RadioBanter - View Single Post - BF964S noise

December 18th 05, 09:38 PM posted to rec.radio.amateur.homebrew

On Sun, 18 Dec 2005 17:15:17 +0000 (UTC), (John S.
Dyson) wrote:

I also question (a little) the need for better than a true 1dB NF
when using a terrestrial antenna in the 2M band.

clip

Given the very wide signal strength range, I'd definitely consider OIP3
of at least 25-30dBm, and try to design for no more than 10-15dB gain.
It is very possible that even a very highly performing preamp will work
worse than no preamp at all!!!

One reason for attempting to get a low noise figure as possible, even
for terrestrial working, is that you can then afford a larger loss due
to input band-pass filtering.

The unloaded Q to loaded Q ratio of the input filter determines the
filter loss and the unloaded Q is mainly defined by the filter
construction (LC, Helical, 1/4 lines etc.). A 15:1 ratio would give
about 0.5 dB loss, while 5:1 ratio attenuates the passband by 1.5 dB.

Assuming a unloaded Q of 100 for an LC filter and loaded Q of 20 would
produce a 7 MHz bandwidth (at -3 dB points) with 1.5 dB loss in the
middle of passband and assuming 0.5 dB preamplifier noise would give a
total 2.0 dB noise figure, with a nice attenuation of broadcast FM and
VHF TV signals that otherwise might overload the preamplifier.

Paul OH3LWR