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When would I use a 1:1 balun?

ChatGPT said:

This is one of those questions that seems simple until you start digging into antennas. The short answer is:

A 1:1 balun is used when you do not want to change the impedance—you only want to keep RF currents balanced.

It is often called a current balun or common-mode choke, depending on its construction.

For your home EFHW setup, a 1:1 choke is useful, but not as the main transformer. Your chain is: FT-891 or TS-430 → ATU-100 → coax → 49:1 unun → 40m EFHW wire. The 49:1 unun is doing the impedance transformation. A 1:1 choke would be used to control common-mode current on the coax.

Best places:

1. Near the 49:1 unun
This is usually the most useful spot. It helps keep the coax shield from becoming part of the antenna.

But with EFHWs, the coax often acts as the counterpoise, so placing the choke immediately at the unun can sometimes make tuning worse unless you provide a separate counterpoise.

2. A short distance back from the unun
Often a good compromise. Put the choke maybe 10–25 feet from the transformer, letting a short length of coax act as the counterpoise while keeping RF from coming all the way back to the shack.

3. Near the radio/tuner
Useful if you are getting RF in the shack, odd tuner behavior, computer/audio issues, or “hot” mic/controls. But it does less to control feedline radiation near the antenna.

For your POTA vertical, a 1:1 choke at or near the feedpoint is often useful, especially if the vertical has radials. It helps keep the coax from becoming an unintended radial/radiator.