75R to 50R converter

Because you want a Un/Un, or unbalanced to unbalanced transformer. I made decent ones with six hole ferrite beads, and a 5 to 6 turn ratio transformer. 50 Ohms * 1.2 * 1.2 = 72 Ohms. Amidon is one source for the six hole beads.

--
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

A TV antenna (or log periodic) is a balanced antenna, just like a dipole. A TV antenna (or real log periodic) is not likely to have feed point impedance of 75 Ohms, it is more likely to be in the vicinity of 200-300 Ohms. It could be that the OP has already attached the 4:1 balun that came in the box with it. In that case, the antenna would then have an unbalanced feed point impedance of roughly 75Ohms.

BTW, I thought you plonked me.

Not in the least.

Mini-Circuits has been making 50 MHz to 5000MHz transformers for over 40 years, not new nor all that difficult.

Only if used uncharacterized for a VNA situation, a mere spectrum analyser will be quite untroubled about group delay and a few 10ths of a dB will not often matter much very often.

This sounds like someone seriously addicted to VNAs. Of course, if you are making amplifiers for 50 MHz to 5000MHz (and higher frequencies) you should be, that is where most of your performance parameters are measured.

Just guessing from the frequency boundaries and the equipment already in use i think that OP is doing EMC compliance measurements.

Spectrum analyzers are also great for for doing third order intercept measurements.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

Yes, cheap TV antennas are 300 ohms, balanced, but a few types are 75 ohm unbalanced, for MATV/CATV service. These have the shield connected directly to the boom. I have built and used the 50 to 75 ohm VHF transformers to use better test equipment to repair CATV headend equipment and to align custom filters with 50 ohm test equipment.

Just on sci.electronics basics, so far.

--
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

Joe -

Can you give me a part#? I didn't find a single part that meets with your above assertion.

Also, I don't know anyone who measures EMI/EMC without a calibrated antenna system. I submit to you that if you just hook a 75 ohm antenna to a 50 ohm instrument, or vice versa, (and making no other measurements or calculations), then you are simply wasting your time and your client's money.

I would not place much confidence in the results. ESPECIALLY for "off-air" field measurements, which is what I believe the OP is trying to accomplish.

-mpm

Gee, I figured the OP was just trying to get a look at what sort of signals are out there. I can't imagine anyone trying to do calibrated measurements without a good known antenna. It sounded like he's just using a couple old TV antennas that happened to be kicking around. It's really unlikely that he'll notice any difference for off-the-air work with antennas that are essentially unknown anyway, if he just connects them directly to the SA. He can use either 75 ohm or 50 ohm line; it won't make a significant difference.

Perhaps Joseph slipped a decimal place on that MiniCircuits transformer frequency range, at least as separate transformers. In any event, the TC1.5-1 is rated 0.5 to 2200MHz, well in excess of what the OP needs. Never mind that it has more power loss than if you just connect the 75 ohm antenna (assuming it really does look like a 75 ohm source) to the 50 ohm load.

Cheers, Tom

Yagis, dipoles, logs etc are typically balanced antennas by nature. I haven't seen any type of TV antennas that were fed straight from coax, but that doesn't mean they don't exist in the commercial world. I do a have a

2mtr beam that has a gamma match built into it so you can feed it straight from coax, but that's only because the matcher is built in. It's still a balanced antenna.

I've done a little antenna matching/tuning/construction myself. :-)

That's cuz you misunderstood what I was trying to say, I wasn't "boasting about being plonked allot". At any rate, keep your head in the sand over there if you like, but I think the whole plonk thing is silly.

Are you asserting that you have "worked" 2 m band and have never heard of a "J-beam"? How about "5/8 wave whip"?

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

I never said any such thing. What do you mean?

It expires before long. Some newsgroups are 75% noise, and need a lot of filtering to look for anything worth reading.

--
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