From: Andrew Ikin
Sent: Sunday, February 3, 2019 10:38 PM
To: antennaware@contesting.com
Subject: source impedance of resistor terminated antennas
Is there any reason why most resistor terminated antennas like the K9AY and the
Flag use a matching transformer to the Rx feed line that has the same impedance
as the termination resistance. The reason for the question is, such antennas
have a ideal match to the 3dB noise generated by the termination resistor and
this resistor has no radiation resistance. Hence, for antennas like a small
size Flag, the performance could be limited by the resistor noise.
However, if the antenna is connected to a high zin amp., then the resistor
noise should be reduced by the miss-match and if the amp. noise figure is very
low, then it may be possible improve the antenna performance?
Winding the clock back 20 years I noticed that the K9AY Rx matching
transformer could be replaced by a high zin amp. with a voltage gain of 11dB.
The overall gain increase compared to using 9:1 z transformer was 20dB. This
increase in gain is what would be expected (11dB amp. plus 9dB by replacing the
9:1 transformer). Thus, the antenna gain compared to one without the amp. had
not been degraded by using high zin amp.. However, then I didn’t consider the
effect of resistor noise, but this wouldn’t be an issue with the K9AY.
Is there something else I need to consider?
73
Andrew Ikin
G8LUG
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