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
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