Roy Lewallen wrote:
Alan Peake wrote:

Roy, are you talking about high gain yagis? I just looked at the
beamwidths of an 11 element DL6WU Yagi on 2m and got 35 degrees
horizontal and 39 degrees vertical in EZNEC. Not much difference there.
Alan


No, I'm not. The OP specified HF, so I'm talking about Yagis of up to 5
or 6 elements or so. Ones with a larger number of elements would
significantly narrow the pattern in both planes. I'm surprised that the
beamwidths approach about the same value, but I haven't done much
modeling or any measurement of Yagis with a large number of elements, so
I'll believe it.


Here's a sort of intuitive approach to the conceptual analysis...


Consider the overall pattern to be like that of a phased array where you
multiply the pattern of an individual element by the pattern of the
array assuming that the elements were isotropic. Indeed, in a Yagi, not
all the elements are excited equally, but bear with me..

So, in a 3 element antenna with a gain of, say, 5dBi, you're getting
about 2 dBi of gain from just the dipoles, and 3dBi of gain from the
fact that you've got 3 elements in an array. The beamwidth is going to
be a combination of the array pattern (which is the same in vertical and
horizontal planes) and the element pattern (which is not)..


In a higher gain antenna (say 12dbi), you've got the same element gain
2dBi) but the array pattern gain is now 10dBi, so the shape of the
latter will dominate.