On Dec 28, 5:18*am, ml wrote:
hi

In *the *case *of a *center *fed *dipole, *typically * you want the
feedline *lets say coax, *drooping downwards

in a case *where *you might not be able to do this and need *the feeding *
coax *to b closer *and *more parrallel *to one of the legs *......

i realize *that this will * make the dipole more inefficient *and *
distort *the *pattern...

but *how *do you even *roughly *calculate * that??

thanks

How about a simulation? NEC is happy to handle this sort of
sitution. It's the outside of the coax that you care about as a
radiator; just add a conductor to the model approximating the position
and length of the coax as best you can. If the coax goes all the way
to the ground, that probably makes the model simpler. You can model
it with and without a choke (balun) to try to isolate the coax from
the antenna feedpoint.

I think you will find that if you put a choke balun at the antenna
feedpoint and another one a quarter wavelength back down the line from
that, the feedline will have relatively little effect on the antenna.
You might model the baluns as, say, 500 ohm inductive reactances (or
other inductance, if you know what you'll actually use). In
situations where I've had trouble with a lot of RF induced on a wire
where I didn't want it, I've found resonant chokes to be very
effective: as much inductance as I could reasonably get, paralleled by
some capacitance added on purpose to make it resonant at the operating
frequency.

Cheers,
Tom