Homer J wrote:
You can match any thing to a transmitter with a good antenna tuner, the
question is the radiation resistance low enough to efficently transfer
energy to free space?

The radiation resistance is simply an imaginary circuit element whose
dissipation represents the energy transferred to free space. So 100% of
the energy supplied to the radiation resistance, whatever its value, is
transferred to free space -- by definition.

That said, the radiation resistance does play a role in determining what
percentage of the power supplied to the antenna is radiated and what
percentage is dissipated as heat. The formula is:

Efficiency = Rrad / (Rloss + Rrad)

where Rrad is the radiation resistance and Rloss the loss resistance.
It's essential that both values be referred to the same point. So you
can see from this simple formula that for a fixed amount of loss, the
higher the radiation resistance, the higher the efficiency. But also
keep in mind that the signal from a 79% efficient antenna is only 1 dB
lower than from a 100% efficient antenna, so there's not much point in
worrying about the last few (or, often, many) percent.

The feedpoint impedance, part of which is the radiation resistance, has
to be matched in order to get power to it in the first place. And most
tuners are efficient only over a limited range of impedances. So very
high or low radiation resistance can result in loss in the tuner,
reducing your overall efficiency.

When using a G5RV that was close but not touching the branch of tree I
got a flat match, could receive well but did not get out worth a damn.
Moving the antenna to an area where there were no trees and using 2 30
Rohn telescopic masts as end poles made a big difference, got a flat
match, RX and TX were great.

A good impedance match doesn't tell you anything about the antenna or
system efficiency. But on HF if you can hear signals a lot better than
they can hear you, it's a sign of poor system (antenna and tuner)
efficiency.

My 2 cents worth on the subject but I as usual someone will say they
transmit great with the branchs laying right on the wire.

The effect of branches touching the wire depends very strongly on where
along the antenna the branches touch relative to the points of highest E
field. So two people could have very different results from touching
branches. Also, you could have very different results on different bands
from the same antenna and same branches.

. . .

Roy Lewallen, W7EL