On May 31, 9:47*am, Jim Lux wrote:
On 5/30/2011 4:25 PM, Joel Koltner wrote:



"Frank" wrote in message
....
How many amateur radio operators use this kind of academic preening
when they are putting up a dipole.

Not many, certainly... but I would offer that those who do are able to
enjoy the hobby more thoroughly than those who don't. Whether or not
that additional enjoyment makes up for the extra time needed for
learning, well, that's up to each individuals.

I personally care about these things, and even for hams who think they
don't... most all of them have probably thoughts to themselves, at one
point or another, "Hey, what happens if you feed a dipole off-center?,"
and it's nice that someone else has already gone through the effort to
figure it out such that the answers are readily Google-able! :-)

And there's a whole raft of "off center fed" dipoles of one sort or
another out there, particularly for multiband applications, so it's nice
to understand that what's really going on is that the feedpoint
impedance you see is basically the same as feeding at the center, but
run through a "transformer".

Or another way to think about it (perhaps closer to what's going on)
is that it's like changing the position of a tap on a resonant tank
circuit. If you get into building RF filters using coupled
resonators, you'll appreciate that changing the tap position on the
input and output resonators changes the filter's operating impedance,
as seen at the input and output, though just as in the off-center fed
antenna, the current distribution in the resonator (or antenna)
changes relatively little. For the case of the antenna, of course,
you have to decouple the transmission line very carefully if you feed
it off-center -- or else just allow for the fact that there _will_ be
antenna currents on the transmission line e.g. an end-fed half-wave).

A bit of theory helps one evaluate all the "secret recipe worked 1000
countries on top band with 1 Watt and a 3 foot long antenna" stories too.

;-)

Cheers,
Tom