300 ohm to 75 ohm impedance matching "transformer"

J

Joe

Guest
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
Click to expand...

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?
Click to expand...
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".
Click to expand...
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
Click to expand...
.... 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
 
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