[TowerTalk] Comparing Antennas

[T A RUSSELL]

Comparing  Antennas   de  N4KG

When signals are above the receiver AGC threshold,
switching antennas will produce LITTLE or NO  AUDIBLE
difference.

This is because the receiver Automatic Gain Control system
works to maintain CONSTANT  AUDIO  OUTPUT for ALL
signals as long as they are above the AGC threshold.
You might SEE a difference on the S-meter, but you will 
NOT  HEAR  any difference.  

To discern small differences in signals, it is best to turn the
receiver AGC  OFF and to adjust the RF gain for a comfortable
listening level after advancing the audio gain.  This places 
the receiver in a LINEAR mode of operation.  One dB of
signal change will result in one dB of audio change.

BTW,  1 dB is close to the minimum discernable difference.
(Many automotive and home stereo receivers use audio control
IC's which step the volume in 0.75 to 1.5 dB  steps.)

The dB difference in signal levels is 10 x LOG (P2 / P1).

1 dB =   1.26 X P1 or a 26% power increase
2 dB =   1.56 X P1 or a 56% power increase
3 dB =   2.00 X P1 or a 100% power increase

Not all receivers use a flat AGC response.   The Collins S-line
receivers were set up close to 1 dB audio increase for every
10 dB of signal increase.

de Tom  N4KG  (receiver design engineer in my other life)


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