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300 ohm vs 75 ohm connections?

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Mr_bill2

A

I have a 300 ohm adaptor with twin leads, that I use on my indoor 75 ohm f type connector wire antenna. I think I remembered once reading that there are advantages to only hooking up one of the two leads to the 300 ohm connection on the back of a radio, is this true or won't it make a difference using one lead or both? Especially when taking into consideration that I'm not really using a 300 ohm wire antenna, it's a 75 ohm f type with a screw on 300 ohm adaptor on the end. My radio only supports a 300 ohm connection terminal.

Thanks!

Follow Ups:

RE: 300 ohm vs 75 ohm connections? - BFitz 08:17:12 03/05/19 (0)
Which tuner is it? Scott tube tuner?

Many antennas have a 300 ohm output, needing a 300 to 75 balun at the antenna to hook up to 75 ohm cable.

At the other end, if you have a 75 ohm input, you simply hook it up. If it's a 300 ohm tuner input, yes, use another balun at the end of the coax cable.

You need the tuner manual to know if the 2 screws on the tuner back panel are 75 ohm or 300 ohm. Usually on older tuners, it's 300 ohm. If you use a low loss balun, there is little loss involved.

If unsure, try it both ways and see what works better. You can hook a 75 ohm coax directly to 2 screws with a bare ends coax cable, or a female F jack with 2 short wires soldered to it.

It will not hurt or break anything if you hook 75 ohm coax directly to a 300 ohm input.

The other thing you may have heard - a really decent and easy indoor antenna can be made from a single stand of wire, about 60" long, only hooked to one of the two antenna inputs. For some people, this can work better (again, indoors) than a wire dipole. Bottom line - use trial and error to see what works best.
RE: 300 ohm vs 75 ohm connections? - Triode_Kingdom 00:39:14 03/04/19 (1)
The adapter should have a transformer to step up the impedance to 300 ohms. If so, then for all intents and purposes, it is a 300 ohm antenna. Twinlead is a balanced transmission line. How much difference there might be connecting only one terminal probably depends on how the connections are handled inside the tuner. I seem to remember that some older tuners actually connect one terminal to ground. If that's the case, the antenna's pattern will be badly skewed no matter what you do, and the results entirely unpredictable. OTOH, if the balanced line is properly terminated inside the tuner, say with a transformer, I think connecting both terminals can provide some advantage regarding both gain and pattern.

--------------------------
Buy Chinese. Bury freedom.
- RE: 300 ohm vs 75 ohm connections? - Mr_bill2 18:29:40 03/04/19 (0)
  Thank you. I've see some older tuners with 2 as well as 3 ternimlas, one of which is a marked ground. That being said mine only having 2 terminals it's not clear that one might be a ground. I'll have to check the service manual to see if it helps me. Thanks again for your helpful information.
  
  Edits: 03/05/19

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