Toni wrote:
"This explains the "not exactly 50 ohms" thing."

The FET input resistance is high and a shunt diode probably has high
impedance at low signals.

The antenna appears as a generator at its resonant frequency with an
internal resistance equal to its radiation resistance. As frequency
lowers, so does radiation resistance. The antenna`s series capacitive
reactance rises rapidly too. Due to lower antenna loading, a high
receiver input impedance does better with a too-short antenna than does
a 50-ohm input. But high-impedance input may be more susceptible to
interference and overload.

Maximum power transfer to a receiver occurs when there is a conjugate
match between the antenna and its receiver. This may, however, not give
the best noise performance.

Best regards, Richard Harrison, KB5WZI