[Hammarlund] ANTENNA MATCHING FOR OLDER RECEIVERS

[Magoo]

Most of the older receivers were designed to use a 300-450 ohm doublet
antenna.  Simply attaching a 50 ohm coax cable to the terminals will cut the
signal way down due to a severe mismatch between antenna and antenna input.

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.

I wound my own matching transformer using an FT37-43 toroid core with 23
close-spaced turns on one side (to the receiver) and 8 close-spaced turns on
the other (to the 50 ohm antenna).  This was much better, showing less than
3:1 SWR at 80M to 1.7:1 at 10 meters.  By playing with the spacing of each
winding and the number of turns I could make the transformer work very well
over a 20 mhz span and acceptably over all the bands 80-10 meters.

Choosing a different mix of toroid would probably yield better results, but
the FT37-43 is all I had available.  I suggest looking at the FT114-43
or -75 mix.  The larger core is less lossy and it is possible to get a
better match over a wider frequency range.

As many receivers undoubtedly set 450 ohms for the input impedance, further
playing around may be necessary.  I placed the transformer in a small box
with banana terminals and an SO-239 jack.

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.  There is no discernable
difference in signal strength between using a 40M open-wire fed dipole (450
ohms) and a coax-fed dipole run through the matching transformer.

Bill, VE3NH