[Antennas] Re: "J" poles

2002年7月12日 21:22:01 -0700

At 12:13 AM 7/13/2002 -0400, you wrote:
> If it is 1/2 wl long and it is end feed we call it a dipole, but that is=
=20
> just a classification, that antenna IS a: (it depends
>on the feed arrangement!) it could be a zepp, or a "J" pole, or with
>a cavity we call it a 1/2 wave cavity matched antenna, or something
>like that.

I had said I was going to QRT but will make one more response.
It depends on how it is fed! That's what I have been trying to say.. A=20
j-pole and the Zepp (really one in the same) are monopoles because way they=
=20
are fed. There is only at ONE path for the RF energy to flow at the feed=20
point...A dipole has two.
Your theoretical free space dipole or the example of a parasitic element of=
=20
a Yagi are dipoles and they are not end fed. There are no transmission=20
lines or matching networks providing power at the end of the elements.
Additionally, a dipole can be a dipole regardless of its wavelength. Be it=
=20
fraction of a wavelength or many wavelengths it is still a dipole.
Even an off center fed dipole ODFD - though lop-sided - is still a dipole.
As a dipole, a monopole can be any length. Just because it may happen to be=
=20
a =BD-wavelength it doesn't go through some strange metamorphous and become=
 a=20
dipole only to change back again when it isn't =BD-wavelength. The theory of=
=20
operation remains the same for monopoles regardless of their length.=20
Self-resonance, or the phase relationships that may or may not exist on the=
=20
antenna's element makes no difference - the theory of operation still=20
remains the same.
It has been confirmed many times by actual field strength measurements the=
=20
performance of ANY (this includes =BD-wavelength) monopole is improved by=20
having it installed over a good counterpoise/radial system than one=20
without.
If you install a =BD-wavelength monopole without a good counterpoise/radial=
=20
system it will work (so will any wavelength monopole), but not as well as=20
one which does. The loop return current will find a path back to the=20
generator. This means the antenna will be working against anything that=20
will provide a current return path back to the generator. The problem is=20
that this random path (earth, masts, transmission line etc..) can have much=
=20
higher losses than a properly installed system resulting in poor efficiency=
=20
and causing distortion of the radiation pattern.
That's it, I don't know how to make it any clearer.
73
Danny
SK