[R-390] Balanced input on higher bands correction

Fri Dec 24 17:46:16 EST 2021

Hi again Jim, Dave and the group...

I have both right angle C and straight Twin-ax connectors, so antenna 
connection isn't an issue.

Over the years, I've tried a number of different baluns, from Hygain BN86 
and W2AU types to 75 Ohm TV type baluns and never found a "one size fits 
all" solution for feeding the balanced input, so more recently I've been 
feeding one side of the Twin-ax balanced connector with the wire antenna 
lead and shorting the other side to ground as described in several online 
texts.

Recently, while going through the RF deck of this S-W 390A, I happened to 
use temporary Twin-ax connections with long "jumpers" that have alligator 
clips at each end for convenience, which lead me to suspect problems in the 
RF deck above 14.5Mhz which aren't really there. It was the 18" long jumper 
to ground that was causing the problem.

Since feeding the antenna to the radio this way works so well, I'm seriously 
considering applying the Shipboard cable-swap mod and connecting signals via 
the C connector.

Happy Holidays and VY 73...Jordan VE6ZT

----- Original Message ----- 
From: "Jim Whartenby via R-390" <r-390 at mailman.qth.net>
To: <r390a.urr at gmail.com>
Cc: <r-390 at mailman.qth.net>
Sent: Friday, December 24, 2021 11:15 AM
Subject: Re: [R-390] Balanced input on higher bands correction

> Thanks Dave,I didn't know about the balun, I wonder it covers the MF and 
> HF bands? I have the UG-971/U here which was what I was thinking about.
> Jim
>
> Too much agreement kills a chat. E. Cleaver
>
> -----Original Message-----
> From: Dave Merrill <r390a.urr at gmail.com>
> Cc: R-390 at mailman.qth.net <r-390 at mailman.qth.net>
> Sent: Fri, Dec 24, 2021 10:07 am
> Subject: Re: [R-390] Balanced input on higher bands correction
>
> IBM used twinax on their System 34 token ring network and at least some of
> the twinax to BNC adapters you'll find are from those networks. They
> contain a balun which may or may not be responsive at HF frequencies.
>
> A safer bet is a UG-971/U and a UG-636A/U C to BNC adapter. Both of these
> are found in the MK-288 Maintenance Kit.
>
> NOS UG-971/U adapters are available very reasonably here: eBay item
> 312574496229.
>
> The UG-636A/U can be found on eBay for about $20.
>
> 73 & Happy Holidays,
>
> Dave N9ZC
>
> On Thu, Dec 23, 2021, 11:11 PM Jim Whartenby via R-390 <
> r-390 at mailman.qth.net> wrote:
>
>> Jordan There are Twinax to coaxial adaptors available so you don't really
>> need to jury-rig the connection. I believe that you have a choice of
>> Twinax to N, BNC or UHF.
>> There is one on eBay, Item # 401412862478 which adapts Twinax to BNC.
>> Another is Item # 132072997494 which is Twinax to UHF. If you are 
>> patient,
>> the perfect adapter for you application will eventually show up, at a
>> decent price.
>> Not my auctions, don't know the sellers, etc. <grin>
>> Jim
>>
>> Too much agreement kills a chat. E. Cleaver
>>
>> -----Original Message-----
>> From: Jordan Arndt <Outposter30 at shaw.ca>
>> To: R-390 Forum <r-390 at mailman.qth.net>
>> Sent: Thu, Dec 23, 2021 12:38 pm
>> Subject: [R-390] Balanced input on higher bands correction
>>
>>
>> Hi group and Happy Holidays...
>>
>> Sometime back I responded to a post stating that my 390As all had an 
>> issue
>> above 15-16Mhz when connected to my ~90 foot wire via the balanced 
>> antenna
>> input...
>>
>> The problem was not the antenna or the balanced input. The real problem
>> was
>> that I was temporarily using an 18" jumper to ground one side of the
>> balanced input connector to the chassis.
>>
>> Once I replaced the 18" jumper with a 2 inch wire grounded to the 
>> mounting
>> screw just below the balanced input connector, the sensitivity was
>> restored
>> from 15 to 32 Mhz. In fact, I'm surprised at how much of a difference the
>> length of that connection made...
>>
>> Live and learn...!
>>
>> 73...Jordan VE6ZT
>>
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