[ARC5] Hmmm...OT....AN/GRC-109 vs RS-6

[Tim]

Hi Ken - you've got it right, two different systems.  The RS-6 has the
antenna changeover relay.
Seems this discussion is going on via ARC5, MILSURPLUS and MRCA lists.
Maybe we are not all on board with each list.

My observations that I sent on the MRCA list regarding this topic:  FYI
------------------
Roger all....
The relay in the RS-6 simplifies all that, but at the expense of a relay
and associated complexity. LOL
Yes the RS-1 and GRC-109 system receivers have a compromise design in the
RX antenna routing.  There is an 18 pf RX antenna signal coupling capacitor
in the circuit that is too small to permit significant signals to get to
the receiver at the lower frequencies (during Key Up), but it is too large
in that it is in parallel with the output variable loading capacitor (upon
Key Down) thus limiting impedance matching range with that fixed value.  18
pf was a compromise value in trying to attain both with QSK ops..

Yes, the GRC-109 TX pi network and tube are in a parallel branch circuit to
ground on receive so they form a low impedance path for some receive signal
to "get lost".  "suck out" if you will.  Mostly a problem on the lower
freqs with a common antenna.  However I think (and in Vietnam MACV-SOG
operations for sure). operational TX and RX were on separate frequencies -
mainly bypassing the problem.
Also, getting the message OUT from a team was higher priority than getting
a message IN to a team, so RX system performance may have been compromised
a bit with the selection of that 18 pf capacitor during the design phase.
Plus they were listening to  URC-32, GRC-106, GRC-26 or some other high
powered TX. "Plenty of signal"...... My speculation on that part...

I just use a separate 50 foot random wire on the receiver and it works
great, plenty of sensitivity.  Usually a dipole or Inv L on the TX.  Good
comms.
73, Tim
N6CC

On Mon, Nov 23, 2015 at 12:23 PM, Kenneth G. Gordon <
kgordon2006 at frontier.com> wrote:

> Well, it looks as though I made yet another "ass-u-me" error. From my
> reading various articles from various places, I thought it was clear (at
> least to
> me) that the AN/GRC-109 was simply a military-nomenclatured version of
> the CIA's RS-6.
>
> But it isn't: it is a militarized version of the RS-1.
>
> I own an AN/GRC-109, but have never seen an RS-6 except in photographs.
>
> The RS-6 is the much later minaturized version of the earlier RS-1 and I
> got
> them confused in my 73-year-old pea-brain.
>
> From what Mike Feher recently told me, there appear to be considerable
> differences in the circuitry. For one thing, the RS-6 has, apparently, at
> least
> one relay in it, and, again, from what Mike mentioned, I now "assume" that
> is
> an antenna-changeover relay.
>
> Having such would, it appears, preclude the -30 dB "suck-out" which occurs
> in the '109.
>
> Anyway, I humbly apologize to Mike for my erroneous assumptions, and for
> horning in on something I knew nothing about.
>
> I should keep my 73 year-old mouth shut.
>
> Ken W7EKB
>
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