Is this 300 Ohm to 75 Ohm Impedance Matching "Transformer" Defective?

"Joe" I bought a 300 to 75 ohm "matching transformer" to hook up to a home built

**Go back to "sci.electronics.basics" - cos your Q has already been answered.

Part of you Q is answered on the link you posted.

Do pay attention.

..... Phil

#snip#

It might. There may be a DC-blocking capacitor present in the connection from the coax center connector to the coil, to ensure that no damage occurs if you were to plug this sort of balun into a coax which was being fed DC power for a mast-mounted preamp. The DC-blocking cap would have a very low impedance at radio frequencies (it would be effectively "invisible" to RF) but will have such a high impedance at DC that your meter will read "infinity".

It's probably wound as a 4:1 "auto-transformer", in which the primary and secondary windings are actually connected together. This establishes a current path for DC from the high-impedance side to ground, and allows any static electricity building up on the antenna to flow safely to ground. If the primary and secondary windings were isolated, you could get hundreds or thousands of volts of charge building up on the secondary side (even in clear dry weather) and the static charge could arc over to the primary windings and damage the insulation on the wire.

Tt's very probably wound on a small ferrite core (toroid or two-hole balun core) rather than with an air core. The high permeability of ferrite means that fewer turns of wire would be necessary,

Since there are only a few turns of wire, the DC resistance would be less than an ohm.

Yes... you could, for example, connect a 300-ohm balanced antenna to

75-ohm coax with one, and then use a second one to adapt the coax to a high-impedance balanced antenna input on a TV or stereo.

--
Dave Platt                                    AE6EO
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NO ONE has addressed the fact that there is NO apparent continuity from the center conductor of the coax to ANY of the other terminals.

Do you, Phil, or anyone else know if this indicates that this "transformer" is defective?

Thanks.

--- Joe

Thank you, Dave. It appears that you and I were simutaneously adding to this thread just a couple of minutes ago.

None of the examples on wikipedia or anywhere else I looked had a capacitor. So I really wondered if there *might* be one inside the transformer.

I appreciate the explanation. Even though these things are physically simple devices, impedance matching of transmission lines sure is currently somewhat mysterious to me.

=== Joe

I looked at a new antenna transformer; it has DC continuity between all terminals. That is, any terminal should have continuity to any other terminal. The resistance will, of course, be very low... in the neighborhood of 0.1 ohms or so.

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Some people are like Slinkies.  Not really good for anything, but they bring a 
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Michael,

I have finished the homemade indoor version of a UHF antenna and hooked up the 300 ohm to 75 ohm "transformer". It works.

When I asked the question about the transformer, the antenna had not been completed, and I really wanted the antenna to be the only variable, thus the attempt to ensure that the transformer was good.

I built the Gray-Hoverman antenna as shown on

My copy of this antenna was really a kluge. Rather than using copper or aluminum tubing or bare heavy gauge aluminum or copper wire, I used 22 AWG or smaller insulated copper wire, Scotch taped to a piece or corrugated cardboard about a quarter inch thick.

The geometry, however, is spot on :)

I have not yet constructed the reflectors which are to be positioned 4" to the rear of the main antenna plane.

All things considered, it works pretty well, seeing that I'm about 45 miles from Mt. Wilson which is where most(?) of LA's TV transmitters are located.

A note about Phil Allison:

I had thought about asking about construction materials for the antenna before I built it, but I thought at least one of the sci.electronics.* A-Holes would take the opportunity to show their "wit".

Sure enough, old toilet-mouth Phil Allison crawled out of the woodwork.

I sure wish he would at least stay out of sci.electronics.basics. He is the opposite of a good teacher and takes joy in his attempts to kill enthusiasm and the joy of learning.

--- Joe