From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Satellite rig: Kenwood TS-790a or Yaesu FT-736r?
Date: 15 Sep 92 13:33:23 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <521@arrl.org> zlau@arrl.org (Zack Lau) writes:
>In rec.radio.amateur.misc, gary@ke4zv.uucp (Gary Coffman) writes:
>>
>>Note that the quality of the receiver is *the* issue with satellite
>>radios. You're dealing with weak signals, and surprising pileups.
>>It helps though, if the transmitter works and the controls make
>>sense. Today I'd say go with the Icom Twins (275, 475) or the
>>Yaesu FT-736R as your best choices. The Yaesu is cheaper and all
>>in one box while the Twins are more flexible. I used to have an
>>older pair of Twins (451, 211) before lightning visited.
>
>
>I've wondered about this myself.  It seems to me that the cost
>effect solution to having great sensitivity is a mast mounted
>preamplifier.  Expensive low loss feedlines or mounting the rig 
>at the antenna usually is less practical.  Usually, one preamp
>is enough to establish a good system noise figure, unless you
>have a really deaf receiver or lots of cable loss.  So, if you
>take this approach, the sensitivity isn't that important. A common
>rule of thumb is to have 10 dB more preamp gain than noise figure for
>the VHF bands and 20 dB on the microwave bands (unless you have 
>interference from strong stations).

This is true. A super noise figure or great MDS capability is not
mandatory for a great satellite receiver. The mast mounted preamp
is the essential tool that sets the ultimate sensitivity of the 
system. However, it's possible to have a preamp that is too hot.
Too much preamp gain can depress the third order intercept of the
receiver enough to cause troublesome IMD products.

>Now, what is the dynamic range of the linear transponders of a
>satellite?  30 dB?  Certainly not telephone quality.  How much
>better does the dynamic range of your receiver have to be?   My guess
>is that going from 50 dB to 80 dB would not improve the intelligibility
>of signals off the satellite very much (system dynamic range).  More
>dynamic range is useful in dealing with non-satellite 
>interference, but I think its safe to assume that signals in a
>satellite pileup are coming through the transponder.

The problem is that not all the signals passed by the preamp are
coming from the satellite. In most urban areas the ether is filled
with strong VHF and UHF signals. Very selective networks can't be
tolerated before the preamp because of their insertion loss. Very
selective networks after the preamp further degrade the receiver
front end's S/N ratio to the point that the preamp has to be very 
hot or the benefits of the preamp are wasted. Hot preamps generally
don't have high intercept and are often unstable. The usual approach
is to have a receiver with a very high intercept and little selective
filtering between the preamp and the receiver. So very high dynamic
range in the receiver is important. The IC275 excels here. The FT736R
is good. On my old IC211 I used the Mutek front end replacement board
for the radio. This board used the Mini-Circuits balanced mixer to
replace the bipolar first mixer in the original radio. It also 
replaced the lossy PIN diode switching network with a Teledyne relay. 
These changes improved both dynamic range and S/N ratio of the unit.

Satellite pileups require very good IF selectivity to separate the
closely spaced, and nearly equal strength signals. The IF filtering
in most contest grade HF rigs is entirely adequate, but most VHF/UHF
multimode rigs *don't* have these excellent IF filters. The FT736R
is good in this respect while the Kenwood 790 sucks because high
performance filters aren't available for the *subband* receiver.

In my old setup, I took the IF output of the IC211 and fed an IC745
in general coverage receiver mode. Using this cascade, I was able to
take advantage of the superior filtering of the HF rig, particularly
the IF shift and passband tuning features. The FT736R has IF shift
and a notch filter, but lacks passband tuning. Often reducing the
IF bandwidth from a nominal 2.1 kHz to 1.8, or even 1.6, kHz will
make a dramatic improvement in readability of an SSB signal less
than 10 db above the satellite noise floor. The availability of
a really narrow filter for data and CW use is mandatory as well.

Naturally, a high intercept converter in front of an HF rig also works
well. One advantage of this arrangement is that the preamp and converter
can be integrated and mast mounted. The resulting HF IF reduces the
requirements on the downlead. Ordinary double shielded flexible coax
is sufficient.

It should also go without saying that superior phase noise performance
is mandatory in all the local oscillators. Some synthesized VHF multimode
rigs still have substantial phase noise, notably the Kenwood 711.

Gary KE4ZV