[CQ-Contest] N4KG: Re: CW Acoustics, Weak Signals, & AGC

[T A RUSSELL]

On Tue, 19 May 1998 11:56 (PDT) Jeffrey Pawlan <jpawlan at pawlan.com>
writes:

>Steve is correct.  I have shown many local hams that by using the old
>fashioned technique of putting the transceiver in manual (AGC OFF),
turning
>up the volume control to about 3/4 then riding the RF gain control to
>control the level, one can discern signals in the noise that are
>virtually impossible to hear in the normal AGC-on mode. Most laugh at
>me when I tell them this, but I can definitely attest that without
>this technique I would not have heard many very weak New Ones such as
>Malpelo, Amsterdam Island, Faroe, 3B7, and others. The only drawback
>is obviously there will be no AGC action on the high power people who
>insist on chatting or making rude noise on the calling frequencies of
>the DX so you should not do this with earphones.
>
>73s,
>Jeffrey Pawlan, WA6KBL
...............................................

YES, this technique WORKS.  I learned this in the mid-50's using a WW
II vintage Hallicrafters S-20R, predecessor to the S-40B.  It still
works today.

Actually, it is not necessary to turn the AGC off.  Reducing the RF
gain raises the AGC threshold as indicated by the S-meter position
with no signals (or high level noise) present.  This leaves the entire
receiver in the LINEAR region where any increase (or decrease) in
signal level results in a corresponding INSTANTANEOUS increase (or
decrease) in audio level.

The benefit of leaving the AGC turned ON, but with a high threshold is
that it will still act to protect your ears from BIG signals or static
crashes, with the additional gain reduction associated with the AGC
action and the associated time constant.  For best reception of WEAK
signals, it is best to always leave the AGC in the FAST (or OFF)
position so that the receiver gain recovers quickly from larger
signals (which reduce the receiver gain for the duration of the AGC
time constant).

Operators who use SLOW AGC in pileups are NOT able to hear weak
signals under the pileup which explains much of the unruly behavior
and continuous calling by less knowledgable operators.  (SLOW AGC is
recommended for SSB with STRONG SIGNALS in NET operations.  It is
counterproductive in weak signal reception.)

Operators who use FAST AGC or NO AGC can hear between syllables or
static crashes giving them a much better understanding of when the DX
station responds, improving their timing and chances of making
contact.  This is a direct result of the fast gain recovery in the
FAST AGC position (or nearly instantaneous recovery with AGC OFF).

This technique of using reduced RF Gain and advanced audio gain to
copy weak signals is one of the tricks that has contributed
significantly to my 9BDXCC country count.

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

--
CQ-Contest on WWW:        http://www.contesting.com/_cq-contest/
Administrative requests:  cq-contest-REQUEST at contesting.com