Re: NEC-LIST: Dipoles above Ground

> Jack:
>
> So I must disagree that the ends up is better than the center up for
> a half-wave horizontal dipole over earth. From physics we both know
> that it is electron acceleration that does the radiating, but with
> NEC it is current moments and their related magnetic vector
> potential that we use in modeling the radiation gain and pattern.

George,

NEC tells me otherwise.

The example below is for an 80M dipole, leg length 19m.

Center height 15 m, end heights 1 m. The gain of this antenna over
average ground is 2.55 dBi. Impedance 47 ohms. Now let us turn the
antenna up-side down, center height 1 m, end heights 15 m. We have to
lengthen the antenna a bit (resonant frequency 3.94 MHz compared with
3.8 MHz) because the tuning changes, but lo-and-behold more gain, 3.38
dBi. Impedance 24.5 ohms. The antenna has more directivity (beam
width 102.6 degrees compared with 107 degrees) F/S is increased, hence
a bit more gain.

The increase in directivity is because V-shaped dipoles exhibit more
gain in the direction which is the bisector of the angle between the
arms of the dipole, in this case straight up.

I interpret this result as evidence that current and voltage along the
whole of the antenna contribute to radiation --- I did not mean to
imply that all the radiation comes from the ends of the dipole ---
only to point out that radiation from a part of the dipole where
current is small is important.

Do not droop multiband antennas, e.g. the G5RV type, they work better
horizontal.

I certainly recognize that describing how an antenna radiates from a
consideration of acceleration of charges is a subject that is beyond
my ken, and a topic that is (perhaps) outside the present discussion.

The point I wanted to make is that one must not loose sight of the
fact that the ends of the antenna are an important contributor to
radiation. A statement that has appeared in ARRL Handbooks for years
follows: "-----Remember that current produces the radiated signal, and
current is a maximum at the dipole center. Therefore performance is
best when the central part of the antenna is straight, high and clear
of nearby objects.---"

But let me return to the dipole on the idealized hill, if you have the
interest model it. As Dana Hoggatt has noted the elevation and
azimuthal pattern for this antenna exhibit very abrupt changes (at
about 19 degrees in the elevation plane; and ± 7 degrees in the
azimuthal plane --- a change so abrupt that it almost looks as if NEC
had gone bonkers. Continuing, it is the environment that is doing
this. The pattern is insensitive to frequency. I can change the
frequency by megahertz and see the same effect.

So we must conclude that this hill model is unrealistic, but it is
interesting that one sees an apparent interaction in the case of a
horizontal dipole, a 20 metre length dipole above a flat topped hill
(60 metres in diameter) with vertical sides. Whereas a vertical
dipole behaves as one would expect.

Regards, Jack, VE2CV

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Thu Feb 17 2000 - 18:01:35 EST