Steve, K7LXX, wrote:
> Also, my recommendation is to put the 40M antenna at the bottom of the stack;
> an additional 10 feet up the mast is only 7% of a wavelength - not enough to
> make any significant angle improvements.
^^^^^^^^^^^ ^^^^^ ^^^^^^^^^^^^
True, if you are talking only about the take-off angle of the main forward lobe.
Not true, if you consider the rest of the antenna pattern, primarily
the high angle part of the pattern. For example: a 3 element 40 meter yagi
at a height of 1/2 wavelength over perfect ground has a perfect null (>40dB)
at an angle of 90 degrees from horizontal. This changes significantly with
small changes in antenna height (see table below).
height approx. strength of pattern
in wavelengths relative to main lobe
at an angle of 90 degrees
--------------- -------------------------
0.600 -15dB with secondary lobe @80deg -12dB
0.575 -18dB with secondary lobe @80deg -15dB
0.550 -20dB with secondary lobe @80deg -18dB
0.525 -25dB with secondary lobe @85deg -25dB
0.500 >-40dB
0.475 -25dB
0.450 -20dB
0.425 -15dB
0.400 -12dB
In this exercise the take-off angle of the main lobe changes from roughly 20
degrees at a height of 0.6 wavelengths to 35 degrees at 0.4 wavelengths.
This, in turn, would change how well you reject QRM from nearby stations - 20dB
change in high angle rejection for a 10% change in height from 0.5 wavelengths
(about 70 feet) is worth thinking about. However, this only one part of a
complex equation one needs to ballance when deciding what is best for your QTH.
For what it's worth...
Cheers - Chuck Claver
de NJ6D
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