Topband: front to back?

Larry Molitor w7iuv at arrl.net
Sun May 25 20:11:09 EDT 2003


At 13:39 5/25/03 +0000, Barry wrote:
>I didn't notice any significant F/S until at least 90 degrees from
>the heading. I've heard others talk about 30-40 db F/B, but this is
>nowhere close. I'd like to hear others' experiences.

Barry,

A while back I evaluated the flag, pennant, diamond, and delta, as well as 
the K9AY. Of all of them the delta configuration consistently produced the 
poorest performance. It was the most susceptible to degradation caused by 
nearby resonant objects and had the poorest F/B even when in the clear.

Yes, I know that the computer predictions differ, but I don't think anyone 
has ever made an EZNEC to EZNEC QSO yet! And yes, I know that the delta 
configuration is the easiest to build, but I'm sure you know there is no 
free lunch. If you need to have a deep null with better F/S, you will 
eventually wind up with the flag configuration. Why not just quit fooling 
around and go right to it?

If you insist on making the most of the delta there are a couple of things 
you can try. First off make sure that your matching transformer is the 
isolation type of a known good design. Also make sure that the winding 
insolation is not compromised. Any leakage between the windings will 
decrease isolation and possibly cause unwanted IMD products in the band. 
The other thing to watch for is to make the antenna as dimensionally 
accurate as possible. Each wire length should be as close a possible to 
EXACT length with no sags/droops in the span. Even small departures from 
perfect symmetry will cause a dramatic reduction the depth of the rear null.

Good luck & 73,

Larry - W7IUV



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