[R-390] Balanced Antenna Input

[n4buq at knology.net]

I wanted to build a "matching" adaptor to use the 125-ohm input, but 
realized the thing would ground one side of the input if I wasn't careful.

I want to feed my radio with twisted pair from the antenna and use the 
balanced input.  It seems this does give a slightly better signal in my 
limited testing with my unbalanced feedline so I'd like to pursue that.

I bought a twin-ax connector, but it looks like I won't really need it 
now.  I plan to build something that I can connect to the balanced input 
using a BNC connector (yes, I realize that's an unbalanced type connector, 
but it should work), but it really should be insulated from the chassis.  
Still thinking...

Thanks,

Barry - N4BUQ


> The balanced to SO239 adaptors do indeed defeat the purpose of the
> balanced input.  The idea behind the balanced input is that if your
> shack is a high-noise environment, you can cancel some of it by
> running a balanced line out to the antenna.  That way, any crap
> picked up by the feedline is applied equally to the + and - inputs
> (my terminology) and cancelled.  This phenomenon is called
> "common-mode rejection".  As soon as you ground either wire, that
> goes away and you're back to the conventional unbalanced line.
> (By the way, in this mode there is no point in adjusting
> the balance caps, since it's not balanced anyway.)
> 
> That said, the majority of today's users (and some in the past -
> witness the Navy mod to swap the rear-panel connectors) go
> unbalanced, simply because stuff is more available.  And
> remember, in the HF range, if you have a decent antenna and
> your shack isn't an EMI hellhole, most of your noise will be
> atmospheric and far-field QRM anyway.
> 
> 73,
> Dave Wise (SWL in Portland OR)
> 
> > -----Original Message-----
> > From: r-390-bounces at mailman.qth.net
> > [mailto:r-390-bounces at mailman.qth.net]On Behalf Of n4buq at knology.net
> > Sent: Thursday, April 13, 2006 8:58 AM
> > To: r-390 at mailman.qth.net
> > Subject: [R-390] Balanced Antenna Input
> > 
> > 
> > I have seen some adapters that mate a given connector type to 
> > the "twin-ax" 
> > style used at the balanced antenna input.  I might be wrong, 
> > but it appears 
> > these typically have one connection to the shell which 
> > grounds that side of 
> > the connection.  The balanced connector is designed to work 
> > with both sides 
> > of the feedline ungrounded, right?  If so, don't these 
> > adapters defeat some 
> > of the proper workings of this input?
> > 
> > Barry - N4BUQ
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