There is a calculation for "free-space path loss" that calculates signal fade over distance :
Loss = 96.6 + 10log(d-squared) + 10log(f-squared) dB
d = distance in miles f = frequency in gigahertz
So if you know your EIRP (transmit power + antenna gain) you can estimate the signal strength at "d" distance.
This formula is incomplete because it ignores receiver sensitivity and noise figure but I suspect it is "tuned" for a particular transceiver where the receiver noise figure is known.
To make it even simpler just think about how much energy reaches the far end. Use light for your thought experiment to make it easier to understand.
At the transmitting end, if you have something like a light bulb (isotropic radiator) that sends the power equally in all directions (an antenna gain of 0dBi or 0 dB relative to an isotropic radiator) you spread the energy equally over a sphere with the transmitting antenna at the center. If you focus the energy with a reflector or lens (directional antenna), more of the "light" gets to the receiver. You can "see" the "light" from the transmitter from farther away.
The amount of energy decreases as a function of the square of the distance (think about how much of that energy falls on a section of the sphere and then make that sphere bigger) so you will need 4 times the power (6dB increase) to provide the same energy over the area of the antenna if you double the distance.
At the receiving end your antenna scoops some of the energy out of space. If you want to increase the amount of energy that reaches the receiver your antenna must capture the energy over a larger area. Dish antennas make this concept very clear. Bigger dish=more energy. If you double the linear dimensions of the dish its area quadruples. So every time you double the linear dimension of a dish, e.g. going from a 2' dish to a 4' dish, you get a 6dB increase in energy and double the distance.
Now if you look at the free-space path loss calculations you will see a 6db increase in path loss as frequency doubles. This seems strange until you think in terms of antenna area as I stated above. When you double the frequency, all the dimensions of an antenna are cut in half. It now subtends one quarter of the area it did before so it scoops only 1/4 of the energy from space that an antenna at half the frequency would.
Now comes the place where we talk about receiver sensitivity. A receiver has a point where its internal noise becomes the limiting factor. Once the signal drops below some minimum value, the receiver can't hear it anymore. If you can eke a sensitivity improvement of 6db from a receiver, you will get twice the range. (Receiver sensitivity is something that seems to be ignored in a discussion of range, probably because most people can't do anything about it.)
There is another limiting factor and that is other noise that is around. Once that noise is stronger than the desired signal, you can no longer hear the desired signal. This is natural noise, man-made noise, or interference from other signal sources. One good thing a directional antenna does for a receiver is that it cuts out noise and signals that don't come from the desired direction.
So, if you want to double your range you need to do one or more of the following:
1. increase antenna gain 6db. You can increase antenna gain 3dB at each end to achieve the same result (or 2dB + 4dB);
2. increase transmit power 6dB;
3. increase receive sensitivity by 6dB.
This is cumulative. You can increase antenna gain by 2dB, transmit power by 2dB, and increase receiver sensitivity by 2dB to achieve the same result.
Remember that coax and connectors cause a loss. 3M of LMR-195 will cause about 2dB of loss which you will have to overcome with transmit power or antenna gain. This is why you want to put your AP or other radio as near to the antenna as possible.
-- Brian Lloyd 6501 Red Hook Plaza [EMAIL PROTECTED] Suite 201 http://www.lloyd.com St. Thomas, VI 00802 +1.340.998.9447 (voice) +1.360.838.9669 (fax)
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