Amos Keag wrote:
Agree! But, the radiation resistance ... ?


The topic of radiation resistance has been pretty much beaten to death
on this newsgroup. A groups.google.com search should provide a good
evening's reading and entertainment.

In brief, "radiation resistance" is the part of an antenna's impedance
which "absorbs" the power actually being radiated. It can be referred to
any point on an antenna. At that point, wherever it is, the power
radiated is I^2 * Rrad, where I is the current at that point. Kraus
generally refers the radiation resistance to the feedpoint; that is, he
calls the resistive part of the feedpoint impedance the radiation
resistance. In AM broadcasting, where monopoles are often higher than a
quarter wavelength, it's common to refer the radiation resistance to a
point of maximum current. Either approach is perfectly valid. But it
means that you have to be careful to say what you mean when you use the
term, particularly if the antenna is longer than a quarter wavelength
(monopole) or half wavelength (dipole).

There's a common equation for efficiency, Eff = Rrad / (Rrad + Rloss).
When using it, you have to make sure you're referring Rrad and Rloss to
the same point. A common error when dealing with folded monopoles is to
refer the radiation resistance to the high-impedance, transformed
feedpoint, while neglecting to refer the loss resistance to the same
point by transforming it. This leads to the erroneous conclusion that
folding the element improves efficiency. It doesn't, except that the
additional wire surface area reduces the wire loss -- but this isn't
usually a significant contribution to the total loss.

So to address the comment --

Changing the feedpoint location can change the current distribution on
the antenna. This in turn will change the radiation resistance referred
to some fixed point along the antenna, although in most common cases I
can think of, it won't be a large change. You could contrive some cases
where it would. If you refer the radiation resistance to the feedpoint,
that is, define it as being the resistive part of the feedpoint
impedance, then changing the feedpoint location can have a major impact
on the radiation resistance.

Again, if you want more about the topic, read some of the threads where
it's been discussed in more detail.

Roy Lewallen, W7EL


Cecil Moore wrote:

Roy Lewallen wrote:

Of course, the feedpoint impedance will be different if you choose
another feedpoint.


Here is the confusing quotation:

The impedance of the antenna doesn't change with the feed method

(assuming of course that it has a single feed point)

A center-fed dipole has a "single feed point". An off-center-fed
dipole has a "single feed point". The feedpoint impedance of the
antenna does indeed change depending upon where that single
feed point is located.