The PA0FRI version that I built did not include the HPF. Maybe it should have and then the IMD would have been much better. But since I am interested in everything from 20 kHz to 30 MHz any HPF is a non starter for me. The LZ1AQ circuit variant I use is from Everett Sharp N4CY, which uses much faster transistors than the 2N2222. Even though it uses UHF transistors, it is stable, well behaved, and works well even down to 20 kHz. Highly recommended.
A link to the late PA0FRI's work is here:
https://www.pa0fri.com/Ant/Actieve%20ontvangst%20antenne/Actieve%20loop%20ontvanger%20antenne.htm
