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

[John KA1XC]

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