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RE: HRD and two radios: was dual IC-706 station



Wayne,

> ALL ham transceivers lack sensitivity in the VHF and UHF bands.

I must disagree with you, not all ham transceivers suffers from poor
sensitivity, in fact some are very sensitive, a good example is the Yaesu
FT-847, with a sensitivity figure of 0.125 micro volts in SSB/CW and 0.2 in
FM this rig it is VERY good in that respect; now, o more common problem with
modern transceivers is the noise floor figure of the receiver, that in some
cases may "mask" weak signals, for the most part this noise comes from
poorly designed PLL and front end circuits, ambient RFI  noise may also
impair RX, so please lest not generalize.

Another thing to note here is that to much sensitivity may produce a "noisy"
receiver, rendering the radio useless for weak signals RX, when working weak
signals the most important factor is the Signal to Noise ratio (S//N).

> That's why we use low noise preamplifiers at the antenna when 
> we need to receive weak signals such as satellites.

Again this is not entirely correct, low noise preamplifiers are intended for
the most part to overcome transmission line loses in an effort to prevent
degradation of the S/N ratio, that's why good quality preamps utilize costly
GaAsFET transistors, to cause the lesser possible impact in the S/N ratio,
and that's also why they should be mounted at the antenna (before the TX/RX
line), if a preamp is mounted at the shack it will be amplifying the signal
and the noise introduced by the line losses, as a result we will see a
degradation of the S/N ratio, the goal is to keep the weak signals above the
noise.

73/DX Jose M. Valdes R. (Joe) YV5LIX
eQSL.cc Advisory Board Member
QSL manager EA7FTR
SYSOP YV5LIX DX Cluster
telnet://yv5lix.org.ve:7300
VHF Packed: 145.430 using C YV5LIX
http://www.yv5lix.org.ve
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