Good morning.
Vertical lobe compression for a (fairly low) horizontal antenna is a real
occurance.
This is an interesting subject, to which I have only seen one printed
article. I have my own vertical track range results that are very clear. The
antenna modelling software will also give a result that will correlate
fairly well to the range data.
For example, a 20 mtr dipole at a height of 35' (half wavelength), will
compute to an energy (I try to avoid using the word "radiation" - a
carryover from convention talks!) peak at 28 degrees. A simple 3 element 20
mtr Yagi at the same height will have a peak at 26 degrees. While the
apparent difference in energy at the peaks is about 5.5dB, at the lower
angles, there is more energy in proportion to that of the dipole.
A bigger 6 element Yagi, will have its (computed) nose at 22 degrees, still
at the same 35' height. Again, there is more low angle energy.
The range tests I conducted were done in about 1986 using accurate (and
recently calibrated) equipment, fine copper mesh for the ground area and a
vertical track to measure from 5 degrees up to almost 45 degrees. The
measured difference at 5 degrees between a dipole at a half wavelength and a
5.5dBd Yagi was between 9-9.5dB. At the actual nose of the dipole and Yagi
(which were not at the same elevation), the difference measured the expected
5.5dB. The range data shows more lobe compression and resultant energy at
the lower angles than the existing software. At least our friendly software
gives us a good indication.
This effect diminishes as the antenna is raised and at about a wavelength is
not very evident.
The original reason for conducting the tests was to validate the angle(s)
from various stacked antennas. Having had many stacks, I did not think the
current books were accurate. For example, they would say that the resulting
main lobe from two Yagis at 1 and 1/2 wavelength would be 3/4 wavelength. My
experience over an extended period of time indicated that the lobe was
lower, to about 90% or so, of the upper antenna. The tests showed that the
observed was correct. If the current software had been around, I could have
saved a lot of time and expense! It shows essentially the same as the
vertical track range.
The vertical lobe compression "discovery" was accidental. The sense antenna
was sitting at the bottom of the track when the antenna under test was
switched from a dipole to a 3 element to get readings at a low height (1/2
wavelength). Just happened to notice the spectrum analyzer reading at a
delta of >9dB and thought we had lost calibration. It was set aside until
the stack data was finished and then some more tests were run.
The one article I recall was a very small one, maybe only a half page. Don't
know what publication.
My "old saying" is, "If you can't get the antenna high, then make it with as
much gain as possible. The results are not necessarily linear."
Have a good day and 73,
Tom, N6BT
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
--
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.htm
|