[R-390] r390a balanced input connector

[Roy Morgan]

At 09:35 PM 6/24/2004 -0400, Bob Camp wrote:
Hi

There have been numerous threads on the 125 ohm input impedance of the 390. 
The simple point is that if you align the radio out of a 120 ohm generator 
then you will have a 120 ohm radio.

Bob and others,

MIL-R-13947B, 26 October 1960
RECEIVER, RADIO
(RADIO RECEIVER R-390( )/URR).   states:

"3.13.3 Antenna input impedance.- The rated input impedance for the 
balanced input circuit shall be 125 ohms. In the range from 500 kc to 16 
mc, the measured input impedance shall not be less than 50 ohms nor greater 
than 375 ohms; for the range from 16 mc to 32 mc the measured input 
impedance shall be not less than 100 ohms nor greater than 700 ohms. (See 
4.9)."

( This is the Military Specification that the receiver was built 
to.  Thanks to Al Tirevold for scanning and OCR-ing this 
document.  Available at
http://www.r-390a.net/
under Documents:
http://www.r-390a.net/faq-refs.htm  )

I have not (yet) measured the input impedance of any R-390 receiver, but I 
would expect a wide variation within the above limits if the thing is even 
moderately well aligned.

All this is even more confusing when you get into receiver design theory 
and they show that a low noise receiver does not general apply a matched 
load to the antenna.

It was well known among VHF folks who were tuning and/or building 
converters and pre-amps that the tuning that gives the highest gain is 
seldom the tuning that gives the best signal to noise ratio.  I would 
expect many things to be different between VHF/UHF and HF situations, among 
them input circuit Q, the related bandwidth, sources of noise and the 
relative impact from those sources, and losses in feedlines but even at HF, 
things are not simple.

The bottom line seems to be that if you align the radio out of the same 
source impedance as the antenna you will be using then the radio works just 
fine.

I have noticed that when using an antenna tuner to get low SWR on transmit, 
another setting of the tuner gives me greater signal strength on 
receive.  It might take lots of thinking of the sort that Bob presents to 
sort out why.

  If you are going to do a true balanced input then there is an extra step 
to the alignment procedure related to properly balancing the input.

As I understand it, that input coil balance adjustment is important to 
minimize common mode noise and RFI interference and for proper functioning 
of direction finding systems.  That would be important when the receivers 
are being used near operating transmitters or other noise sources.  I have 
an odd direction finding receiver that has two separate coax inputs to a 
balanced input transformer, so there may well be some direction finding 
systems that need to have the input transformer well balanced.

Another item on the to do list here is to experiment with that balance 
adjustment to find out if it affects receiver performance (sensitivity or 
selectivity) when one side of the input is grounded in the way that most of 
us use it.

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 at nist.gov --