Owen Duffy wrote in
:

....

If I take a half wave folded dipole immersed in some environment where
the ambient noise temperature is T, and attach a load directly to the
feedpoint, the load power will be maximum when it is about 300 ohms
rather than about 75 ohms, and the noise power density due to ambient
noise would be K*T*300 W/Hz rather than K*T*75 W/Hz. If Rr is the
(virtual) resistance due to coupling of the antenna with distant
space, then surely this example suggests that Rr is 300 rather than 75
ohms.

(If I performed the same experiment with a plain half wave dipole, the
load power will be maximum when it is about 75 ohms, and the noise
power density due to ambient noise would be K*T*75 W/Hz.)

Sorry, that is plainly wrong. Clarity struck whilst having breakfast, the
received power of a matched system should be independent of R.

Noise power density is simply K*T W/Hz. There is no R term.

The text should read...

If I take a half wave folded dipole immersed in some environment where
the ambient noise temperature is T, and attach a load directly to the
feedpoint, the load power (due to ambient noise) will be maximum when it
is about 300 ohms rather than about 75 ohms. If Rr is the (virtual)
resistance due to coupling of the antenna with distant space, then surely
this example suggests that Rr is 300 rather than 75 ohms.

My apologies.

Owen