On 3/16/13 7:55 AM, Jim Brown wrote:
On 3/15/2013 8:19 PM, David Gilbert wrote:
No way that the respective wavefronts will add in all of those
directions. Geometry alone would tell you they are much more likely
to cancel in some directions.
Yes, we can drive two or three Yagis pointed in the same or different
directions, but HOW they combine to form a pattern can be as close to
the infinite number of monkeys and typewriters as it is to how the
operator thinks he is aiming them.
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when you start spacing at more than a wavelength, you're really building
an interferometer, so you'll get some interesting combination of grating
lobes and the element patterns.
There are some generally useful cases, though.. two fairly high gain
antennas pointed in fairly different directions. There's not much
interferometer/grating lobe effect because in the main lobe direction of
antenna A, the fields from antenna B are small. So want you get is
something with two big lobes and a lot of little stuff.
Maybe you're in a geographic location and you want to set up HF links to
two other places and don't want to have to worry about switching
antennas, etc.
A similar use case at VHF is where you have two big cities that are
90-100 degrees apart from your distant location, and you want OTA TV.
Two fixed LPDAs one pointed at each city and a power combiner and you're
good to go. Say, if you were in Orange County and wanted to receive
the Los Angles and San Diego stations. (in this case, the "LA" antenna
could probably be a piece of wire or the back lobes of your SD facing
LPDA, though)
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