711,
A couple of things. None of them are all that important, but might give you a clearer 'picture' of what's happening.
Instead of a figure '8' type pattern, think of it as a sort of elongated football, sort of. The pattern really doesn't 'pinch' down in the center like with an '8', where the antennas are. How much 'pinch' depends a lot on the distance between the two antennas. That distance is measured in 'wave lengths', not in feet/inches, and the further the two are apart, the more 'pinch' there is. It never gets to the point of there being no signal to the sides of the 'pinch'.
One antenna being shorter than the other will make a difference in the radiation pattern. That also depends on if the antennas are the same, or if one of them is 'naturally' shorter than the other if they aren't the same. If both antennas are resonant at the same frequency (close), then it's almost as if they were the same type of antenna. Otherwise, the shorter antenna is the 'director', the longer one the 'reflector'. The radiation pattern typically goes toward the shorter of the two. If you mix a little 'phasing' into that you get some kind of random pattern that you really don't have any idea of where it's going (without a LOT of testing).
Phasing isn't exactly something you want to do with a directional antenna like a beam. You can, but you won't have any idea of how it turns out. Phasing is normally done with antennas of the same 'size', usually two or more of the same antenna. With there being no difference in the radiation pattern of each antenna, phasing them means the result is predictable. The more 'difference' between the two (or more) antennas used, the less predictable the result.
Oh and now the fun part! The whole mess is dependent on the "ground" of the phased antennas. That does NOT just mean the size of the vehicle, since the vehicle isn't the only 'ground' used by a mobile. The other part of the 'ground' used is the capacitive ground of the 'dirt' under that vehicle, which changes ALL the time. That means that the resulting radiation pattern is always changing (unless the vehicle is sitting still). The direction of the pattern isn't changing so much, but it's overall size/shape is. This is at HF. VHF and UHF aren't affected so much because the typical vehicle is almost always more than just a single wavelength in size. But they are affected to some extent, just not enough to worry about, sort of. [About ready to give the whole thing up as a bad idea? Don't, it's not really as bad as it sounds, sort of. But you should be aware of it.]
There are two ways to get a directional antenna, beams and phasing. They are not the same even if you end up with the same results. Don't mix the two unless you want a ~lot~ of aggravation.
And while we're at it, you can also 'steer' a phased antenna system, vary it's phasing to 'turn' it in the directions you want. One way is to use switchable phasing lines, or 'lump' the circuitry into one box and twist dials. If that's interesting to you I'd suggest getting a copy of the amateur 'Handbook', it has phasing in it (or the 'Antenna' book(s)). It really isn't all that simple, but it also really isn't all that difficult!
- 'Doc
(Wish I got paid by the word. ... hell, just wish I got paid!)
(No I don't. Higher tax bracket, ain't going there.)
A couple of things. None of them are all that important, but might give you a clearer 'picture' of what's happening.
Instead of a figure '8' type pattern, think of it as a sort of elongated football, sort of. The pattern really doesn't 'pinch' down in the center like with an '8', where the antennas are. How much 'pinch' depends a lot on the distance between the two antennas. That distance is measured in 'wave lengths', not in feet/inches, and the further the two are apart, the more 'pinch' there is. It never gets to the point of there being no signal to the sides of the 'pinch'.
One antenna being shorter than the other will make a difference in the radiation pattern. That also depends on if the antennas are the same, or if one of them is 'naturally' shorter than the other if they aren't the same. If both antennas are resonant at the same frequency (close), then it's almost as if they were the same type of antenna. Otherwise, the shorter antenna is the 'director', the longer one the 'reflector'. The radiation pattern typically goes toward the shorter of the two. If you mix a little 'phasing' into that you get some kind of random pattern that you really don't have any idea of where it's going (without a LOT of testing).
Phasing isn't exactly something you want to do with a directional antenna like a beam. You can, but you won't have any idea of how it turns out. Phasing is normally done with antennas of the same 'size', usually two or more of the same antenna. With there being no difference in the radiation pattern of each antenna, phasing them means the result is predictable. The more 'difference' between the two (or more) antennas used, the less predictable the result.
Oh and now the fun part! The whole mess is dependent on the "ground" of the phased antennas. That does NOT just mean the size of the vehicle, since the vehicle isn't the only 'ground' used by a mobile. The other part of the 'ground' used is the capacitive ground of the 'dirt' under that vehicle, which changes ALL the time. That means that the resulting radiation pattern is always changing (unless the vehicle is sitting still). The direction of the pattern isn't changing so much, but it's overall size/shape is. This is at HF. VHF and UHF aren't affected so much because the typical vehicle is almost always more than just a single wavelength in size. But they are affected to some extent, just not enough to worry about, sort of. [About ready to give the whole thing up as a bad idea? Don't, it's not really as bad as it sounds, sort of. But you should be aware of it.]
There are two ways to get a directional antenna, beams and phasing. They are not the same even if you end up with the same results. Don't mix the two unless you want a ~lot~ of aggravation.
And while we're at it, you can also 'steer' a phased antenna system, vary it's phasing to 'turn' it in the directions you want. One way is to use switchable phasing lines, or 'lump' the circuitry into one box and twist dials. If that's interesting to you I'd suggest getting a copy of the amateur 'Handbook', it has phasing in it (or the 'Antenna' book(s)). It really isn't all that simple, but it also really isn't all that difficult!
- 'Doc
(Wish I got paid by the word. ... hell, just wish I got paid!)
(No I don't. Higher tax bracket, ain't going there.)
