Originally Posted by
Graybeard
Here's a curve that maybe wasn't considered before:
The HF antenna and antenna couplers are in the leading edge of the vertical fin in most modern transport aircraft. It is a shunt antenna which is fed by the coupler that converts transmit power up to 400 watts at 50 ohms impedance to very low impedance - on the order of ten milli-ohms. The current, therefore, is very high, and it is carried a short distance from the coupler to the vertical fin on a heavy copper cable. The coupler excites the fin and makes the whole airplane into the antenna, meaning that very high currents are flowing in the vertical fin and its junction with the fuselage.......
It's 0 ohms near 0 Hz (DC) but once you get to a few hundred KHz the impedance is much higher. Don't forget that you have real and inductive impedance which 'add' up. 400W into 50 ohms is only 2.8A rms.