Since setting up my Yaesu FT-710, I’ve been fighting with a problem. When transmitting over 20w on 30m/40m, I will lose CAT control on my computer. I have no problems transmitting 100w on all other bands.
My theory is that RFI is making it back into my shack and causing interference with the USB connection to my laptop. My justification is that EFHW antennas without a dedicated counterpoise or a choke uses the coax shield as a counterpoise of sorts. This brings RF to wherever the feedline is routed. Also, the interference is power dependent (less power means less RFI).
My first attempt at a solution was to install clip-on ferrites on both ends of the feed line, USB, video, and power cables cables coming from my laptop. After testing, this did not appear to improve my issue.
My next attempt was two-fold: build a common mode choke for my feedline, and install a dedicated counterpoise on my 49:1 EFHW balun.
I ordered a toroid (type 43, 2.4in OD), and following G3TXQ, I wrapped the end of my feedline (RG58) around the toroid 12 times. I then enclosed the ferrite with the windings inside a small plastic project box. I sealed the hole at each end with some plumbers putty. See below for pictures of the final CMC assembly.
I also added a counterpoise to my balun. The length should be approximately 8.7% of the antenna length. In this case, for an 80m EFHW, (40m long radiator) the counterpoise came out to be around 3.5m, or approximately 12 ft.
After adding the CMC and counterpoise, my RFI issue has stopped. I can now transmit full power on all bands without losing CAT control. See below for a photo of the balun and the final setup.
References:
Clip-on Ferrites: https://www.dxengineering.com/parts/DXE-CSB31-275-10
Type 43 Toroid: https://www.mouser.com/ProductDetail/623-5943003801
Common Mode Choke: http://www.karinya.net/g3txq/chokes/
EFHW Counterpoise: https://forums.qrz.com/index.php?threads/new-end-fed-antenna-ideas.906844/page-3#post-6829826