[R-390] Proper Ant Connection for an R-390a

[Roy Morgan]

On Nov 12, 2012, at 1:42 PM, Mack McCormick wrote:

> My R-390a restoration is complete and working great!
>
> Here are some pictures

Mack,

WOW!  You have done a superb job on that radio.. congratulations.   
It's an inspiration.

> ... how should a typical ham antenna be connected to the receiver?

Simple way: ground one twinax pin and feed the other one with your  
coax, as you seem to be doing now.

The NAVships re-arrangement simply accomplishes the simple way above  
by moving the connections around in the radio and modifying a UG  
adapter so you can use the single type C connector for convenience  
(and so as to not damage the antenna relay and adapter.)

Expensive way: spend $30 or more for the UG-970 or -971 adapter which  
does the same thing.

Homebrewed way:  cobble up a bit of coax, a female SO-259 wire mount  
connector and a twinax connector to do the same thing.

Really nice home brewed way: get a nice twinax connector, and a female  
single hole chassis mount  connector - or preferably a right angle  
one, buy a 5/16 X 24 tap*, thread the twinax connector  clamp bushing  
for the BNC threads, and assemble the thing with with a couple bits of  
wire.  (I did this - very nice since most/all of my receive and low  
power coax is BNC terminated.)
*This may not be the correct thread specification, do check it if you  
decide to do this.

Improved home brewed way: do the above but include a small ferrite  
toroid selected for the HF spectrum and wound to match the 50 ohm coax  
to the nominal 125 ohm balanced input.  (I have not done this, but  
would like to to see if it makes any difference.)

More complicated way:  Run your balanced line all the way to the rear  
of the radio, use both twinax pins.

Slightly more complicated way:  buy or scrounge a fairly modern TV  
type receiving balun, feed the twinlead end to the balanced connector  
and your 50 ohm coax to the type F end. One fellow claims this makes a  
significant improvement on older radios meant for balanced antenna  
setups.

Similarly more complicated way: rework your 4:1 balun to match the  
balanced line to 125 ohms.  Use the twinax connector.

Yet more complicated way: find and use some twin conductor coax (not  
so easy to find now) with 125 ohm impedance and make some sort of  
impedance transformer or network to match the balanced line you have  
to the twin coax.

Finally, do report what you do and how it works!

Roy

Roy Morgan
k1lky at earthlink.net
K1LKY Since 1958 - Keep 'em Glowing!