[R-390] The R-725 and the DF story?

[Dan Merz]

John,  I liked your typing into the wind,  as you humbly put it.  This provokes
the idea that there may be more than one truth (= the best exposition of the
known facts) about the R-725.  I have excerpts from the "Direction Finder Sets
AN/TRD-3,  AN/TRD-23A and  Direction Finder Sets AN/TRD-15 and AN/TRD-15A"
TM-115825-231-24,  August 1973,  that pertain to the R-725.  I don't know what
else is in this manual outside of the parts I have on the R-725 or what the type
of direction finder equipment is involved.  This information was provided to me
by Tom when I became interested in modifying the 390 i.f. chassis to fit my
390a,  which was completed.  Maybe this will help,  if someone else can look at
the complete manual for details on the df equipment.   Dan.

John KA1XC 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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