[Antennaware] Fw: source impedance of resistor terminated antennas

[Andrew Ikin]

From: Andrew Ikin 
Sent: Sunday, February 3, 2019 10:38 PM
To: antennaware at 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