[amsat-bb] Re: Receiver sensitivity
kl7uw at acsalaska.net
Sun Jan 6 18:17:57 PST 2008
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
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 at hotmail.com
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