The terminating resistor needs to be equal to the impedance of the
Beverage antenna wire against the ground. The goal is to have signals
traveling towards the terminating resistor completely absorbed by the
resistor, not reflected. When you achieve that goal your front to back
ratio will be the best you can get from the Beverage.
If your feed transformer is the wrong impedance ratio you will not find
the right termination resistance using minimum SWR at the feed point,
because there will be reflections right there between the transformer
and the Beverage wire. In that situation, adjusting the terminating
resistor for minimum SWR will result in the value of terminating
resistor which produces a reflection that best cancels the reflection
caused by the transformer/Beverage mismatch. That will definitely cause
less than optimum F/B ratio.
What I suggest is that you "sweep" the antenna. That is plot an SWR
curve over several megahertz. Say from 1.5 to 5 MHz in 250 kHz
increments. Then adjust the terminating resistor for the flattest SWR
curve, NOT for the lowest SWR. When the curve is the flattest, you have
the minimum reflection from the terminating resistor. Your power
transfer from signals in the desired direction to your feedline will not
be optimum because of the impedance mismatch of your wrong ratio
transformer, but the reflections from it should be absorbed fully when
they get back to the terminating resistor, along with signals from the
"wrong" direction. This is a lot more work, making a lot more
measurements and jotting them down and plotting a curve for every
terminating resistor value you try. Sorry there is no free lunch.
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|