[TowerTalk] [Bulk] Re: G5RV vs 40M dipole

[Jim Brown]

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