[R-390] Using/Aligning Balanced Input (WasThe R-725 and the DF story?)

Tue, 9 Dec 2003 14:06:38 -0800

Using an unbalanced antenna feed necessarily introduces the ground (chassis)
of the receiver into the antenna circuit.  This allows currents from the
power line and other connections to become part of the antenna.   The test
is to connect a short coax line to the receiver, shorted or preferably
terminated with a 50 ohm resistor  - the set should remain silent.  If it
doesn't, then the receiver obviously has an antenna.  Unfortunately this may
not be the intended antenna, and all sorts of spurs and spurious signals can
get into the set through "back doors".

One solution is to build a small balun to present a balanced feed to the
antenna connector.   I have used a TV 300-75 ohm balun sucessfully, but
presently use an unknown (junkbox) core with two windings of twenty or so
turns, each.  The coax feed line shield and one end of a winding is grounded
to the receiver.  The other end of this winding is connnected to the "hot"
or center coax conductor.  Each of the two wires from the other winding are
connected to the reciever's balanced input.

The coax ground connection is surprisingly critical, it must be short and an
optimum grounding location on the set may need to be found by trial and
error.   This is especially true in the R390 because of the grounded
center-tap of the antenna winding, where the chassis still plays a part in
the antenna circuitry.  A good earth ground is also desirable.

An alpha-delta antenna switch is used to select several receviers here, and
when the switch is switched away from any set, that set falls silent (with
exception of strong local AM stations).  The primary motiviation for all
this is to reduce interferance on 160 meters from AM broacast.  Local
computer and other electronic noise reception from inside the shack has been
greatly reduced.

73 de Bill, AB6MT
[email protected]

----- Original Message -----
From: [email protected]
To: Roy Morgan
Cc: R-390 List
Sent: Tuesday, December 09, 2003 10:49 AM
Subject: [R-390] Using/Aligning Balanced Input (WasThe R-725 and the DF
story?)

Your mention of the balanced input trimmer alignment raises a question for
the list (not related to DF).  I connect to the balanced input connector in
the manner documented on most 390 web pages, that is ground one balanced
input and feed the center conductor to the other.  Hence the second trimmer
cap in each RF input coil has no effect when it is tweaked.  Is this normal?
Is any sensitivity lost by doing it this way.  Some discussions have talked
about using two 56 ohm resistors (one in each input pi), tied together to
feed signal for alignment.  Any benefit to doing this for normal operation?
Have also read discussion of using dual center conductor coax (like they use
for ethernet cables I believe) to feed a balanced dipole antenna in the
manner which was intended, which tends to help reduce common noise.  Any
thoughts on whether or not this is worth the trouble to build up such an
antenna?

----- Original Message -----
From: Roy Morgan <[email protected]>
Date: Tuesday, December 9, 2003 11:21 am
Subject: Re: [R-390] The R-725 and the DF story?

> At 03:15 PM 12/8/03 -0500, JMILLER wrote:
> >  ... I believe that DF can be determined by measuring phase or
> time of
> > aarrival differences from multiple receivers / and fixed
> antennas at
> > known positions...  ... I suspect there were 3 or more
> receivers, with
> > matched IFs, and one IF post processor doing the bearing
> > calculations.  Any thoughts?
>
>
> Yes, some:
>
> In one system I read about, the multiple antennas were fed to a
> phasing
> network which was adjusted by hand or by automatic system to
> produce two RF
> signals exactly in phase.  The R-390 balanced input circuit
> allowed for
> in-phase signals to cancel in the input transformer. One part of
> the R-390A
> alignment is to adjust the input transformer trimmer cap to
> achieve maximum
> balance and thus maximum null for two equal in-phase signals. It
> could very
> well be that the IF output was used in such an automatic nulling
> system and
> that flat phase vs. frequency characteristics at the IF were
> needed to make
> it work well.
>
> One article that tells of early DF equipment was in October, 1944
> QST.  It
> tells about the radio stations used by the Army Air Corps to guide
> aircraft
> over Canada and Alaska to the western Pacific. Adcock arrays were
> used.  I
> can't remember if they were steerable or if they were fixed with
> adjustable
> phasing networks in the feed system. In any case, my reading of
> that
> article made me want an example of the SX-28's shown in use.   Now
> I have
> two of them, both awaiting restoration.  I have no plans for
> Adcock arrays
> here.
>
> Sooo many projects, sooo little time.
>
> Roy
>
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
> [email protected] --
>
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>

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