Most HF receivers do not have enough gain for decent AGC action on weak CW
signals. (The insertion loss of the CW filters can reduce IF gain
considerably.) This is particularly so on 10M, even with a beam in an urban
location. These receivers are sensitive enough, but for any signal that
doesn't
move the S-meter, you have to crank up the audio gain. We all know what
happens
next, when a loud signal appears.
I prefer that the overall gain be high enough that the noise just moves the
S-meter, so any signal will result in AGC action. The proper way to achieve
this is to have sufficient IF gain, which most receivers lack. If there's an
IF
gain adjustment, one can turn it all the way up. This will change the S-meter
calibration, but probably not have any other adverse effects. It's usually not
enough, though. Unless you want to modify the receiver, a preamp is the only
alternative, but you'll have to switch it out when there are extremely strong
signals around. (I sense the purists cringing.) The front end of most
receivers provides adequate sensitivity and excellent dynamic range, and a
preamp will almost certainly compromise the dynamic range. Still, I find I use
the preamp almost all the time on 10 and 15M with either the FT-1000D or
TS-930.
I probably don't hear any better, but it's a lot more comfortable, and maybe
I'll still be able to hear 30 years from now.
Needless to say, the preamp is often required with lossy receiving antennas.
I agree with Tom that your receiver is sensitive enough if you can hear a
significant increase in noise when the antenna is connected. Adequate
senstivity, however, does not guarantee sufficient gain for good AGC on weak
signals. The AGC threshold could still be 20 dB above that noise level. On
the
other hand, if your noise level moves the S-meter even slightly, you're better
off without the preamp.
73,
Scott K9MA
--
Scott Ellington
Madison, Wisconsin USA
sdelling@facstaff.wisc.edu
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