[R-390] Balun for balanced antenna connector

[Bill Smith]

A question, and a goal at the station here.  I use a Alpha Delta antenna
switch, which shorts unused switch positions to ground.  Don't know if that
is necessary, but the goal is to have all receivers fall absolutely silent
when they are not connected to an antenna.  One reason is to insure that the
power line and local devices such as computers don't find themselves acting
as signal sources.

The only way I have discovered is use of balanced feeds into the receiver.
I run unbalanced (coax) line to the set, then ground the coax shield to the
receiver chassis, and connect a balun primary to the shield and center of
the coax.  The balun secondary is connected to the balanced inputs of the
receiver.  Short leads are very important.

I use TV baluns (300-75 ohm) on some receivers where the AM band isn't very
important.  From measurements here, they don't work very well below 2.5 Mhz
or so, and really dive below 1 Mhz.  That can be an advantage, and usually
the receiver gain can overcome performance on 1.8 Mhz, where 160 meter
operation appears normal.  A homespun balun is used on the R-390 that works
over a wider range of frequencies, in fact I am still suffering overload on
the AM broadcast band.  The antenna is a multi-band, parallel dipole where
160, 80, 40, and 20 meter dipoles are all connected to a single RG-213
feedline.

The grounding system here should be ok, the radio room is only three feet or
so above ground.  I run 2-1/2" copper strap to two ground rods and to the
house water pipe where it comes in from the street.  The house is old, so
there should be metal at least to the meter.

Receivers are dead on all bands except the AM broadcast band, where some
stations can still make it weakly into a receiver (use Hallicrafters SX-62,
HRO, BC-779, AR-88, R-390, TS-440)

The question is can you silence your receivers by switching away the
antenna, and if so, what approach to you take?

73 de Bill, AB6MT
billsmith@ispwest.com


----- Original Message -----
From: "Thomas W Leiper" <twleiper@juno.com>
To: <r-390@mailman.qth.net>
Sent: Sunday, July 07, 2002 6:22 AM
Subject: Re: [R-390] Balun for balanced antenna connector


> On Sat, 6 Jul 2002 23:09:24 -0500 "Richard Biddle" <theprof@texoma.net>
> writes:
> ...
> > The signal generator ... seem to generate a stronger signal
> > on 10.5 MHz then when I ran an unbalanced antenna into
> > the balanced connector.
>
> What happens when you run the unbalanced antenna into the
> unbalanced input?(What a shocking and avante-garde concept!)
> Also, there is a simple Navy mod (somebody else can elaborate)
> that involves putting a shorting connector into the balanced input
> to ground one side and simply swapping the min-bnc cables to
> put the other balanced side onto the unbalanced connector. I do
> this on all of mine and it works excellent with 75 ohm dipoles and
> feedline. I also have a radial array that uses four diploes oriented
> in the four cardinal directions and, rather than using a remote
> switch, I simply ran a piece of CAT5 network cable and used the
> four twisted pairs contained therein. A simply DPQT rotary
> switch at the radio end feeds the unbalanced input on my Non-A
> directly wih excellent results. These things really like twisted pair
> feedline and doublets...an easy field configuration.
>
> Twit Hammarlund
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