[Hallicrafters] SX-122

[Glen Zook]

There was an article written by me in Electric Radio
over a year ago about using TV type baluns.  The
article is available from either of my websites

http://home.attbi.com/~k9sth

http://home.attbi.com/~zcomco

According to another Bill Smith, W5USM, who was in the
cable TV business for many years, most TV baluns will
make it down into the broadcast band before they start
"falling off".  They definitely have to make it below
5 MHz since there are various cable TV signals in that
range.

On receivers that were designed with the balanced
feedline input, they definitely help.  On those that
were designed for coaxial input, don't even try using
one!  They won't help, and sometimes hurt.

I have found that using a TV type balun can make from
a few dB to over 20 dB difference between a direct
coaxial input and the balun depending on the
frequency.  This was measured using service monitors
with accurate calibrated attenuators.  For example, on
a Collins 51J2 use of the balun at 700 KHz made about
a 6 dB difference.  Going up in frequency (the 51J2
has 30 individual 1 MHz ranges), depending on the
individual band, it made up to slightly over 20 dB
difference in sensitivity.

Anyway, TV baluns are cheap.  In fact, a lot of people
have one, or more, around the house since a lot of
accessory TV items (i.e. VCRs) came with one.

By the way, the "normal" connector for TV use is the
type "F".

Also, do not connect the shield of the coax to the
chassis of the receiver.  This can unbalance the
balun.

Glen, K9STH


--- Bill Smith <billsmith at ispwest.com> wrote:

Receivers with balanced inputs work much better if the
antenna fed with coax is connected to the receiver
through a balun.  We can get into a long story about
this (have on other mail lists), but I have used a TV
75-300 ohm converter.  The two 300-ohm twin-lead lines
are connected to the A1, A2 antenna terminals.  The
antenna coax is connected to the S-connector (or
whatever the TV coax plug is called) and the shield of
the coax is connected to the receiver ground using a
very short wire.

 Such TV adapters are lossy below about 3.5 MHz, but
the receiver is able to overcome the loss except at
the bottom of the AM broadcast band (700 KHz and lower).

=====
Glen, K9STH

Web sites

http://home.attbi.com/~k9sth
http://home.attbi.com/~zcomco

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