On 1/10/02 12:25 PM, Dave at email@example.com wrote:
>My understanding is a yagi performs as good as it possibly can when it is 1
>electrical wavelength above the ground. Any additional height is redundant.
I think that over-simplifies the rule of thumb. Think of it this way --
horizontal yagi's just start to become effective at about 1/2 wavelength.
Lower heights tend to distort the pattern. Heights above about 2.5
wavelengths tend to show little change in pattern, so there's no
advantage of going higher.
That leaves a range of about .5 to 2.5 wavelengths in height to place
Antennas can sometimes be too high for conditions -- placing a null near
a choice arrival angle. That's why we usually try to place yagis in the
.75-1.5 wavelength height.
A multi-band antenna like a tribander w/40m is a hard device to place.
100 feet might be too high for some 10m conditions (3 wavelengths), but
it's pretty effective for 40m (3/4 wavelenth). Putting it lower will
introduce some compromise on 40m, but will improve 10m.
Rather than try to find the right height for all conditions, many
operators with tall towers like this place other yagis at lower heights,
effectively covering several angles (and different conditions) at once.
In general, it's easier to place an antenna lower than higher. That's
generally why higher antenna placement is considered desirable.
Bill Coleman, AA4LR, PP-ASEL Mail: firstname.lastname@example.org
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