Steve Olney wrote:
G'day All,
I have a question about something that has puzzled me for some time now.
It concerns the procedure for giving signal reports. I understand the
scale for readability - no problem. The problem I have is with the the
signal strength part. The R part is a relative indication - relative to
QRM/QRN etc, as I understand it. But what about the S part?
In the case of a receiver which at the frequency and time of the received
signal has an S-meter reading where the needle is just moved off the stop in
the absence of a signal - an S-meter reading of 5 on a signal would seem to
warrant a report of S5.
However, the same receiver which might have a preamplifier added and at the
time and ferquency of the received which has a no-signal reading of say 3.
If that receiver now receives a signal which moves the S-meter up to a
reading of 8 - what is the appropriate report for the signal? S8 or S(8-3)
= S5?
It seems to me that it is possible to have a high level of noise with a
small signal which is barely readable but giving a S-meter reading of 8.
Isn't closer to the truth to be giving S-points above the noise or am I
missing the point?
73s Steve Olney (VK2ZTO/AXSO - QF56IK : Lat -33 34 07, Long +150 44 40)
Steve,
Ideally one would use field strength to describe incoming signals, as
field strength is independent of the antenna and receiver. However, the
need for a calibrated antenna and receiver is a bit over the top for
amateur operators. The antenna has a changing "k factor" with
frequency. The field strength of the signal and noise could be
determined. As well, most field strength meters have linear meter
scales and no AGC applied to the RF or IF stages.
A communications receiver usually has internal AGC, and an "S meter"
that is operated by AGC voltage. A receiver with AGC can not avoid
having a compressed S meter scale that is approximately logarithmic, and
is an advantage if a wide range of strengths is to be displayed on a
single meter scale, where accuracy is nominal. Ideally, the S meter
should read S9 for 50 microvolts applied to the RF input to the receiver
at the tuned frequency. Each S point below that is supposed to be -6
dB per point. On most ham gear the S meter is gratuitously known by
some as a Guess Meter.
Bob ZL2CA
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