[Antennas] Re: "J" poles

2002年7月11日 19:38:37 -0700

At 09:15 PM 7/11/2002 -0400, DD wrote:
 >Dan, end fed 1/2 wavelength dipoles are very common. They are fed with
 >600 ohm open wire line using tuned feeders.
A =BD-wave end feed antenna is not a dipole. The one you describe is a Zepp.
Also an end feed =BD-wave antenna's radiation pattern is not exactly the=
 same=20
as a =BD-wave dipole. The point of maximum gain tilts away from the feed=20
point. This should also serve as a clue that it is not a dipole. If it was=
=20
the radiation patterns between the two antennas would be identical - which=
=20
they are not.
If you were to analyze the differential mode currents in the 600 ohm feed=20
line in your example you'll find that 1) the amplitudes and not *exactly*=20
the same and 2) that the phase differential is *exactly* 180=BA. Thus there=
=20
is *some* common mode radiation present from the transmission line.
Think about it, at the point where the transmission connects to the antenna=
=20
the current, the unconnected conductor side of the transmission line=20
(bitter end) the current is zero, however, the other conductor of the=20
transmission line which connects to the =BD-wave element the current is at=
=20
its lowest point but, is not zero. If it were zero at that point no power=20
would be transferred to the =BD-wave element. (P=3D I X E) In other words=
 they=20
(the parallel conductors of the 600_Ohm line) are not completely balanced!=
=20
Though the differences are not great they do exist and that difference is=20
reflected all along the transmission line; hence common mode radiation. How=
=20
much effect that may have on the antenna's radiation pattern would depend=20
upon on the length and routing of the transmission line.
>The end is a high impedance load. Therefore, the feed is voltage mode
>not current mode. One side of the transmission line drives the antenna
>while the second side of the line carries the complimentary wave so the
>line does not radiate. This end fed dipole works on all even harmonics.

I don't dispute that may the antenna be resonant and/or have a high feed=20
point impedance. That's not the issue here.
A simple question. Where does this complementary wave's current come=20
from? There must be a current loop. What is it route?
73
Danny