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 radio.
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.
thanks, jeff
Yes.
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.
73, Ed - KL7UW ====================================== BP40IQ 50-MHz - 10-GHz www.kl7uw.com 144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w DUBUS Magazine USA Rep [email protected] ======================================