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300 ohm to 75 ohm impedance matching "transformer"

Started by Joe May 22, 2008
I bought a 300 to 75 ohm "matching transformer" to hook up to a home built
TV antenna.

The matching transformer looks exactly like the one on this URL:

http://www.solidsignal.com/prod_display.asp?PROD=WMXFMRPT

I have a couple of questions:

Can these matching transformers be used in either direction?  That is, my
antenna will get the 300 ohm end, and the TV antenna input will get the 75
ohm end, is this okay?

Also, I'm not sure if this part is defective.  I put a DMM meter on the
various connections and measured the resistance:

Between the two 300 ohm conductors = a fraction of an ohm  (0.4 - 0.3 =
0.1 ohm).

Between each of the the two 300 ohm conductors and the outer coax = a
fraction of an ohm  (0.4 - 0.3 = 0.1 ohm).

Between the inner coax and each of the other three terminals = infinite ohms.

Does this make any sense?  I originally checked these resistances thinking
that this "transformer" would be two coils wound on an air core, so that I
would measure a small resistance between the two 300 ohm leads, and a
small resistance between the outer and inner of the coax connector.

I have another DMM that can measure capacitance, but it's not currently in
operation.  If recommended, i could try to measure capacitance between the
various terminals.

FWIW the package, but not the part, is marked "Philmore 300-75 OHM
Matching Transformer.  No. MT73"

The part itself is marked: UHF/VHF MATCHING TRANSFORMER CHINA

Thanks.

---  Joe
Joe wrote:
> > I bought a 300 to 75 ohm "matching transformer" to hook up to a home built > TV antenna. > > The matching transformer looks exactly like the one on this URL: > > http://www.solidsignal.com/prod_display.asp?PROD=WMXFMRPT > > I have a couple of questions: > > Can these matching transformers be used in either direction? That is, my > antenna will get the 300 ohm end, and the TV antenna input will get the 75 > ohm end, is this okay? > > Also, I'm not sure if this part is defective. I put a DMM meter on the > various connections and measured the resistance: > > Between the two 300 ohm conductors = a fraction of an ohm (0.4 - 0.3 = > 0.1 ohm). > > Between each of the the two 300 ohm conductors and the outer coax = a > fraction of an ohm (0.4 - 0.3 = 0.1 ohm). > > Between the inner coax and each of the other three terminals = infinite ohms. > > Does this make any sense? I originally checked these resistances thinking > that this "transformer" would be two coils wound on an air core, so that I > would measure a small resistance between the two 300 ohm leads, and a > small resistance between the outer and inner of the coax connector. > > I have another DMM that can measure capacitance, but it's not currently in > operation. If recommended, i could try to measure capacitance between the > various terminals. > > FWIW the package, but not the part, is marked "Philmore 300-75 OHM > Matching Transformer. No. MT73" > > The part itself is marked: UHF/VHF MATCHING TRANSFORMER CHINA > > Thanks. > > --- Joe
Those are simple baluns. A 2:1 transforemer. Yes, they work in both directions. They are simple HF transformers and depending on the design continuity between all terminals is normal. -- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
On Thu, 22 May 2008 00:30:28 -0700, Joe wrote:

> I bought a 300 to 75 ohm "matching transformer" to hook up to a home built > TV antenna. > > The matching transformer looks exactly like the one on this URL: > > http://www.solidsignal.com/prod_display.asp?PROD=WMXFMRPT > > I have a couple of questions: > > Can these matching transformers be used in either direction? That is, my > antenna will get the 300 ohm end, and the TV antenna input will get the 75 > ohm end, is this okay?
Yes - it's called a "balun" (which nobody knows how to pronounce) which is an abbreviation for "balanced to unbalanced". And, as you can see from the first image here, it does have DC continuity: http://en.wikipedia.org/wiki/Balun Hope This Helps! Rich
Rich Grise wrote:
> Yes - it's called a "balun" (which nobody knows how to pronounce) which > is an abbreviation for "balanced to unbalanced".
OK, so how do you pronounce it? I pronounce it exactly like the first syllable of each of the two words.
> And, as you can see from the first image here, it does have DC continuity: > http://en.wikipedia.org/wiki/Balun
... unless a blocking capacitor has been deliberately inserted into the coax path in order to avoid shorting out any preamp power supplies that may be in the downstream equipment. -- Dave Tweed