[R-390] Antenna Connectors

[David Wise]

It's necessary to involve both pins.  If you don't, the coil
primary is essentially in series with either the fixed or trimmable
balance caps.

Unlike most radios, the R-390x have a balanced, or differential, input.
The balance caps contribute to circuit operation in an interesting
way ONLY if you are using a balanced antenna lead.  In that case,
when the caps are trimmed right, the receiver will respond only
to the difference between the two conductors, and will ignore
common-mode noise.  The way most of us have the input set up
(single-ended, i.e. one pin grounded), this feature is defeated
and the trimmers don't do a thing.  Note that use of an "un-bal"
(opposite of a balun) at the receiver is futile, since the only
common-mode noise that will be rejected is what's picked up between
the un-bal and the receiver!

Dave Wise (SWL in Portland Oregon)

> -----Original Message-----
> From: Barry [mailto:n4buq at aol.com]
> 
> Along the lines of antenna connections, I "discovered" 
> something interesting
> the other evening.  For testing purposes, I have been 
> connecting my RG-58 to
> the unbalanced input.  I decided to switch to the balanced 
> input.  When I
> connect the center conductor, the radio jumps to life; however, when I
> connect the outer shield to the other pin (not grounding 
> either pin), the
> received signal becomes much quieter and about a 10dB 
> increase in signal
> strength (according to the Carrier Level meter).
> 
> I figure this is because I aligned it to the balanced input (with a
> lower-than-125-ohm impedance), but I was kind of surprised at 
> the difference
> I could detect between the two inputs, especially when I 
> disconnected one
> pin on the input (the coax's shield).
> 
> One other thing: the antenna input caps (the ones in the 
> first transformers
> that go across the first transformers) don't do ANYTHING.  Is 
> this simply
> due to too much impedance mismatch?