http://www.qsl.net/n1bwt/chap5.pdf

On Wed, 12 May 2004 16:09:13 -0700, DaveC wrote:

|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? )
|
|Setting up on the roof, finding the wireless access point's signal, I moved
|the wireless adapter around the focus point a little to maximize signal
|strength (I use the word "maximize" loosely...). I then aimed the dish around
|in the general direction of the access point, looking for a leap in signal
|strength (using a signal strength utility program to verify my aim ).
|
|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 tried rotating the dish on its axis to account for polarity mismatch (
|would this really be an issue? ). No joy.
|
|The wireless adapter can receive signals well, generally, w/o a dish, but
|because of my location, reception of the desired signal is not great.
|
|I'm aware that reflections can be strong from nearby objects, so that could
|account for some pickup of signals in directions other than from their
|origin. Also, I suspect that the shallowness of the dish is a contributor.
|But the lack of directionality of the dish, in general, has me stumped. Have
|I miscalculated the focus? I understand that the ratio of focus-to-diameter
|of the dish is important; for 2.4 GHz, is best in the 0.25-0.55 range (this
|one is 0.69). Is this contributing to my problem?
|
|I used this web page as a reference:
|
|http://www.usbwifi.orcon.net.nz
|
|Any help would be greatly appreciated.
|
|If this is off-topic for any NG here, please let me know what is a more
|appropriate forum. Are there mailing lists for such topics?
|
|Thanks,