Re: Critical Dimensions for Parabolic

Jeff Liebermann <jeffl@xxxxxxxxxx> wrote in
news:cd2ga35svcgbg26bmrd014a26b233hq3oh@xxxxxxx:

doofus <doofus@xxxxxxxxxxxxxx> hath wroth:

I built one of these for a unstable weak connection a couple miles
away.

One of these whats did you build? Photo?

Thanks for the reply.

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:


I improved the connection, but it is still not stable; transmission
appears to be better than reception using a orinoco pcmcia card (one
of the best tested for receive sensitivity; power 200mw).
Interestingly the software reports a fair 33% - 40% signal strength
and link quality, which is as high as I got when I was alot closer
with a good stable connection, but NOW that I am much farther away the
connection is made to a web page but the data transfer stalls and I
get nothing. It is related to signal strength cuz when I am close to
some other access point, everything works fine.

33-40% is marginal for any connection. You may be trying to go too
far. Any particular distance in feet or meters? I kind prefer real
numbers to "alot closer". Anything in the way along the line of
sight? Are you very close to the ground, in which case you might not
have sufficient Fresnel Zone clearance? Any particular model Orinoco
card? How is your dish positioned between the card and the antenna?
The dish reflectors don't work very well with PCMCIA cards. Photo?

Guestimate, stable connection at 33-40% was at about 1km, now at about
4km. 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. The Senoa card, the one
most are using with the prism chipset.


Here is the template I used, but I doubled the size, EXCEPT I made the
heighth about 1/2 the width (curve of parabola remains the same) The
template from http://www.freeantennas.com/projects/template/ calls for
height and width to be the same-a square.

That kind reminds me of my high skool math problems, where the purpose
was to obscure the actual measurements. Since this isn't a quiz,
could I trouble you to supply actual measurements in feet or meters
instead of the math quiz format?

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.


Since my math only goes through Trig, how critical is the height/width
ratio and does it have to be a square to be maximally effective.

Assuming vertical polarization, the height has to be at least 1 full
wavelength for a dipole (1/2 wave) feed. That's 12.5 cm at 2.4GHz.
Larger is better, but less will be a problem. If the feed is longer
than 1/2 wavelength, the total height should be at least 1/2
wavelength longer than the feed.

Thats greek to me, haha. 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.


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 ;-). Please take a look
at the "parabolic reflector template v.3" at the url above. Double the
size of it, as I enlarged it by a factor of two. 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?


I am
just on the borderline now of getting a stable signal. What can I do
to increase the strength, particularily the receive end?

Lose the PCMCIA card unless it has an external antenna connector.
Replace with a PCMCIA card that does have an external antenna
connector. Build or buy a real parabolic dish antenna, yagi, panel,
biquad, cantenna, Franklin, AMOS, whatever antenna. Buy a pigtail to
connect between the PCMCIA card antenna connector and the dish antenna
connector. The antennas on FreeAntennas.com were designed to somewhat
improve the signal by redirecting the RF in some general direction.
The gain is not huge but certainly useful. It's also cheap and easy,
which makes it a good starting point. However, you are apparently
going for the DX record over some unspecified distance. That's going
to take a more complex system, possibly using commerical antennas.

Can someone take a
look at the template at the url above and tell me where I am going
wrong or what I can do to improve the antenna?

The template is fine. Post a photo of what you've done and some
numbers and we'll try to help.


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

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