Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of those
thumb-sized products) and an old 18-inch satellite TV dish.

Calculating dish's focus point as f = ( d^^2 ) / ( 16 * c )
f = focus point above center of dish
d = diameter of dish
c = depth of dish at center

With a circular, symmetrical parabolic dish, I presume that the focus is
directly above the center. I've located the tiny antenna within the adapter
as close to this point as i can.

( Odd thing about this dish -- this one is a DishTV brand -- is that in the
original design for satellite reception, the driven element [ the "can", the
input to the LMB ] was located low on the dish. In other words, the focus
used in this design was off-center, about 30 degrees below the center axis of
the dish. This would mean that aiming would be off-axis, too, yes? What is
the reasoning behind this design? )

Yes, these dishes are designed/intended for an off-center LNB. They
have a different radius of curvature in the horizontal and vertical
dimensions. You won't be able to place an LNB (or access-point
antenna) at an on-axis location and get a clean focus.

The reason for an off-center focus arrangement is that it keeps the
LNB or other driven element, and its support strut(s), out of the path
between the satellite and the dish. This reduces diffraction effects,
and results in a stronger signal and cleaner focus.

I find 2 things:
1. It doesn't really matter where I point the dish. The signal varies a
little bit when I approach the direction of the access point, but no "leaps
and bounds" in the signal strength between being "dead on" and way off.

2. I pick up signals that aren't even in the direction of the access point.

I'd guess that this is due to a combination of three effects:

- You aren't getting a clean focus, because you're trying an on-axis
focal position for a dish which isn't circularly symmetrical. Try
re-attaching the LNB support arm which came with the dish, and
mount the access point on this arm so that its antenna is at the
location of the original LNB.

- The dish was designed for frequencies higher than 802.11b - it's a
bit small (in terms of wavelengths) to get a really clean focus at
2.4 GHz. Some people have had better results using the larger
dishes originally intended for use with the lower-frequency
Primestar system.

- The access point's antenna is probably rather omnidirectional in at
least one dimension - you're getting significant reception "off the
back" of the antenna, via direct radiation which isn't being
reflected from the dish.

Commercial parabolic-dish antennas for 802.11b use are usually
circularly symmetrical, with an on-axis feed arrangement consisting of
a dipole or (often) a two- or three-element Yagi arrangment aimed
towards the dish.

I've gotten reasonably good results using a Dish Network dish and LNB
support arm, with the LNB replaced by a homebrew biquad antenna and
sheet-metal reflector.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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