Topband: Magic 'T' Combiners

[Rick Karlquist]

Joe Wilkowski said:
> Folks, has anyone had experience in constructing a Magic "T" combiner ?
> If so, can you provide me with your empirical data ?
>  I have used BN202 binocular cores and using the impedance formulas for
> the core material  and constructing the transformer as a hybrid in-phase
> combiner with centertap  going to a 37.5 ohm resistor to ground and a
> carbon 150 ohm resistor across the entire transformer at the feed points.
> /joe k8fc

I built an 8 element vertical array a few years ago that
used both in-phase and out of phase hybrid combiners built
with those binocular cores.  They worked perfectly, just like the
textbook says.  The F/B of the array was around 30 dB.

An in phase combiner consists of an autotransformer
and a transformer.  The autotransformer has
150 ohms across it (for a 75 ohm system), and the inputs at
the ends of the winding.  The center tap then drives a 37.5 ohm to 75
ohm conventional (unun) transformer  No 37.5 ohm resistor is involved.

For an out of phase combiner, you simply need a transformer.  You insert a
37.5 ohm resistor between the center tap and ground (of the primary), and
take the output from the secondary.  The transformer should be a 150 ohm
to 75 ohm transformer.

Also remember that the isolation of a combiner is only as good as the
return loss of the load (receiver or preamp.)  Many or most of those do
NOT have low return loss.

I tried to recreate my previous array of passive verticals using
active verticals.  This didn't work at all, because of stray phase
shifts in the active verticals.  I used the same combiners.  So the
conclusion was that it was the fault of the antennas, not the combiners.

I wouldn't be surprised if you are in the same situation.

Rick N6RK