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
_______________________________________________