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

[JMILLER1706@cfl.rr.com]

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 <roy.morgan@nist.gov>
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
> roy.morgan@nist.gov --
> 
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