"Not very much" means a barely perceptable difference in signal quality when
used with a standard portable shortwave radio, if any perceptablel
difference at all. At any freq in the shortwave spectrum, but mainly from
approximately 6 MHz to 12 MHz. The commercial product I was copying is
supposed to help pull in weak signals, but I have heard it doesn't do this
very impressively, so suspected the schematic I was working with was just
not designed very well. Makes use of two N-channel JFETs and a UHF high
speed switch, but left a lot to be desired on my part. I added a
Q-multiplier taken from Joe Carr's Practical Antenna Handbook, which
improved things, but I'm still not satisfied as I want it to work with the
on-board whip as well as it does with the 110' longwire antenna I currently
use it with. I am guessing I need another 20 or 30 db out of it, at least.
I am still fairly new to RF however, so it goes slowly.
Connecting a preamp directly to a receiver will not have a usefull
effect at all unless the preamp has a lower noise figure than the
front end
of the receiver. The noise figure, not the gain, is usually the
limiting factor.
When the preamp does have a usfull effect you will only notice
an increase in signal quality when the received signal would be
near the noise floor of the receiver without the preamp.
Shortwave is noisy, when listening to a signal that stronger than
about
10dB more than the sensitity figure of your receiver you will not
get any improvement from a preamp.
If connecting a preamp directly to a shortwave receiver has a
significant
effect it generally means that your receiver is deaf.
Preamps are usefull where there is significant loss in the cable
between the antenna and the receiver but only if the preamp is
at the antenna, not at the receiver.
Adding 30dB of wideband gain is unlikely to have a usefull effect. It
is likely to overload your receiver and produce lots
of noise due to intermodulation and harmonics when you
drive the receiver into nonlinearity.
Bob
