Towertalk
[Top] [All Lists]

[TowerTalk] Beverage performance VS term resistor

To: "tower and HF Antenna Construction Topics." <towertalk@contesting.com>
Subject: [TowerTalk] Beverage performance VS term resistor
From: VE6WZ_Steve <ve6wz@shaw.ca>
Date: Thu, 23 Jan 2020 14:05:06 -0700
List-post: <mailto:towertalk@contesting.com>
The recent experiments with my Beverages in the cold weather has raised some 
comments about the termination resistor and the antenna performance.  I 
received a few emails asking about F/B. I made the comment that it really is'nt 
worth the effort trying to get the termination value exact. Close is good 
enough.

Changing the termination resistor value has only a limited effect on the RDF of 
the antenna.  The F/B can change some what, but the zenith and side rejection 
is mostly preserved independent of the termination value.
This is well described by John in the the Lowband DXing book. (5th edition, pg 
7-64 “conclusions”)

Today I did some modelling on a 920’ wire.  RDF with a 500 Ohm term is 11.7 dB, 
with 200 Ohm term it is 11.3 dB, and with 800 Ohm term it is 11.4 dB.  So only 
about .4 dB of RDF change with a very significant (+/- 300 Ohm) resistor 
change.  Looking at the azimuth and Zenith plots, the front-to-side remains 
mostly unchanged while rear lobes develop that diminish the F/B.  However, 
unless there is a specific noise, or QRM problem from the back of the 
Beverages, this will likely go un-noticed.  The important parameter is RDF….the 
rejection of noise in the full 3D hemisphere compared to the max forward gain.

In fact, I modelled no termination (by inserting 1 e6 Ohms as a term resistor) 
and the RDF of the bi-directional Beverage is still 9.7 dB. (almost zero F/B) 
This is pretty decent, and is better than many other small loop antennas and 
smaller arrays.  The azimuth pattern still shows significant side and zenith 
rejection which is why the RDF calculates so well.  This is perhaps why many 
ops have had good success with bi-directional Beverages.

If you are unfamiliar with what the RDF metric, Greg has a good explanation 
here: 
http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/RDFMetric.htm
 
<http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/RDFMetric.htm>

73, de steve ve6wz
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
<Prev in Thread] Current Thread [Next in Thread>