[R-390] TRD-15, Was The R-725 and the DF story?

[Tom M.]

John and group,

The DF set used with the R-725 was the TRD-15 (and one other that escapes me at
the moment).

The R-725 was prototyped by Motorola, and then the rigs were modified by Arvin
and Servo.

Arvin and Servo took existing 390A depot dogs, removed the IF decks and
salvaged certain parts (such as the BFO, crystal filter and a few others) and
built up new IF decks called "SERIES 500" decks to drop in.  They also added a
transformer to use for hum-bucking on the PTO.

Important:  The SERIES 500 deck and the R-390 deck are not interchangeable. 
The filament voltages and IF hookups are different, but mechanically they are
95% the same.

I'm told the reason the R-725 was sold for DF purposes and not the R-390 was
that Collins and the Signal Corps jumped into the R-390A with both feet.  The
use of the R-725 over the R-390 was a face saving measure to stay within the
R-390A concept.  This was probably also justified by training, documentation
and parts drivers as well.  The Army cut the cord on the R-390 and didn't go
back.

Servo is still in the DF business.

see:

http://www.servo.com/

  

73 Tom N5OFF
--- John KA1XC <tetrode@comcast.net> wrote:
> Since we're on the subject, I'd like to bring up a question that's been on
> my mind about the reported purpose of the R-725 and its DF friendly IF deck.
> Is DF use really the case, or is this just a story that gets repeated?
> 
> While I've often read this explanation, I've never seen any
> documentation referring to what actual DF equipment or systems the
> R-725 was used with. I'm very familiar with the need for carefully
> characterizing the IF phase or group delay characteristics for particular
> applications, but I am unfamiliar with any needs for DF. Maybe I don't
> understand this requirement but could there be another explanation? The
> following is my reasoning.
> 
> DFing to me means determining the location of an emitter
> From what I know about the major HF DF systems used (such as the giant
> Wullenwebber arrays that were deployed world-wide) they could be simplified
> into three parts:
> 
> i) the antenna array which is used to receive and resolve the emitter
> bearing
> ii) the receiver which allows you to listen in and provides a conditioned
> IF output
> iii) the display processing equipment which takes the IF signal and extracts
> the amplitude information and puts it in a form which can be used to create
> a rotating polar display. This gives you the familiar DF scope with the
> propeller shaped display indicating bearing.
> 
> The whole point of this is that it is the emitters amplitude which is being
> plotted against bearing, and I just don't see the emitters phase components,
> or the IF's, coming into the equation. I've also heard stories told by DF
> and intercept operators of such centers being filled with racks and racks of
> R-390's, R-390A's, and R-391's, but don't recall R-725's being mentioned
> much if at all.
> 
> Perhaps I'm missing something in the above explanation, but I DO have some
> applications in mind where linear IF phase would be handy.
> 
> 1) Radio-location, defined as determining where the *receiver* is located
> based on known transmitters.
> 
> The transmitted signals contain precisely timed pulse information which can
> be translated to distance, so having an IF that preserves the phase (and
> therefore the timing) of the pulses is important. But there was lots of
> specialized radiolocation receiving equipment built and sold;  why would you
> use an R-725 (plus other equipment), and why buy an expensive receiver that
> covers all of HF when radiolocation utilizes lower frequency ground
> wave?  This doesn't make much sense to me.
> 
> 2) Data communication, involving something more complex than the usual
> multi-channel TTY, but have not seen any references to this.
> 
> 3) ELINT, Electronic Intelligence gathering, a big time Cold War activity.
> This is where you are interested in the actual  RF signal itself so that it
> could be analyzed and information extracted, or so that it's "signature"
> could be determined.
> 
> Back when the NSA was formed, they (and their various agencies) started
> analyzing all the foreign signals they could from DC to light, and continue
> to do so today.
> 
> Wide-bandwidth analog recorders had arrived on the
> scene and remote listening posts, planes, subs, and ships hugging other
> countries borders were filled with all kinds of receiving gear quietly
> listening in and feeding these wideband recorders signals straight from
> their IF outputs.
> 
> Miles of tapes were routinely recorded every day, then rushed to
> centralized analysis labs for study. That is one of the main reasons
> surveillance receivers have IF signal outputs.
> 
> This is one application where keeping all of  an unknown signal's amplitude
> and phase components intact would be the highest priority, since the goal
> would be to record the desired signal in its original form with the minimum
> distortion possible.
> 
> For this special purpose the R-725 would fit the
> bill, the few hundred built could have been all that was needed. ELINT was
> routine on VHF up through the microwaves, so why not HF too?
> 
> Could HF ELINT have been the real purpose for this radio? The DF story might
> be just that, a very believable cover story used to explain the procurement
> contract, brought to you by the cloak and dagger folks that
> like to keep their real business quiet.
> 
> Perhaps I've just been typing into the wind, but just maybe there is someone
> reading who could chime in and perhaps shed some light on this.
> 
> thanks,
> John
> KA1XC
> 
> 
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