RadioBanter - View Single Post

April 29th 06, 11:45 PM posted to rec.radio.amateur.antenna

wrote:

Verticle polarization has less power headed toward space (up for a
H-dipole) and still benefits from ground effects. The model for free
space nets the same gain as dipole. The problem with free space is
my backyard has dirt in it.

It appears you're comparing a half wave horizontal dipole with a (half
wave) J pole. Ground does have a major impact on the performance of the
antennas, but the reflection results in loss of a lot of signal power if
the wave is at a low angle and vertically polarized. Horizontally
polarized waves, in contrast, reflect with very little loss at low
angles. I haven't tried modeling it, but would guess that in a typical
line-of-sight path where there can be one ground reflection in addition
to the direct path, the signal would go through fades and reinforcements
as you varied the antenna height. Because of the lower loss of
horizontally polarized reflections, the fades would be deeper and the
reinforcements stronger with horizontal polarization. On the average,
the horizontal would be better (that is, the average signal for all
heights would be stronger) because no significant loss is incurred in
the reflection. But getting a decent signal would be much more of a gamble.

Those looking at modeling results from EZNEC or similar programs should
realize that the far field analysis isn't a good simulation of this
situation. Probably the best way to model it would be to model two
antennas, one at each location, over ground. Put a source in one antenna
(the transmitting antenna) and a conjugately matched load at the
feedpoint of the other (the receiving antenna). Then look at the power
delivered to the load for various receiving antennas. There's a
practical limit to how far apart the antennas can be before numerical
limitations are reached, but with double precision calculations they can
be quite a distance apart.

The conventional J pole with half wavelength radiator has the same gain
as a vertical half wave dipole, again assuming you can keep the current
on the outside of the feedline to a low value for both antennas.

However for free space groundplane compared to free space verticle
halfwave there is gain. Better radiation angle as well.

A quarter wavelength vertical ground plane antenna has the same gain and
pattern as a half wavelength vertical dipole, and the same as a J pole
(if you can keep the current on the outside of the J pole feedline to a
reasonably small value). This is true whether in free space or over
ground, and it can be demonstrated by theory, measurement, and modeling.

Roy Lewallen, W7EL