L8Nighter

Thorn thanks for the image in the FAQ to visualize how you increase db the pattern gets flatter.

Now my question: Say one has a 5db mag mount on the roof of their car. In the manual of most cards they will get Up to 1500 ft outside. How high in elevation does that go, im confused on height vs distance. Don't worry about stuff getting in the way I just want to get the basic logic down. Does that mean 1500 ft distance and 1500 ft up into the air? And if you put a 10db antenna on your car that looks like the elevation of the signal would go down but your distance you cover would go up?

Is there any formula to calcuate height vs distance?

So with your 5db antenna driving around in the city, are you going to be able to reach access points that are 50 some stories in the air or would you have to tilt your antenna a certain way.

Thanks if you can clear any of that up for me.


A:Thanks, I'm glad the diagrams helped. I can't take too much credit for them though; they're hardly unique. Most books on rudimentary RF stuff have better ones.

Under perfect (theoretical) conditions, the signal would originate from a single point in space, and radiate outward in all directions, giving a sphere of signal. So -in theory- the 1500' would reach in all directions (BTW, take that 1500' with a grain of salt. Hell, with most manufacturers, take a whole salt shaker...)

In reality, the signal orginates along the length of the antenna, hence the doughnut shape, and that's for an omni. Cards with a built in antenna, would probably have a pattern closer to a wedge.

As to formulas to calculate height vs. distance, I'm sure are some, someplace, but I don't have them. They would most likely be highly theoretical. Check the ARRL antenna book. In reality, the problem with what you are asking is it is highly dependant on the strength of the transmitted signal, which is an unknown. And remember, we are talking 2 two-way radios here: NS transmits a beacon request, which the AP must receive. The AP transmits the response, which your stumbling rig must receive. Two transmitters, two receivers. In addition to the signals, noise levels between them will also affect the whole equation.

Since dB is logarithmic, replacing a 5dB antenna with a 10dB antenna should give a distance almost 4 times the diameter of the 5dB unit, at a corresponding decrease in the height. But it is still dependant on the signal transmitted to you.

In actual wardrivng, an AP 50 stories up is probably effectively shielded by the building. Using a directional, you should be able to scan a building window by window that high up, but just driving by would not pick it up in all likelihood.