April 02, 2023

Choking out RFI on an EFHW antenna

(By Keith / KN5G) While the new End Fed, Half-Wave antenna has worked well on the HF bands I did notice that it had some RFI coming back into the shack and causing some issues with the USB cable’s signal between the computer and the radio. The EFHW antenna is an unbalanced antenna so it is normal to see some RF coming back down the outside shield of the coax towards the shack. 

The fix is a Common Mode Current Choke in the coax before it gets to the radio.

You can buy a CMC Choke but they are kind of expensive considering what they are made of and how simple they are to make. 

Here is one I built with a 240-43 Toroid core wrapped with 12 turns of RG58 coax. I need to put it in a box and solder the coax to the PL-259 connectors.

You can see from the VNA sweep that it has -36.4 dB of attenuation at the bottom of the 40 meter band which is pretty good. If I am not mistaken a 240-31 mix core would provide more attenuation on the higher bands, 20 meters - 10 meters.

Hopefully this will take care of the RFI issue I was seeing with the EFHW antenna.