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Owen Duffy · July 15th 10, 11:10 PM posted to rec.radio.amateur.antenna

Hi Steve,

steveeh131047 wrote in news:8a4ba365-af4e-40fc-abaf-
:

Owen,

I like the IRE definition, which according to W8JI is: "The total
power radiated in all directions divided by the square of the net
current causing the radiation".

That definition draws a distinction between Rrad and the resistive
component of the feedpoint impedance; it makes the Rrad of a folded
dipole 75 ohms, not 300 ohms; and it avoids some of the errors folk
make in assuming that "folding" a vertical can reduce ground losses.

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.)

To some extent, my question comes to is Rr at a 'defined' point, and is
that point the interface between non-radiating feed line and the
radiating structure.

Yes, you could argue as Richard states that a folded dipole includes an
integral transformer, but then, the generalised outcome of that argument
is that all antennas are impedance transformers, Rr could be some agreed
fixed quantity located at a great distance in space, or even more
confusing, an non-shared arbitrary value, and that key to it all is some
new concept of the antenna impedance transformation ratio.

The notion that Rr is defined at a virtual point that is between any
deemed integral impedance transformation and the rest of the radiator,
rather than defined at the physical feedline / radiator interface doesn't
seem too sensible.

Owen

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