I am setting up a small wireless network that will have to cover a bit of distance. I have a 40 foot tall tower and clear shots to the buildings I have to get to.
I am mounting two antennas on top of the tower, one horizontal and one vertical polarized antennas.
My question, is there a standard as far as antenna placement using a configuration like this? Distance apart? Should one be higher than the other? I will be using a Proxim AP-4000 to run the show and have been told that a 1 watt amp on each antenna would be a good thing. Still wondering about that.
Anyway, I am new to this and would like to get this straight.
Excellent questions. Typically you would have antennas separated by "X" number of feet (with your accent maybe I should say meters), dependent on the frequency being used. But since you are using two different polarizations you will have isolation between them already. For antenna placement of the same polarity, you can get away with less vertical separation between antennas as you would with Horizontal separation (for most directional antenna). Here is a link that will calculate the isolation you will get for the separation between the antennas.
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For horizontal separation you can use
22+20log(d/ f) - (G1+G2) - (S1+S2) d = distance f = freq g1 and g2 are the main lobe gains of the antenna s1 and s2 are the side lobe levels of the antenna
For vertical separation you can use
28+40log(d/f) d = distance f = freq
From experience and at the freq you will be using 3 feet vertical and 4 feet horizontal separation should be good to go (1 meter vertical, and say 1.5 meter horizontal)
So long as you do not violate any emission standard from your FCC type people should not be a problem. This depends also on the gain of your antenna, and how far you are trying to go.
I am in the San Fransisco Bay Area, I will check to see what the FCC has to say about the power output before I put any amps to the system.
The Horizontal antenna is 12 dB and the Vertical is 15.4 dB. I am looking to cover about 3/4 of a mile with this setup. The AP-4000 output is about 18-20dB if I read the spec sheet correctly.
No doubt, the salesman that's selling overpriced 1 watt amplifiers toly you it would be a good thing. The AP-4000 already belches +20dBm (100mw). The AP-4000 is also a dual band access point, therefore requiring an amplfier for each band. The AP-4000 also support PoE meaning you install it in a waterproof box on the tower near the antenna.
The problem with amplifiers is that excessive power on one end of a wireless link creates an "alligator". That's an animal with a big (transmit) mouth, and small (receive) ears. Your AP-4000 will be heard over a much larger areas than it can hear. If your clients are using +17dBm (35mw) MiniPCI cards inside laptops, your AP-4000 isn't going to hear them at extreme range. I consider such systems not much better than a jammer. Try to keep the EIRP fairly symmetrical, so you don't create this problem.
The best way to extend coverage is with antenna gain. The range improvement is symmetrical, unlike the aformentioned alligator.
Dana explained the antenna positioning calculations clearly. I've had isolation problems that could only be solved by installing some wire screening between the antennas. Much of the leakage came from bounces off of the tower, coax cables, back of other WISP's dishes, etc.
Hi, glad to be of help. Even though the equipment you are using is supposed to be unlicensed, the FCC still limits how much power can be radiated. And this limit is usually specified after the gain of the antenna, basically what is called the EIRP, which is the output of your transmitter, minus the feedline loss, plus the gain of your antenna. Jeff also gave a more detailed explanation of the power concern, so I will not repeat that here.
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