[AMRadio] Receive Preamp question


Jay Rusgrove JRusgrove at comcast.net
Sun Nov 4 16:06:10 EST 2007


> "One way you can know if you have enough gain is to note the background
> noise your receiver picks up with the antenna connected, versus not
> connected.  If the background hiss (not lightning crashes or the various
> buzzes from local noise sources, but just the steady even
> background hiss) is louder with the antenna connected, then you already have
> enough RF gain.  If it doesn't jump up with the antenna connected, then you
> need more gain, either thru repair of the receiver or an extra stage."

For this test it's better to terminate the receive in a 50-ohm load rather than leave it open - this
will properly terminate the rf amplifier and give a more accurate reading of whether or not the
receiver is hearing atmospheric noise.

The FT101 series of receivers has a typical noise floor of -136 dBm so with any decent sized antenna
you should be able to hear the atmospheric noise (even on 10 meters). If the receiver can't hear the
atmospheric noise I'd hunt down the problem in the receiver and fix it before adding an external
preamplifier. The FT101 is not known for it's strong signal handling capability (two-tone IMD
performance is pretty low - IIRC around 59 dB) so additional gain will only compound that problem.

Try to stick with the same or similar device the manufacturer used if possible. While most dual-gate
MOSFETs are similar, those with higher forward transfer admittance (like 3N211) may become unstable
and require some re-engineering of the amplifier.

Jay W1VD







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