[Hammarlund] ANTENNA MATCHING FOR OLDER RECEIVERS

[Glen Zook]

It has been my experience that virtually every TV type
balun works down to at least 160 meters.  This is
because in the cable TV business there are, according
to a good amateur friend of mine who was in the cable
TV business for over 30 years and was also the VHF
editor of QST, various frequencies as low as 5 MHz are
used in the cable TV business.  He has run the
"curves" on these baluns and has found that virtually
all of them work down into the AM broadcast band.  My
personal experience has also been the same.

I have measured over 100 receivers (with the
3-terminal antenna-antenna-ground screw connections)
the difference between coming directly into the
receiver with 50 ohm coax (with the 2nd antenna
connection jumpered to ground per "normal" use) and
using a TV type balun (with the second antenna
connection removed from ground).  These measurements
were made using at least one of my 3 service monitors
with well calibrated attenuators that have a 50 ohm
output.

There was not a single case of showing a loss by using
the TV balun.  The effective increase in sensitivity
between using the balun versus coming directly into
the receiver varied, depending on the individual
receiver (and especially on general coverage
receivers, the actual band and frequency thereon) from
about 3 dB to well over 20 dB for the same "S" meter
reading.

The "nominal" input impedance of the TV balun coils is
75 ohms and the "nominal" output impedance is 300
ohms.  However, this is not "exact" in any of them. 
They do a pretty good job of a 4:1 impedance matching.
 This is not "perfect" by any means.  However, most
receivers do not have a constant input impedance over
their entire tuning range.  Also, the input impedance
can vary all over the place even on a single band on a
general coverage receiver.

For an inexpensive "fix" for the impedance matching
problem the TV balun is certainly a viable
alternative.

Back in 1966 I was given Collins 75A1 s/n 4.  It was
almost a basket case when I got it and I eventually
rebuilt the receiver.  Frankly, the sensitivity when
connected to a coaxial fed antenna was very poor.  I
eventually built an external antenna coupler (band
switching) and then the receiver sensitivity became as
good as any of my more modern receivers.  But, it was
a real "pain in the posterior" to have to retune the
antenna coupler every time I changed frequency very
much and especially when I changed bands.  I traded
off the 75A1 in the mid-1970s when you almost had to
pay for someone's gas to get them to haul off the old
"boat anchors" (sure wish I had it back!).

A few years ago, after I had acquired a number of the
older receivers that had the higher impedance antenna
feeds that I finally tried the TV balun.  The
difference in most cases was "night and day".  All of
my receivers that had the 3-terminal antenna
connections worked much better.

Since that time I have serviced numerous receivers
with the 3-terminal connections and have found in most
of these that the use of a TV balun is definitely
warranted.  Now, in certain receivers, like the
Collins 75A2 and the 75A3 that were designed when
coaxial fed antennas were becoming popular, that the
TV balun doesn't help at all.  But, in all of my
Hallicrafters receivers through the SX-100, on my
Hammarlund receivers, on my National receivers, etc.,
that the TV balun coils definitely do help.

I have not "run" any TV baluns on an analyzer, etc. 
What I do is to check the sensitivity of the receiver
with a service monitor (a fairly expensive equivalent
of a signal generator) between the direct input to the
unit and with a TV balun.  This is the "proof of the
pudding".  If the sensitivity improves with the TV
balun then the balun is doing its job.

There have been several Collins 75A1 receivers that
have been acquired within the past couple of years by
local amateurs.  Every one of them complained about
the effective sensitivity of their receiver.  In every
case the use of a TV type balun improved the
sensitivity to where all of those amateur radio
operators said that they could hear at least as well
as any of their more "modern" receivers.  In a couple
of cases they said that the 75A1 was even more
sensitive.

Now, there are probably some TV baluns out there that
will not work.  However, and I may have been
fortunate, every TV balun that I have tried does work
down to at least 160 meters.  This includes those that
came with accessory items like VCRs, etc., those
bought at Radio Shack, and those that are supplied by
the cable TV companies.  Also, I have suggested the
use of TV baluns to many people and, so far, every
person has told me that the effective sensitivity of
their receivers has been greatly improved.

I definitely agree that a better match can be achieved
by building some sort of device.  How much difference
doing this will accomplish is open to question.  The
best way is to build a small antenna tuner that can be
"peaked" for each and every frequency.  But, this is a
real "pain" to use as well as being fairly costly (if
all new parts are used) to build.  

Building a balun (or purchasing one made for the
amateur frequencies) is definitely another option. 
Again, when compared to the cost of a TV balun, this
is going to cost more money (and if you have more than
one receiver this adds up!).  In addition, if you
"roll your own", this takes time as well as the
expense of the various items.  I am definitely a fan
of "rolling your own" and have built all sorts of
equipment over the years.

But, for the improvement that using TV type baluns
accomplish versus the cost versus the efforts
necessary to build your own balun, TV baluns are
definitely a viable alternative.  Frankly, I am a firm
believer in the principles of the conservation of my
finances and labor (you can read "cheap and lazy" if
you want to!).  If I can find something that does the
job for a minimum of cash outlay I am definitely in
favor of using it!

Glen, K9STH


--- Magoo <magoo@isp.ca> wrote:

You might think that using a standard TV balun would
solve the problem, transforming the 300 ohms of the
receiver terminals to 75 ohms, which is close enough
to 50 ohms for our kind of business. And I guess it
works to some degree.  But how good is this really?
 
I ran a number of TV baluns on my MFJ analyzer, using
300 ohms as the load. In every case the SWR was at
almost infinity over the range of 3.5 to 21 mhz.  At
28 mhz the SWR dropped to about 5:1!  The TV baluns
are made to operate from 54-960 mhz...there is not
enough inductance to work into the lower frequency
ranges.


So, what differences did I notice?  Well, I can run
any of my station 50 ohm antennas into the receivers
and they perform about the same as if I hooked up to a
signal generator and matching nework.

=====
Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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