phlogiston wrote:Sorry about the links. I understand now.
phlogiston wrote:So, tell me if this makes sense:
The calculated focal point from a parabola assume that the incoming waves arrive perpendicular to opening of the dish. That's middle school math. Well, it dawned on me that this is not the case when you are gathering radiated waves from an (effectively) single point such as an antenna. With my relatively deep reflector I noticed that the signal from near APs was less then that of far ones. That had me stumped until I tried moving the receiver back and forth and found that the focal point was a little different for APs at different distances. After some practice, I was able to mark focal points for different distances and either predict, very roughly, the distance of an unknown, or set it for best reception of a known. This effect will be the same for different shaped reflectors, but seems to be more pronounced when the shape of the parabola situates the focal point even with the opening of the dish. If my math is correct, then having a very shallow antenna (like almost all of those you find on the market) would be best because the focal point wouldn't "move" as much.
Which makes me think that this is already common knowledge. Man, I hate being the last guy in the room to get something.
It makes sense, although I wouldn't say it is common knowledge within the WiFi community. It probably is common knowledge with designers of satellite communications, Point-to-Point microwave links, or similar communications gear.
As an example, look at satellite TV reception dishes: They are very shallow, and are designed to receive a signal anywhere on a given continent with no tuning, except for alignement. "More forgiving," as Scruge put it.
The opposite would be PtP microwave links on towers: Deep dishes, with a very tight beam. Some even have a short waveguide on the edge of the dish, as mentioned by Airstreamer. They tend to look like a hemisphere attached to a pipe of the same diameter, with the length of the pipe being about the depth of the hemisphere.
phlogiston wrote:I wonder if there is any use to being able to measure the distance of an unknown AP without GPS.
Distance measuring (with or without GPS) without triangulating is something that gets discussed ever couple of weeks.
You can't do it for two reasons: You don't know the power being transmitted, and you don't know what materials and objects are between you and the AP that are attenuting the signal.
In a purely theoretical situation, where you know the power level of the transmitter and where the signal is only lost in free space, then yes, you could measure the distance. It's actually pretty trivial, as it's the same formula used to compute the free space signal loss.