[Hallicrafters] More on use of baluns in BA receivers (my experiments)

[Philip Atchley]

Hello all
In light of the postings on this reflector awhile back concerning the
increased sensitivity of certain BA receivers if a balun was used to couple
the (nominal) 50 Ohm antenna to the balanced inputs, often labeled A1 and A2
of the receiver.

So I decided to do some experiments.  As my listening interests span the
spectrum of BCB through the shortwave broadcast ranges, I concentrated
primarily on the range of 530kHz to 22MHz.

First, as suggested in the earlier posting I tried a couple TV baluns but
found the performance below 10MHz dropped too dramatically.  I figured that
a balun designed for HF use would probably be much more suitable to my
needs.  So I went searching for an economical antenna tuner that had an
internal balun.  As it turned out, I had an MFJ 901B tuner given to me.
This is a compact tuner containing a balun and no power meter.  Ideal for
receiver use.  I installed a 'bypass' switch which left the balun in circuit
but disabled the tuner function when desired.

The below charts indicate relative performance with and without the balun
(tuner bypassed) and with and without the tuner (balun in use).  For the
second chart the signal was normalized to S4 without the tuner prior to each
measurement to make it somewhat meaningful.

As you can see, the balun has significant loss below 2MHz, though this may
be partially due to input coil problems on the SX-71, which is pending
replacement. But in truth, I suspect I may need to rewind the balun with
additional turns of smaller wire to be functional down into the BCB. That
will be my next step.

Also, "real world" tests with the tuner and real world antennas indicates
much more than the "nominal" improvement shown by the following charts, due
to the widely varying characturistics of the antennas with frequency.  With
my antennas this is especially noticable at 17.5MHz and 6MHz where there are
very dramatic differences of several S units.

Test equipment used.  Xtal crystal calibrator with variable attenuator and "
forced" 50 Ohm output through Pi attenuator. All readings were taken twice
and where slight differences observed were averaged.  Marker frequency used
was 1MHz

50 Ohm line vs Balun alone. (S meter readings)

kHz    50 Ohm line      With Balun
1000   S7                     S5.3
2000   S9+10               S9+10
3000   S9+18               S9+20
4000   S9+12               S9+12

5000   S9+11               S9+13
8000   S9+7                 S9+12
12000  S8.8                 S9+14
14000  S6.5                 S7.2
20000  S6.5                 S5
30000  S4.3                 S5.2

Readings taken with Balun and Balun/tuner, normalized to S4.

1000   S4                     Zip
2000   S4                     S3.5
3000   S4                     S4.6
4000   S4                     S4.5
5000   S4                     S4
8000   S4                     S4
12000  S4                    S4.5
13000  S4                    S5
14000  S4                    S5
20000  S4                    S5.3
30000  S4                    S5.5

73 de Phil  KO6BB
Loving home provided for wayward Boatanchor Receivers
k06bb at elite.net
Merced, Central California
37.18N  120.29W  CM97sh







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