Aiming the antenna upward does work in some cases. Have had mixed success.
The best is to have the low element (i.e. reflector) several feet above the
ground. A friend of mine has a tuning method for LPDA's wherein this method
is used; however, the #1 element was specified at being 1/4 wavelength above
ground. A source element could also be placed behind the #1 element to tune
for maximum F/B on the array.
Placing a Yagi antenna a few feet above ground (or rooftop) is certainly not
a good idea, except to maybe check the coax. The coupling to mother earth is
way too much. If there are any favorable readings, they will probably be
coincidental at best. These comments are for full size elements only.
If a 20 mtr Yagi can be placed about 12', the elements can be checked and
they will most likely be close; however, the feedpoint impedance will still
change as it is raised to its final position. Most all antenna handbooks
will have a chart in the fluctuations of feedpoint impedance of a dipole as
a function of height above ground. Have had quite a lot of success in tuning
linear loaded 40 mtr elements at 15'. Linear loaded 80 mtr elements like to
be about 25'.
One way to do tuning at lower heights is to raise the low band element to
its final height and note the frequency. Lower it to a convenient working
height and note it again. Adjustments made at the lower height will track
fairly well to the final height. The one caveat is to be sure the antenna is
in exactly the same low location every time it is checked. Changes of a foot
(even a few inches in a sensitive environment) can make significant changes.
Of course, the higher the better, especially in regards to matching the
Have a good day and 73,
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
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