On Sun, 12 Oct 2014 05:41:37 -0000, wrote:

Jeff Liebermann wrote:
On Sat, 11 Oct 2014 18:04:07 -0000, wrote:

Jeff Liebermann wrote:

snip

Speaking of dipole antennas, I did this study a while back:
http://802.11junk.com/jeffl/antennas/vertical-dipole/index.html
Animated version:
http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/animated-v-dipole.html
It's a 1/2 wave dipole at various heights above a real ground. Any
semblance to textbook dipole pattern is long gone.

Yep, ground has a huge effect on some types of antennas.

An instructive slide show would be the vertical pattern of a horizontal
1/2 dipole at .1, .2, ... .5 wavelengths over ground.

Another one would be a 3 element beam at those heights.

I can do both of these, but I'm busy/lazy this weekend. I also can't
find the program I used to create the annimated GIF file. Argh. It
would also be helpful if someone would specify the frequency range of
interest.

It doesn't matter if everything is done in wavelengths.

That's why I ran the numbers at the traditional 1 meter (299.8MHz)
wavelength. Everything can be easily scaled by frequency.

The basic idea was to minimize the number of potential variables and
effects. I was only interested in the effects caused by the length
and/or height of the monopole, and didn't want to get into anything
that was frequency or material dependent. I think I've successfully
demonstrated that short dipoles have almost the same gain as a proper
1/2 wave dipole, if one only looks at the antenna, and ignores
literally everything else.

I guess there are some who would want to see that a 160 meter dipole
at say .2 wavelengths high has the same pattern as a 2 meter dipole
at .2 wavelengths if for no other reason than to be assured the effects
are frequency independant.

As long as the frequency, matching, coax cables, skin depths, ground
characteristics, mounting structures, etc are the same, they'll be
identical. However, when frequency, site, and construction specifics
are included, such as the operating frequency, the height in meters
instead of wavelength, or the use junk wire are included, the model is
no longer frequency independent.

Somewhere on my computah is a series of models that I built for a
simple 20 meter dipole, that started with an idealized free space
model, and progressed towards a real installation which by coincidence
resembles my house. I threw in everything that might have an effect
on the pattern to see what might happen. I got stuck at including the
sloping hillside because NEC2 seems to only include a flat earth. I've
also done similar studies for commercial antennas mounted on very real
and quite cluttered towers. Doing these incrementally is an excellent
introduction into the difference between ideal antenna patterns, as
found in the literature, and the nightmarish reality of real antenna
installations.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558