>
> I am looking for a UHF antenna design to null out
> transmissions roughly in an easterly direction, but I would
> like to keep the antenna as unsightly as possible since it's
> located on a building where it could be easily seen, not on a
> tower where it wouldn't matter. Any websites out there with
> ideas I could review? I looked quickly through my antenna
> book, but didn't see much. It shows a cardioid pattern, but
> not sure how to make one myself. I just don't have the funds
> available to purchase one commercially. Any ideas? Thanks...
> Larry N1MIW
A little confused on the first sentance - you want an antenna that only
covers in one direction, or only nulls in one direction? I think you're
asking for the latter, i.e. you want something with 270ish degrees of
coverage.
How much of a null do you need? 10 dB? 20 dB? More?
If I did read it right and you're looking for 270 degrees of coverage,
probably the best you will be able to do is to use an omni or quasi-omni
(like an offset dipole array) and use the mast, tower, or whatever to create
a cardiod-ish pattern to attenuate the signal to the east. But, depending
on the geometry of the structure behind the antenna, the null may be as
little as a few dB. If this is on a rooftop, an omni located off one side
of, and below the roof of, a mechanical room or chiller/cooling tower or
similar obstruction to help shield the antenna in one direction may be an
alternative.
Building a mostly-omnidirectional antenna with a single, narrow null in the
pattern 10 dB deep or more isn't very easy. With phased antennas it's
possible, but given the desire to keep it on the low-down, that's probably
not practical, especially on UHF where a small phasing or mechanical error
(such as flexing in the wind) is going to change the pattern and degrade the
theoretical null significantly when trying to use two (or more) phased
omnis.
If you're looking for something much more narrow than 270 degrees (actually
well under 180 degrees), some common UHF directional antennas include:
Yagi - good for a beamwidth of anywhere from about 120 degrees on down
depending on the number of elements. Not very clean pattern, typically lots
of sidelobes.
Log periodic - beamwidth varies with number of elements like a yagi, but
much cleaner pattern. Logs typically have less gain than a yagi of equal
boom length, but the cleaner pattern is often advantageous when you're
trying to attenuate off-axis signals.
Corner reflector, parabolic reflector, or truncated parabolic - beamwidth
usually in the range of maybe 60 degrees on down depending on the
size/geometry of the reflector. Typically a cleaner pattern than yagis. On
VHF/UHF, the driven element is often a dipole, or a dipole with a reflector
(a la a two-element yagi aimed toward the reflector).
Panels - available for a range of beamwidths, typically 160 degrees or less,
often with a good pattern. Quasi-flat design makes them less conspicuous
than yagis or logs. Often enclosed in a fiberglass, FRP, or similar radome.
Reflector can either be a plate (flat or bent), screen, or screen embedded
in an insulator (like fiberglass, similiar to a fiberglass satellite dish).
The radiator can be a dipole, log cell(s), etc. depending on the desired
pattern, beamwidth, bandwidth, etc.
If you want to experiment with one of the above, a corner reflector is
probably the easiest of all of the above to design and build, both
mechanically and electrically; you should be able to find recipies in any
half-decent antenna book.
--- Jeff WN3A
