Re: Critical Dimensions for Parabolic

doofus <doofus@xxxxxxxxxxxxxx> hath wroth:

See the url, there is a diagram and photos of the one I built. Looks
like half a tin can with rubber ducky places at focal point. More below:

The URL is for someone elses reflector. I wanna see what you've done.

Guestimate, stable connection at 33-40% was at about 1km, now at about
4km.

I think you're doing just fine if you can get a connection at that
distance, especially without line of sight. That's much too small an
antenna to do that range reliably. It's just too far. The small
dishes are good for perhaps 300-500 meters maximum. Any further
improvements will need to be in either the size of the antenna, or the
construction (i.e. feed illumination angle).

Line of sight, hahahaha, u jest. Just walked around with the laptop
to see where I could find the strongest signal. Cannot actually see the
transmitter, but the signal must be pretty strong, cuz I am marginally
connecting now, even at 4km, but it is too iffy.

Well, thats a fair test. However, you'll probably notice that the
signal goes up and down radically with traffic and position. That's
because of the Fresnel zone and reflections.
<http://www.terabeam.com/support/calculations/fresnel-zone.php>
At that range, you should have some altitude. For example, at 4km
(2.5 miles), you should have 9 meters (30ft) of clearance which means
both ends should be at least 9 meters off the ground.

The Senoa card, the one
most are using with the prism chipset.

Ok, no model numbers. Sorry, no help.

Just double the size of the template diagram at the url. Cut thin ***
metal, fold around foam supports to form the parabola in the template
diagram given. Only they are using a square piece of *** metal and
mine is twice as wide as high, roughly 12 inches by 6 inches high.
I don't have a ruler handy but the exact dimensions can be gotten off
the template diagram at the url. I am not at home, so I don't have a
ruler with me here, but the dimensions can be gotten off the template at
the url, only I doubled the size and made the height half as high as the
width.

Got it. 12 inches long and 6 inches high. That give you an aperature
size of about 6". Maximum gain for a 0.15 meter (6") parabola is
about:

Maximum gain for a 0.15meter diameter dish:
gain = 9.87 * Dia^2 / wavelength^2 * (feed efficiency)
gain = 9.87 * 150mm^2 / 125mm^2 * 0.4
gain = 5.7
dBi = 10 log(5.7) = 7.5 dBi gain
This is the best that can be done with such a small dish. The 40%
efficiency is probably optimistic. Because your feed method is very
inefficient, it will be much less than 7.5dBi.

Using a single rubber duckey 6 inch 5Dbi whip
as the focal point foam supported in a half cylinder parabola (similar
to 1/2 tin can but shaped more precisely) See approx. dimensions above.
Connector between antenna and card is about 6 inches long, so loss
minimal there.

That's fairly close to the original. The problem is that the rubber
ducky feed radiates in all directions. Only a part of that radiation
hits the reflector and goes in the desired direction.

The 6 inch 5dBi rubber ducky is another problem. The original 2dBi
version only radiates in the upper half wave (6.25cm) section, so the
vertical height of the reflect can be as short as 12.5cm. However,
the 5dBi mutation radiates over perhaps 12.5cm in length. It should
be at least 15.6cm (1/2 wave plus 1/4 wave above the feed tip) wide.
18.8cm would be better if you can extend the reflector another 1/4
wave below the ground end. Translation: Make the height somewhat
longer than the feed antenna.

The width is a function of the f/D (focal length to diameter) ratio
and the feed illumination angle. It can vary substantially. Since
neither of these are particularly easy to determine with a PCMCIA card
feed, the optimum size is currently mostly guesswork. I can go more
into detail once I determine what you've actually done so far.

You might as well be speaking Lithuanian to me ;-).

Will broken Polish, French, Hebrew, Yiddish, Russian work any better?

Please take a look
at the "parabolic reflector template v.3" at the url above.

Done.

Double the
size of it, as I enlarged it by a factor of two.

Is there some problem with supplying your dimensions? I can't tell if
you doubled the width, height, volume, depth, f/D ratio, wire
thickness or what.

Now bend your metal
over foam supports conforming to the template curve and insert a roughly
6 inch rubber ducky into the foam vertical at the focal point of the
parabola. That is what I have, EXCEPT that my metal parabola is one
half as high as it is wide, yet still covers the complete heighth of the
rubber duckey, whereas there's is a square-as long as it is wide. Does
that explain what I have done any better?

No, but I can guess that you're really asking is does the aspect ratio
have to be 1:1 or does the height have to be the same as the width?
The answer is no. The height should be 1/4 wavelenth (3.125 cm) above
the tip of the antenna, plus possibly 1/4 wavelength below the base of
the antenna to insure sufficient illumination. The width will need to
follow a parabolic curve. If you extend the width of the reflector,
just follow the general pattern for the parabola, or extend it with a
new and larger template. There is no direct relationship between the
height and the length.

Mine looks similar to this:
http://www.freeantennas.com/projects/template/gallery/Ezreflector.jpg

If you're getting connections at 4Km with that arrangement, I am
really impressed. Whatever you're doing is working just fine.

Incidentally, I once built a WX satellite parabolic reflector out of a
long piece of aluminum roofers flashing. The frequency was about
1.7GHz instead of 2.4GHz. I used a piece of 4x8ft plywood, jig sawed
into a parabolic shape. I then stapled the flashing to the parabolic
curved cutout forming a reflector that was about 12" high, but perhaps
12ft long. Initial measured gain was about 18dBi, which is far less
than the 26dBi I had expected. It worked amazingly well, but was
mechanically unstable, difficult to aim, and was far too ugly to sell
commercially. However, it might give you an idea. Sorry, no photos.

--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.