Toni wrote:
"---if I parallel a 300 ohm resistor to the antenna input the received
signal drops considerably."

Toni says he is not interested in high accuracy, just an idea of what
his receiver impeance is. He is on the right track. If two identical
resistors are paralleled across a voltage source, each takes the same
current. The portable radio antenna is probably a current source due to
its high capacitive reactance in the too-short antenna. That makes the
voltage out proportional to the impedance loading the antenna.

Forget the .7 vs. .5 of the power vs. volts relations. All you want is a
rough estimate of the receiver`s input impedance. Just use common carbon
resistors to find which cuts your received level from a weak signal (not
to get AVC involved) to about half what it is without the parallel
resistor.

Best regards, Richard Harrison, KB5WZI