On Apr 24, 9:26*pm, "Tom Donaly" wrote:
However, according to Cecil's favorite text, a
folded dipole's behavior can be analyzed by assuming that its
current "is decomposed into two distinct modes: a transmission
line mode and an antenna mode." (From Balanis, _Antenna Theory_)
You've got EZNEC; you can try this, too. It won't cost you anything,
and, if nothing else, you might get the satisfaction of telling me
how wrong I am.

Tom: I was unaware of "Cecil's favourite text" and did my own work on
the folded dipole some time ago:
http://www.karinya.net/g3txq/folded_dipole/

I too found that you can resolve the currents between the two wires
into a common-mode radiating component and a differential-mode stub
component. My particular interest was in situations where the velocity
factors of the two modes is very different - for example if you use
zip cord for the construction - and discovering where you then need to
place the stub shorting links for minimum effect on the feedpoint
impedance.

I was disappointed to see that recent editions of the ARRL Antenna
Book don't recognise this effect, but earlier editions do. I have also
found that Cebik's analysis of the topic is incomplete, as a result of
which he reaches the wrong conclusion about shorting link position. I
wrote to ARRL about it some time ago, suggesting they re-instate the
original text, but they seem disinclined to respond.

Without modelling it, my guess is that a ""dual Zo" approach will
require the shorting links to be placed much closer to the centre of
the antenna, but it will do nothing for the overall length required
for common-mode resonance. After all, what you require of the stubs is
that they be very high impedance across the feedpoint, and with "dual
Zo" that would require them to be much shorter.

Now, if you can think of some way of forming a large discontinuity in
the COMMON-MODE Zo we may be onto a "breakthrough"

Steve G3TXQ