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

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

In article
,
K7ITM wrote:

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

thanks very much surprized to hear ya say it might have little
effect, my gut told me as the feed wire got anywhere near one of the
legs something would go really wrong either pattern , or swr or
etc etc

i guess the feedline wouldn't go to ground per say at least not
physically thou i guess the chassies of the rigs are technically
grounded

I wasn't worried about (thou it's important) my 'antenna'
disturbing the feedline, more the other way aroound due to proxmity,
so i wonder if your comments regarding chokes were with respect to
that meaning i wouldn't think chokes would make the feedline
less 'visable' to the dipole leg then again maybe i am wrong
on that


really appreciate your taking the time to post thanks much and
happy holidays

ML,
Any/every thing around an antenna is going to affect it's radiation
pattern to some extent, including feed lines. How much affect those
things have is the practical aspect of that. The 'classical'
radiation patterns, typically shown, are just that 'classical', best
case possibilities. The more 'mundane'(?) patterns are usually never
going to be the same, and in most cases, not really all that
important, sort of. It would be nice to be able to predict things by
using the 'classical' patterns, but just not a very practical thing to
count on. Depends a lot on which bands you're talking about too.
'Directional' is typically more important for the higher bands than
the lower ones, propagation and all that 'stuff'. Which isn't to say
that you shouldn't worry about it, just only to a 'practical' extent.
The easiest/hardest solution is to just move the thing till it's
'right' for you.
Yeah, I know, sort of a totally useless post, right? Oh well...
- 'Doc

In article
,
wrote:

ML,
Any/every thing around an antenna is going to affect it's radiation
pattern to some extent, including feed lines. How much affect those
things have is the practical aspect of that. The 'classical'
radiation patterns, typically shown, are just that 'classical', best
case possibilities. The more 'mundane'(?) patterns are usually never
going to be the same, and in most cases, not really all that
important, sort of. It would be nice to be able to predict things by
using the 'classical' patterns, but just not a very practical thing to
count on. Depends a lot on which bands you're talking about too.
'Directional' is typically more important for the higher bands than
the lower ones, propagation and all that 'stuff'. Which isn't to say
that you shouldn't worry about it, just only to a 'practical' extent.
The easiest/hardest solution is to just move the thing till it's
'right' for you.
Yeah, I know, sort of a totally useless post, right? Oh well...
- 'Doc

nah i get the point, that was really what i was wondering, in
a way what the practically of it all is, meaning if i have to for
certain reason hve the feed wire very close and sorta parrallel to a
leg how much 'worse' the dipole overall will be

vs say does it pay to switch to another type of design say ocf
and or even end feed it would be a 'easy' question to answer if i
really knew the effeciencies of each compared, in my particular
situation

the freq i typically use are 160-10 and sometimes 6m

do appreciate the reply