[R-390] Fw: Balanced Twinax Connector Usage

[Doug Hensley]

George was a senior ETN, along with 2 others, that kept our ships radio signals "on the air".
His input is below:

________________________________
From: George Frederick <gefred08 at gmail.com>
Sent: Sunday, July 26, 2020 3:06 PM

That same exact plug & jack ended up on every UHF satellite multi-channel RTTY receiver installed  throughout the fleet back in the 70's.  The AN/SSR-1A had up to four outlying 'front-end' units called 'amplifier/downconverters' that look like little beer kegs, always very close by an all-aluminum 'turnstile' antenna. The twinax runs could be up to 400 feet away from the main receiver unit in Radio Central. I measured the first model noise floor at -134 dbm, 4 db noise figure, at a 1200 baud datarate @ zero bit errors. Incredible RF performance numbers in 1973 when compared against anything we all worked with on CAMP..... But in all fairness, at HF the R-390 even today is still considered a benchmark unit. If I remember correctly, it was something like .16 microvolt, not too shabby!

On Sat, Jul 25, 2020, 7:33 AM Doug Hensley <w5jv at hotmail.com<mailto:w5jv at hotmail.com>> wrote:
Message: 2
Date: Wed, 22 Jul 2020 21:35:55 -0500
From: Tisha Hayes <tisha.hayes at gmail.com<mailto:tisha.hayes at gmail.com>>
To: R390A <r-390 at mailman.qth.net<mailto:r-390 at mailman.qth.net>>
Subject: Re: [R-390] TV Balun for R-390A

If I recall correctly the balanced coax connector on the back of the R-390A
is for connection to a doublet style antenna with a characteristic
impedance around 125 ohms (working from memory here).

The connector and the coax is still available; it used to be used on very
early networking for IBM AS-400 systems. In fact, if you look for it in
vintage computer surplus it is way cheaper than buying it as a radio (RF)
component.

The cable was commonly referred to as "twinax" and is a fairly thick,
double-center conductor, shielded coax cable.

There is another advantage to using the balanced 125 ohm twinax connection;
In the RF deck it routes the incoming signal through an additional set of
RF transformers and that may improve selectivity in some bands. The "C
type" (unbalanced) connector did not really appear to be the preferred way
of antenna-ing (antennaing? now I am making up new words), the radio. If
you have access to some of the installation manuals for the R-390A it shows
the doublet antenna as the preferred installation.

Of course we know that the R-390A ended up installed in all sorts of
situations; from expedient installations with a whip antenna or a chunk of
wire for an antenna, up to installations with rhombic or elephant cage
arrays.

So, depending upon your band of interest (500 KHz up to 30 MHz) there are
some ferrites that work pretty good for making baluns. (technically a
bal-bal, not a bal-un or an un-un, but I digress). You could match that 125
ohm balanced connection up to a 50 ohm unbalanced (for the traditional ham
radio type of antenna) or a variety of balanced, wire antennas (my
favorites).

*Ms. Tisha Hayes, AA4HA*
*Lookout Mountain*

*Gadsden, Alabama*