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On 1/23/10 12:14 , bpnjensen wrote:
On Jan 23, 5:06 am, "D. Peter 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? That would be my thinking. Experimentation is the lifeblood of the hobby, so I'd try it either way see what you get. 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 DX-160 is a pretty interesting radio to toy with. I've had two, now, with dramatic inconsistencies in performance. Some, here, have done extensive numbers of small modifications and gotten a pretty decent radio out of it, so the raw material is there. The front end of DX-160 is going to be close, but not really at, any standard impedance. And it varies significantly from unit to unit. It's very likely the scenario you first described in your original post will be more than sufficient. Trial and error will get you on target, but the practical differences in performance may be trivial. |
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