From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: About transverters (was Weak signal VHF)
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group
Date: Thu, 17 Dec 1992 08:46:40 GMT

In article <9212161401.AA05515@ucsd.edu> MROWEN%STLAWU.BITNET@cunyvm.cuny.edu (Michael Owen W9IP) writes:
>
>>Well that *used* to be true, but with the advent of the FT736R and
>>the IC275/475 class multimodes, it's now a tossup as to which performs
>>better. The most lacking quality of early multimodes was receiver
>>selectivity. That's now much better with narrow filters available
>>and with IF shift/passband tuning and tunable notches on the newer
>>multimode rigs. They're the equal of a transverter and HF rig, and
>>a lot cheaper if you're buying everything new.
>
>>Gary KE4ZV
>
>Rx selectivity may have been the limitation of _early_ multimode
>rigs, but today the weakness is dynamic range.  Single-band and multi-
>band VHF-only rigs still don't stack up to a good HF transceiver +
>transverter combination when it comes to DR.  The only exception may
>may be the Icom IC-575 (6 meters) which is really pretty robust.  A
>good transverter (w/DBM) whose overall gain has been reduced to about
>10-12 dB, mated with an '850-class transceiver, will give you <<2 dB noise
>figure and +8 dBm 3rd-order intercept.  These figures are much better
>than can be gotten with an IC-275 or FT-736.  For some of the gory
>details, see NCJ a couple of months ago as well as next issue (or maybe
>the one after that?).

That's true. Dynamic range could be better on the top end with the
multimodes, though the newer ones are better, but Microwave Modules 
tranverters won't give it to you either with their archaic mixer design. 
At nearly $1000, the SSB Electronics units are rather rich for a converter, 
but they are robust. They, like the MMTs, however, have a very broad front 
end.  On a mountaintop with other high power radios, things tend to get
nasty without cavity filters in front of them. Then the low end of
the dynamic range is impaired. They are better than the multimodes,
but not significantly IMHO. When operating from a less hostile RF
environment, like for most home satellite stations, there's no real 
difference between the transverters and a multimode. There's lots
of signals to be separated, but none are very strong. Note, I
have 3 of the SSB Electronics transverters, 432, 902, and 1269,
but I wouldn't spend that money again for the value received.

>Another factor to consider in _really_ weak signal work (ie EME) is RX
>synthesizer noise.  Although close-in noise spectral photos have been
>appearing in QST for a while, it's still hard to compare numbers.
>Nevertheless, the VHF-only boxes have notoriously dirtier synthesizers
>than, say, a TS-850.  Transverters, with their crystal LOs, generally
>don't degrade this figure.

That's certainly true of the Kenwood 711, 811, and to a lesser extent the
790A, but the IC275/475 are much better in that regard and the FT736R 
also seems adequate. I've been told, can't bring myself to spend that 
much, that the IC970 also suffers from a dirty synthesizer. On the
other hand, the MMT boxes are just plain noisy and must have a strong
preamp in front of them. Again the SSB units are wonderful, but oh so
dear.

>Lastly, Geoff Webb asked about sources for transverters.  For 6 meters,
>the Tokyo Hi-Power HX-650 looks marvelous.  50 watts, dual switchable
>preamps (high/low gain), DBM, $500 +/-.  On 2 meters, the SSB
>Electronics transverter is available commercially, but its price is
>absurd (ca. $1,000).  Sinclabs (Ontario) makes old-style (dual-gate
>MOSFET, I think) units for 144 & 222.  They're much like the now-
>defunct Microwave Modules.  For 432, Down East Microwave makes a great
>no-tune transverter, kit or preassembled.

I think that Sinclabs, their units were MMT copies, has also discontinued
their line of transverters, but may still have a few on hand. I heard this
through the gravevine for what it's worth. The MMT design is very old, and 
not that good today. I've got one of the Down East units, a kit, but haven't 
gotten the oscillator clean enough yet. It starts hard, and the spectrum has 
spurs. I've ordered another crystal and we'll see if it will clean up. It's 
a MMIC design and the NF is rather poor, but then antenna mounted preamps are 
the rule, not the exception in weak signal work.  It's output is also only 
+10 dbm and requires further amplification.

The Sinclabs and SSB units are 20 watts out, the MMTs claimed 10 but went
non-linear at 7, at least out of 15 units measured, and as I said, the
Down East unit is in the 10 milliwatt range. Lastly, there are the Hamtronics
kits, the TV2 and TV4. They make about a watt, but are transmit converters
only. The less said about Hamtronics receive converters, the better. It
really boils down to a multimode, the outrageous SSB units, or a homebrew
design. Note that Mutek (England) has some wonderful replacement front end 
boards for the Icom and Kenwood multimodes that really make a difference.
I used one in my IC211 and it lowered the MDS 8 db and improved the dynamic
range to boot. It uses a Mini Circuits DBM. The 211 had a very clean
synthesizer (slow tuning), but originally had a weak front end design.
The lack of selectivity, however, still made it less than ideal. I would
often cascade it's IF output into a general coverage HF rig to get the
necessary selectivity. Currently my main rig is a FT736R and I haven't
found it lacking for satellite use.

Gary KE4ZV