It's a combination of a few factors.

I found that using two or more loop counterpoises isolated from the ground greatly reduced capacitive coupling to nearby electrical systems compared to radials and straight counterpoises.

The isolation transformer is used to isolate the antenna and loop counterpoises from the incoming co-ax feed that will be connected to a RF noisy electrical ground at the transceiver.

Using both together reduces the noise floor dramatically as I show in the table in the article showing the measured noise levels across the HF bands.

What I couldn't add to the article is that two weeks ago we had a power cut where a digger cut the supply cable to the local substation. All power in the surrounding area had gone so I used the opportunity to use a 12 V battery to measure the HF noise levels and compare it again when the power came back on. There was only 1 or 2 dB difference in noise level between everything in the local area being off or on so it is really effective at reducing local RF noise.

A loop counterpoise can also be used to improve the performance of a static mobile installation as it increases the capacitance to ground over the vehicle body to ground capacitance. In that situation it doesn't need the isolation transformer as the vehicle is isolated from ground by the tyres.

Regards,

Chris