Possible Cause for Odd HIGH SWR ?

Hi Andy,

I would recommend testing into a dummy load first to see if you can isolate the issue to the antenna system or the radio/software side. If SWR is normal into the dummy load, the antenna system is the culprit. Also worth noting: were all the problem occurrences on the same band? The pattern you're describing, where it resolves overnight without any physical changes, is a classic clue.

Now, about EFHW antennas. They require some care and feeding, and the sales pitch often glosses over the details.

End-fed half-wave antennas are voltage-fed at a high-impedance point, typically 2,000 to 5,000 ohms, which the matching transformer steps down to 50 ohms. That transformer is doing a lot of work, and it has an Achilles heel: it needs a small amount of current to flow somewhere on the "cold" side of the feedpoint to complete the circuit. Without it, the feedline itself becomes part of the antenna, and common mode current starts flowing back down your coax into the shack.

That's where your symptoms get interesting. RF in the shack can do all kinds of mischief, including corrupting audio streams, triggering software behavior, and producing reflected power readings that look like a sky-high SWR but are actually common mode current being sensed by the radio's protection circuitry, not a true impedance mismatch at the antenna.

A few things to address this:

Choke at the radio end. You want a common mode choke at or very near the radio, using a high-permeability core like Mix 31 or Mix 43. This is your first line of defense against RF coming back into the shack on the outside of the coax shield.

Counterpoise or choke at the feedpoint. The EFHW matching transformer wants to see a low-impedance path to ground at the feedpoint. You can provide this either with a short counterpoise wire (1/20 wavelength or so is often enough) or with a choke on the feedline at least 1/4 wavelength down from the feedpoint. Without one of these, the transformer is "floating" and the antenna system becomes very sensitive to nearby objects, feedline routing, and what your software is doing with the transmitter.

Matching transformer quality matters. Some EFHW transformers on the market are wound on undersized or inappropriate cores. Under sustained TX load they can saturate, shift impedance, and in extreme cases arc internally. If the transformer gets warm after a transmission, it's working too hard. Check the SWR both at the start of a transmission and after a few seconds, since a failing transformer will often show a clean SWR at the start that climbs as the core heats.

The overnight recovery is also consistent with this picture. Thermal effects in the transformer core, or RF-induced charge settling on nearby objects, can resolve themselves given a few hours. Moisture at the feedpoint transformer can also shift SWR but typically gets worse with sustained rain rather than better, so I'd move that lower on the suspect list. And, were you on the same band?

Start with the dummy load test, then audit your choke situation at both ends of the feedline. That should point you at the root cause.

73