On 2/20/07, Jim Cicirello <[EMAIL PROTECTED]> wrote: > It is important to note that antenna gain is different than amplifier > gain. Antennas do not have a power source that allows the antenna to > create additional energy to boost the signal. An antenna is similar to > a reflective lens in principle - it takes the energy available from > the source and focuses it over a wider or narrower area.
Antenna gain finally "clicked" for me one day when someone used the example of one of the original non-LED small MagLite flashlights. Stand about 20 feet from a wall in a semi-dark room... Take the lens off... the MagLite looks like a little candle... light radiating in all directions. Note the amount of light on the wall at a particular spot. This is your isotropic radiator... well kinda... close enough for this demo, anyway. A "point" of light". Screw the lens back on and aim the flashlight at the wall. Turn the focus (more gain) to make the spot on the wall smaller and smaller... note how the spot gets "brighter". (Well the reflected light off the wall back to your eyes is what you're really seeing... but anyway, ignore that part...) The flashlight is "transmitting" the same amount of light throughout this entire exercise, but as you focus the lens on the spot you want the light to illuminate you send more light that direction. Same thing works the other direction (careful not to hurt eyes, here if you actually try any of this stuff!... of course). You can have someone look at our little tiny MagLite across a mile or more with a telescope -- a REALLY "high gain" light antenna (lens)... so to speak. If you can make the mental leap here and pretend these are all your favorite high and low gain antennas (for whatever band...) you start to get the idea... In your imagination, it helps show why the guys with big beams can always still hear the pip-squeaks with bad antennas but they can't hear off to the side -- 'cause they're looking with a TELESCOPE for them! -- and how the gain of a particular antenna squeezes its pattern out to the horizon (we hope)... and not the sky... Just imagine your antenna radiates light (not RF... but hey, it's still the all part of the spectrum) and where all that "light" is going to go, and what little lights you'll be able to see "out that direction" if you're using highly directional antennas... makes for a decent (but not 100% perfect) mental model... I always thought it'd be neat (if I were a good graphics design coder, which I most definitely am NOT) to build a path prediction software package that would show each transmitter as a 3-dimensional light source. 3-dimensional instead of 2-dimensional like most path prediction software is now. But talk about a nightmare to code. Sure would look nifty, though. Especially if you could simulate real system activity in a linked system and "fly around the radio system" in 3D, like Google Earth... see where all that RF is really going... add topographic data... etc etc etc. I suppose the big boys (commercial or government) who can afford to pay someone to write such code, probably have toys like this, somewhere... Nate WY0X
