Total posts : 45366
Increasing the OD of the 3-m Part 15 AM radiator reduces its reactance, and also reduces the rate at which that reactance changes when frequency changes. OD doesn’t affect radiation resistance, as that is a function of electrical length.
Here are some numbers for 1700 kHz.
1/2″ OD = 0.12 -j2500 ohms
8″ OD = 0.12 -j945 ohms
The reactance change when going from 1700 to 1710 is 0.7% for the 1/2″ radiator and 0.4% for the 8″. It would be similar when going to 1690, but I didn’t calculate it.
The lower reactance for the 8″ OD radiator means it can be resonated with less reactance in the loading coil, which will reduce I^2R losses in the coil, and improve the radiation efficiency of the antenna system somewhat.
The rate of change in the reactance of the radiator with frequency, along with the Q of the coil sets the impedance bandwidth for the antenna system. So the 8″ OD radiator can maintain a better Z match over more r-f bandwidth, and will radiate higher modulating frequencies better.
Any conducting path leading from the tx chassis to true r-f ground on/in the earth is part of the radiator, so the statements above apply equally to it, as well.