[AMPS] What's a dB or Two?

[T. A. Russell]

Is a 2 dB improvement worth the cost and effort?

Several have indicated that for a signal above the background noise
level, it will be barely discernable.  This is probably true if there is
only
one signal present.  Most modern transceivers have tight AGC systems
which produce zero increase in audio once the signal is above the AGC
threshold.  A stronger signal produces more AGC (receiver gain reduction)
and therefore suppresses the background noise and weaker signals.

When you are pushing the limit in propagation or competition (pileups),
2 dB is a significant improvement.  This represents a power ratio of 
1.585 which translates to 950 Watts vs. 1500 Watts.

Other worthwhile examples of a 2 dB improvement include

-    changing from a 3 element Yagi to a 5 element Yagi

-    stacking two identical Yagi's

In each case, there is considerable testimony that the change produced
a significant improvement in performance.

Tapes of contest pileups early in the competition taken from PJ2
sound like white noise with an ocassional single voice which will 
seem to ride over all the rest.  I have been told by persons familiar 
with detection theory that a small difference in signal level can have
this kind of effect.

When trying to hear a weak signal under a stronger signal, it helps to
disable the AGC, placing the receiver in a linear mode, and controling
the receiver gain with the RF gain control. This prevents the stronger
signal from "pumping" the receiver gain.  Another technique, which is
what I prefer to use, is to simply increase the audio gain and reduce
the RF gain (observe the S-meter needle rise) which raises the 
AGC threshold level to that indicated by the S-meter needle.  This
still protects my ears from being blasted by the appearance of a 
strong signal while providing a wider linear range.  This technique is
especially useful in extracting weak signals from the atmospheric 
noise on the low bands.

de Tom N4KG

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