Re: Modifying old dish network dish for wifi

gregbr549 <gregbr549.2scpbd@no-mx.wirelessforums.org> hath wroth:

>I am new here. I am trying to modify an old dish network dish to get
>better wifi reception. I gutted the reciever part of the dish, hogged
>it out and made room for my USB Wireless Dongle. Will this work? Will
>it be any better than a tin can? Do I need to paint or cover the dish
>with aluminum foil? Here are some pics,
>Thanks
>Greg
>|Download: http://www.wirelessforums.org/attachment.php?attachmentid=63|

Nice idea, but I don't think it will work very well.
Where to start....

No need for aluminium foil cover. There's plenty of wire mesh under
the fiberglass. Or, if it's a metal dish, it will reflect 2.4GHz as
well as the original 12-13GHz.

The big problem is that you don't have a feed that is matched to the
dish. In transmit, the USB dongle radiates in all directions. Most
of it will get reflected to who knows where by the metal horn. My
guess is about 1/10th of the radiated RF actually hits the dish and
goes in the desired direction. In receive, it's not so bad. The USB
dongle probably gets all the RF that is reflected from the dish.
However, that means you'll have more gain in receive than in transmit.

To see how this works, scan through this article on feed design.
<http://www.w1ghz.org/antbook/contents.htm>
<http://www.w1ghz.org/antbook/conf/high_efficiency_prime_feeds.pdf>

The next problem is lack of gain. The maximum gain of a dish antenna
can be calculated. Assuming you did everything perfectly, and the
feed was properly matched to the dish f/D ratio, then:
Maximum gain for a 0.6meter diameter dish:
gain = 9.87 * Dia^2 / wavelength^2 * (feed efficiency)
gain = 9.87 * 600mm^2 / 125mm^2 * 0.4
gain = 91
dBi = 10 log(91) = 19.5dBi
The 40% efficiency (that's the 0.4) is probably optimistic.
Again, that's if everything is lossless, perfectly matched, built
correctly, and properly designed. I don't think your feed qualifies.

Let's say that the USB dongle works as I described, where only 10% of
the RF gets to the dish in transmit. 1/10th is:
10 * log(0.1) = -10dB loss
So your final gain will be about:
19.5dBi -10dB = 9.5dB gain.
That's not much more than a much smaller and easier to build biquad or
coffee can antenna. That's also why you want to spend some time
designing a proper matched feed.

I don't really agree with these, but they might be of interest.
<http://www.findmorecollectibles.com/wifi.html>
<http://www.weijand.nl/wifi/>
<http://www.qsl.net/ki7cx/wgfeed.htm>
<http://www.engadget.com/2005/11/15/how-to-build-a-wifi-biquad-dish-antenna/>
<http://www.nodomainname.co.uk/parabolic/parabolic.htm>

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