[TowerTalk] Re: [DX] [Fwd: Low yagi question]

force12@interserv.com force12@interserv.com
Fri, 4 Jul 1997 09:56:03 -0700


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 )





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