. It can be if 10M is the
highest band -- I remember getting about SWR under 2:1 from 28 to about
28.7 from my 20/15/10 fans.
Eznec models are reasonably accurate if you follow the recommendations
on modeling by Cebik, #108-111 at http://www.antennex.com/w4rnl/
re VE7RF's conjecture that the center of every dipole may not need to
be connected: Since I had a working EZNEC model, I disconnected the
transmission line connections to the 40 and 20m dipoles and fed only
the 80m. There is coupling but very poor swr; 4:1 on 40m, 10:1 on
20m, 8:1 on 15m, so it's not a good idea.
After studying the chapters in N6BT's book that described this
technique, I spent time trying to model it, with only limited success.
As I recall, I could get good coupling (both match and radiation
efficiency) with two bands (one directly driven, one parasitically
coupled), I didn't get a third band working. It seems intuitively
obvious that while the technique is elegant and the performance of Tom's
antennas prove that it works, the design is NOT trivial. :)
73, Jim K9YC
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