> Gary Myers inquired about modeling high dipoles vs vertical antennas, noting
> the dipole shows higher gain.
>
> LB where are you?? My recollection is that horizontal polarity gets all the
> benefits of ground reflection gain, 5db or so being typical of the 6db
> theoretical maximum. Vertical polarization gets none of this benefit but I'll
> be danged if I can explain why. Perhaps one of you scientists out there will
> enlighten us on this subject!
>
> 73 de Gerald Williamson, K5GW
Gerald,
I'm right here. My own reply to Gary appeared on the list a micro-second
before your note.
Horizontal antennas do indeed enjoy the benefit of downward radiation
being reflected back upward and combining with the normal upward radiation
from the antenna. Height determines where those two radiation sources
combine in phase for max gain and where they combine out of phase for a
null--when looking at the pattern as an elevation plot through the max
gain azimuth angle.
Looked at from on top, that is, as an azimuth plot, and comparing the
horizontal and vertical antennas, we can see why (for one reason) the
vertical has less gain: even if the areas of the two plots are the same,
the vertical's field is equal in all directions while the horizontal's is
focused into a couple of lobes.
If we think about self-contained verticals, such as a vertical dipole,
even with a minimum height the same as that of the horizontal dipole, we
can largely neglect ground losses under the antenna (no, not absolutely,
but close enough for the moment). Both antennas will have similar ground
reflection components adding to the far field. However, the vertical's
will be distributed, at least along the highest current portion of the
antenna, and hence, there will not be an exact comparability between the
effects of the reflected radiation. However, one useful technique for
modeling is simply to move one or the other antenna up or down until
their net angles of maximum radiation are the same. Easy to do with
models; much harder I am told with real antennas.
Even so, the vertical distributes its radiation equally in all directions,
while the horizontal has its focused lobes and also its focused nulls.
Still, this is 2-dimensional thinking, useful for an example, but in
reality, the overall field shapes are more complex.
It is the focused lobe effect that we should not overlook here in thinking
about the gain. As a simple exercise, take a VHF Yagi more than 10 wl up.
Horizontal and vertical orientations result in the same elevation angle of
max radiation. Look at the azimuth pattern at this TO angle. It is a
very nice Yagi pattern with a forward lobe having good gain and a beam
width in the 60-70 degree range (to -3 dB points). Let's give this beam a
20 dB F-B ratio. Now flip the Yagi on its side for vertical polarization.
The F-B remains about the same. However, the forward lobe shows less gain
but much greater beam width (sometimes more than 110 degrees to the -3 dB
points). Quibble with my exact numbers, since I am doing this from
memory, but the overall pattern differential still exists. The Yagi
focuses more side-to-side than up-down (when seen horizontally), and that
difference shows up when we flip it for vertically-polarized use. (It is
possible to design a vertically polarized parasitical beam with the same
characteristics of pattern as the horizontal Yagi, but it is not the usual
Yagi design. Some time in the future, Communications Quarterly will
publish some designs along these lines.)
Result: when analyzing the difference between horizontally and vertically
polarized antennas, do not forget to include azimuth lobe information
(which I have summarily called "focusing") along with ground-effect
information in determining the total outcome.
Hope this clarifies (or, as I sometimes say in my primary field, confuses
in a useful way).
-73-
LB, W4RNL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|