[R-390] r390a balanced input connector

[Jordan Arndt]

Is this not a case of using a 600 ohm antenna and a 4:1 balun..???
73 de Jordan....


Roy Morgan wrote:

> 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 --
>
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