On Sun, 1 Jan 2012 11:44:45 -0800 (PST), klem kedidelhopper
wrote:
http://www.prism.gatech.edu/~wn17/
That is a very interesting idea. Thanks for the pictures.
Nice idea. Basically, it's a 600MHz waveguide, with a yagi inside.
Something like a horn antenna. It should work for frequencies about
600MHz, but won't do much for the lower UHF channels. The main thing
that it does is block the side lobes, which are where the reflections
are probably coming from.
The author thinks the multipath is coming from the buildings in the
distance. I disagree as my guess(tm) of the increased path length is
insufficient to cause cancellation (frequency selective fading). The
wire screen horn won't do much for eliminating reflections from
something that's almost straight ahead anyway.
What the wire screen horn does do is block reflections from the side
and most important, from the back. However, my guess(tm) is that the
multipath is coming from BEHIND the antenna, not in front. It's
really easy to test where it's coming from if you get near the
antenna. Take a sheet of anti-static IC foam (mostly carbon), or a
wet towel, and move it around the antenna. When the ghosts disappear,
that's the direction of the reflection. The idea behind the foam and
wet towel is that you want an absorber, rather than a reflector. That
would also apply to the horn construction. Instead of wire screen
(reflector), it really should be made of something that absorbs RF.
But I'm just
curious though, is there any reason for using the screen (except for
cost that is) as opposed to say sheet copper or even in the case of a
prototype, aluminum foil?
It will work the same with solid metal. You could use aluminum foil
on box cardboard as long as all the edges are connected. If you're
lazy, shove the antenna into a 55 gallon drum.
What about any detuning of the antenna that
might occur when placing ground in close proximity with the antenna?
Detuning will probably be severe. I can do an NEC2 model to see how
bad, but I couldn't find an antenna model close enough to clone:
http://www.hdtvprimer.com/ANTENNAS/comparing.html
I'm at home with a cold or flu. Thinking hurts. Another time.
I
had to relocate the mast on a 75 MHZ antenna some time ago and needed
to install 5 ft. of fiberglass mast so as not to detune the antenna's
driven element.
What about the feed line? If the 75MHz antenna is horizontally
polarized, and you run the coax down the boom, and then down the
fiberglass mast, you may have problems. You'll have fewer problems if
the antenna has a balun to keep the coax from radiating. The only way
you can keep the feed line out of the pattern with a yagi is to
cantilever mount the antenna by the boom behind the reflector, and run
the coax down the boom.
How did you determine that this would work? Is this
increasing the "Q" or is it just functioning as a shield? Lenny
Shield.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558