[amsat-bb] Re: Receiver sensitivity

[Jeff Mock]

Edward Cole wrote:
> 
> The R8500 was designed to operate over a very wide frequency range but 
> in doing so some compromises result.  Also, the manufacturer has to 
> consider the RF environment that the radio may be subject and made some 
> trade-offs.  One was trading sensitivity for resistance to interference 
> from strong off-frequency signals.  Back in the 1960's 0.50 uV was 
> standard for a UHF radio but today 0.25 uV is usual for commercial radio 
> equipment.  Often ham radios have better sensitivity than commercial 
> radios.  0.15 uV is equivalent to about -122 dBm.  A good satellite 
> receiver will receive down to -135 or -140 dBm.
> 
> The parameter that most determines the weakest detectable signal is call 
> noise figure.  A low noise figure is needed to achieve -140 dBm.  Most 
> commercial and manufactured ham radios only have noise figures like 2 to 
> 6 dB.  That makes them not very sensitive.  A good preamp for satellite 
> operation will have a noise figure of 0.5 dB.  The improvement is 
> sensitivity that results is in the region of 3 to 6 dB.  IN other words 
> signals twice to four times weaker.  Adding 20-dB gain increases this 
> even more.
> 
> Many hams are surprised to find this out.  But this IS the reason, that 
> we keep recommending the use of a good preamp for satellite reception.
> With a good GasFet preamp like the P144VDG for VHF or P432VDG for UHF 
> (from ARR) your R8500 will hear all the way to the horizon with an Arrow 
> antenna.
> 

Thanks so much, that's a great description.  I was already planning on 
getting a preamp.  I thought that I would wait until I had a long feed 
line and an antenna on the roof, but I think that I'll get a 70cm preamp 
right away.

jeff