[R-390] using the balanced input with unbalanced antennas

[Bob Young]

I use UG-970/U connectors which are twinax to SO-239 90 degree connectors 
with one pin grounded for unbalenced antennas, you get a lot more gain this 
way than using the unbalanced input as the radio is aligned using this 
input. I forgot where I read it but it matters which pin is grounded. If you 
face the receiver from the back the 90 faces right on a 390A and that is the 
correct way. If I remember I'll check which pin in the connector is 
connected to ground tomorrow and post it. These connectors are still 
available at the RF connection BTW and are really nice, I ordered 4 today. I 
do believe the righthand pin as you face the back of the radio is the one 
that is supposed to be grounded, not positive though which is why I'll check 
the connector.

Bob Young
Millbury, MA
KB1OKL


>From: Roy Morgan <roy.morgan at nist.gov>
>To: "n4buq at knology.net" <n4buq at knology.net>, r-390 at mailman.qth.net
>Subject: Re: [R-390] (no subject)
>Date: Fri, 23 Mar 2007 16:25:19 -0400
>
>At 03:31 PM 3/23/2007, n4buq at knology.net wrote:
>>Gang,
>>
>>Is there a right and a wrong way to feed the balanced input with 
>>unbalanced
>>coax?  I found two different versions and wonder if it makes any 
>>difference
>>which way it's done.
>>
>>http://www.qsl.net/k5bcq/r390/r390.html
>>
>>http://www.r390a.com/html/feedpoint.html
>
>Barry:
>
>Those two methods wind up with the same result.  Here are some comments:
>
>It may matter which pin you ground.  Here's why:
>
>MIL-A-27434/24, 28 September 1960 describes the UG-971, twinax to type C 
>adapter.  This  MIL-Spec show the index slot in the inner shell of the 
>connector as being to the LEFT as you view the pins of the twinax connector 
>and the other end of the connector downwards.
>
>MIL-A-27434/23, 28 September 1960 describes the UG-971, twinax to "UHF" (or 
>SO-239) adapter.  This Mil Spec shows  the index slot in the inner shell of 
>the connector as being to the RIGHT as you view the pins of the twinax 
>connector and the other end of the connector downwards.
>
>HOWEVER, I have a photo showing one each of these adapters and both of them 
>have the slot to the RIGHT as you view the pins of the twinax connector and 
>the other end of the connector downwards
>
>What this means is that depending on which of these two adapters you have, 
>OR which way your particular adapter is made, you will be grounding one or 
>the other of the twinax terminals and the other goes to the center of your 
>coax.
>
>Whether this matters, I do not know!  It could be that the adjustment of 
>the Antenna Input Balance capacitor on the RF input transformer would make 
>a difference. It could be that if the thing is adjusted per the book (RF 
>fed equally to both pins and adjust for a null in received signal) that it 
>won't matter which one you ground.
>
>  I suggest you try it both ways and let us know. An  easy way to change it 
>once you have your coax lashed up to the twinax connector, no matter how 
>you do it, is to reverse the two coax wires to the RF deck from the antenna 
>relay  which carry the balanced RF.
>
>Roy
>
>
>- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
>13033 Downey Mill Road, Lovettsville, VA 20180
>Phone 540-822-5911   Cell 301-928-7794
>Work: Voice: 301-975-3254,  Fax: 301-975-6097
>roy.morgan at nist.gov --
>
>
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