[amsat-bb] Re: Making progress

[Scott Townley <nx7u@xxxxxxxx>]

Scott,
You point out an interesting difference between HF and UHF...

<quoted> The preamp was a concern for me in that since I've been working 
HF, I've
found that I receive much clearer with the preamp disabled and the
attenuator enabled and just pushing up the AF gain. I realize this is
due to noise in the area and the fact that the IC-7000 preamp is a
broadband preamp, and generally will just increase the noise floor and
lower the sensitivity of the frequencies I'm actually interested in. <end 
quoted>

The main difference is the noise of the exterior environment.  Without 
going into the gory mathematical details, essentially the noise figure of 
the exterior environment is on the order of 40-60dB on 40m, decreasing to 
roughly 5-10dB on 10m.  Most radios have RX noise figures on the order of 
~10dB.  Basically the radio only needs to be a bit more sensitive than the 
external environment--which is why attenuators are more appropriate for 
40m, and sometimes (but not always) a preamp becomes helpful up on 10m.

At 70cm the external noise is on the order of a couple of dB...but the coax 
and radio push the total up to ~12dB...so a low-noise amplifier at the 
antenna can make a LOT of difference.  I once made a calibrated off-air 
measurement of the sensitivity improvement rendered by an ARR GaAsFET 
preamp on an M2 eggbeater (another omni antenna) and found 8-10dB improvement.



Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u 


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