On Jan 23, 5:06*am, "D. Peter Maus"
wrote:
On 1/23/10 01:28 , bpnjensen wrote:

Can the venerable 9:1 matching transformer (for matching a 500-ohm
random wire to a 50-ohm coaxial cable) also be used the inverse way,
that is, to match an incoming 50-ohm coax to a radio whose only
connection is a Hi-Z pair of screw terminals?

Further on, is there any reason why such a matching transformer could
not be installed inside said radio behind the screw terminal strip so
that a permanent SO-239 could be connected and mounted on the radio
chassis?

Thanks,
Bruce Jensen

* *A transformer impedance ratio is a function of the turns ratio
and the terminating impedances on each side. As long as the
impedances being transformed are the same ratio and the number of
turns on the core are the same ratio, the transformer doesn't care
one way or the other which direction it's used. So, there's no
reason you can't use a 9:1 in an inverse manner. Mounting it
internally, with proper care to minimize coupling to other hardware
within the receiver will work correctly.

* *The only consideration worth mentioning, is that a random wire
isn't exactly a 500 ohm impedance. Impedance of a random wire
changes with frequency. The 9:1 un-un is used to even those
variations out, more or less, to an impedance that's reasonably
uniform across the spectra in use for the receiver input to accept.

* * The application you're suggesting isn't going to have those
variations, and the 9:1 ratio is fairly close to the actual
impedance ratios being applied. Most screw terminal inputs are
actually closer to 300 ohm, and may or may not be balanced. And
radios so equipped, which also have a 50 ohm SO-239 are also
impedance matched from the S0-239 to the front end of the receiver
through a transformer, precisely as you describe.

* * So, with reservations, the answer to your questions a yes,
you can do it. And, no, there's no reason why you can't mount
internally.

* * The only reservations being that the impedances being
transformed are not precisely in the exact ratio as designed for the
9:1, which may result in irregular losses, and resonances--
comb filtering. Which you may or may not be able to discern in
casual listening.

Ah, thanks for this Peter - so, if the screw terminals are closer to
300 ohms, then a 6:1 transformer, with perhaps a 2.3:1 turns ratio,
could be a better choice?

The radio in question, BTW, is a simple DX-160 - it's front end is
pretty wide open anyway, but heck, why not? As long as I don't send a
big static charge through it, I figure I cannot do much damage :-)

BJ