On Apr 11, 2019, at 6:09 PM, Jack Brabham - KZ5A <kz5a@...> wrote:

Antennas like the OCFD, EFHW , really even a plain dipole will utilize the coax shield as part of the antenna's counterpoise if they can,  resulting in  RF in the shack, excessive noise pick-up, and pattern distortion.   The EFHW requires a counterpoise of ~10% of the main element length, the others not so much.

The way to prevent this problem is a choke unun at the antenna feedpoint.   This can be as simple as several turns of coax in 8 to 12 inch loops,  or at qrp levels several turns of  RG-174 wound around something like a FT240-43 core. 

I had a Balun Designs 2KW Unun that was the center of a 160/80 trap dipole, after a few years the electrical box it was mounted in deterioated and it went to the junk box.

Recently it's "parts" have now been resurrected and it now forms the input choke unun for my 4 core 49/1 EFHW transformer, all in the same 4x4x6 junction box.

Fighting RF in the shack problems never ends until you find a way to keep it out.

73 Jack KZ5A

On 4/11/2019 1:44 PM, Joe Puma wrote:

I can try that on the short patch from tuner to meter or radio as I have LM240 there. but the line going to antenna is RG213. So I don’t have a clip ferrite that big.  

On Apr 11, 2019, at 3:40 PM, Evan Hand <elhandjr@...> wrote:

I had similar problems when connected to an off center fed dipole.  i solved my issues with clip on ferrite on the coax going to the antenna.  That may work for you.  Maybe a ferrite between the uBitx and the tuner, or an isolation balun

The issue was RF on the braid of the coax.

FWIW
73
Evan
AC9TU