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Re: Receiver sensitivity

At 02:10 PM 1/6/2008, Jeff Mock wrote:
>I have an Icom R8500.  I've had this receiver quite a long time and I
>really like it.  For me it's more like a piece of test equipment than a
>I've been using it to listen to satellites with so-so results.  With
>high overhead passes I get good results, but not so good lower than say
>30-degree elevation with an Arrow-II antenna.  Maybe I'm getting some
>desensing by being in a big city, any number of things.
>My question is about sensitivity of the receiver.  I'm looking at the
>specs, and it quotes 0.5 uV for FM at 70cm.  This is quite a bit worse
>than other 70cm radios and HTs that quote 0.17 uV sensitivity.
>Is this difference for real?  Does it make a big difference? It looks
>like even the big expensive receivers like the R9500 have the same 0.5
>uV sensitivity as the R8500.


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.

Ed - KL7UW
  BP40IQ   50-MHz - 10-GHz   www.kl7uw.com
144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
DUBUS Magazine USA Rep dubususa@hotmail.com

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