From: kludge@grissom.larc.nasa.gov (Scott Dorsey)
Newsgroups: rec.radio.shortwave
Subject: Re:  spectrum display integrated with computer software?
Date: 9 Sep 92 16:26:38 GMT
Organization: NASA Langley Research Center and Reptile Farm

In article <9209091343.AA25319@ucbvax.Berkeley.EDU> wmartin@STL-06SIMA.ARMY.MIL (Will Martin) writes:
>Speaking of spectrum displays...
>
>I once had a chance to sit in an operator's seat in the back of an
>electronic-surveillance-equipped Army U-21 aircraft (only while it was
>parked on the tarmac at the airport, unfortunately) and tune the receiver
>in that console. It was a rather neat toy -- a long linear visual
>display, like an inch high and 10 or so inches horizontally, which
>was displaying a chunk of VHF spectrum at the time. A horizontal line
>with peaks at the frequency of each signal, their height varying
>with the signal strength. The most interesting feature was that the
>tuning knob drove a cursor on the display, so you could move the cursor
>to any peak you wanted to monitor, and the audio from that signal would
>then appear in your headphones.
>
>Are receivers like this available on the commercial or surplus market? 
>What sort of prices do they bring?

The receiver in the U-21 is a Watkins-Johnson wideband receiver, with
a really amazing panadaptor display and a computer control system.  You
can purchase one from W-J; I know the receivers are about $10,000 per
channel and the control computer is in the $30,000 range, but I don't
know how much the spectrum display is (since it's an extra-cost option).
These creatures come with a number of different receivers from VLF to
microwave, all of which can be controlled by the same computer head.
The receivers are just black boxes, and pretty good performers.
>
>I suppose that this specific piece of equipment might not really be
>all that great for a ground-based DXer; I have heard that aircraft radios
>are not very sensitive, since they are designed for operation high up,
>where there is practically a clear line-of-sight to any transmitter. So
>maybe this same piece of equipment, operated on the ground with a
>house-mounted antenna, might be poorer at picking up signals than a
>commercial scanner. But the display and the cursor-tuning are still a
>nice touch. 

You are not talking about aircraft radios here, but surveillance receivers.
Generally the aircraft gear isn't all that sensitive, but equipment designed
for monitoring weak signals does an excellent job as ham gear.  Just witness
the amount of WWII vintage surveillance gear that hams are using these days;
in another thirty years or so, the W-J machines will be coming up on the
surplus market for next to nothing (and I will be one of the first in line
to bid on them).

>Is any ham gear made with this cursor-tuning feature?

Not really.  The problem is that these receivers take an enormous chunk out
of the spectrum with a very wide IF, and then demodulate a chunk that is
4 MHz wide all at once.  You can't do this economically yet, although I
suspect that you might be able to do some amazing stuff with a modified
TV tuner and some DSP gear.

If you don't mind a much narrower window, you can get a panadaptor display
for most HF or VHF receivers; look around at hamfests for the Heathkit
units which show up in the $20-$50 range (and make sure that the center
frequency on it matches your reciever IF).  You'll only be able to see as
much of the spectrum as your IF strip will pass (less than 20KHz in the
case of my R-390A), but it's an amazing aid to weak signal DX.  I have been
looking for one in good shape for years and finally bought one, and I can't
imagine what I did without it.
--scott
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