Playing with an end fed half wave on 3.5mhz.

Having trimmed the length it offers a respectable match on 80,40 and 20 and sounds quite lively on rx, however it is very noisy, particularly on 80. Grounding the earthy side of the coax at the transformer to an earth stake reduces the noise by 3 S points, it does seem to reduce the just the noise not the signals. Adding the ground has little effect on the antenna matching. On air reports reveal that adding the ground also reduces the radiated signal by around 3 S points. Could it be that the ground is altering the radiation angle.

The receiver is nice and quiet with no antenna connected and is also quiet with the antenna when running on a battery so it seems to be related to the power supply however the noise is only there with the antenna connected. It is a linear psu and the effect is not apparent with a half wave dipole Connected.

I wound the matching transformer primary like an autotransformer rather than bifilar, can't see that is relevant however.

Where do I go to track down the noise ?

It could be noise getting down the outside of the coax which is being picked up by the antenna, you probably need to try a ferrite choke at the shack end of the coax. I had the same problem with my 160M loop antenna, the ferrite choke stopped the noise by about 12dB.

I will add, the electrical earth is very noisy.

Is the power supply -ve connected to the chassis/case and also to the house earth?