Is that a pair of "trucker" CB antennas, mounted on the rearview mirrors of the cab? As I recall, that's pretty directional, right down the highway.
I caught and passed a guy who had the antennas properly spaced (he said) but in line, at the front and rear of the roof of his van.
He had great range to the sides of the highway, notsomuch down the road.
The proper length of the interconnect is... 3/4 wave, which is 1/2 wave of physical cable? Or did I say that backwards ;-)
Heh-heh. That's very similar but not the same. What you're thinking of are co-phased" CB antennas. There's a very good reason for having two antennas on a big semi-truck trailer or tractor. One can't mount the antenna high enough to not hit the bridges. But if the antenna is shortened (with a loading coil) and is mounted on one side, the truck blocks the signal on the other side. So, installing two antennas, one on each side with a "co-phaseing" harness was a good solution. The two antenna system works better than one antenna, but not by much.
The harness is suppose to be an odd multiple of 1/4 wavelength 75 ohm lines, which roughly form a power splitter. In theory, a station directly ahead should have the signal arrive at both antennas at the same time where the signals add together forming a slight peak. To the sides, it also works nicely because only one antenna will hear the signal and therefore there can be no peaks and nulls.
That works great for big tractors and trailers, but really falls apart on passenger cars. I've seen an amazing array of "co-phasing" harnesses all claiming to have more gain than others. Depending on the length of the cables, it is possible to put a null either dead ahead or to the sides. The right lengths are about 70" on each side of a T connector that goes to the radio. That's a bit long for most passenger car trunks, so the creativity begins.
Yuck. Actually, the spacing isn't terribly critical, but the coax cable lengths make a big difference. If he had a null in front and back, then he really goofed somewhere.
So much for CB antennas.
The scheme I use looks like this:
| | Ant #1 | | Ant #2 | | | | Antennas are spaced | | 1/4 wavelength apart | | and are identical radio =======|==========|
^ ^ ^ | | | any length | | coax here | 3/4 wave electrical | coax length T Connector
The way this works is that it creates a cardoid pattern with a big deep null toward Ant #1. A signal coming from the left appears at the T connector. The same signal arrives at Ant #2 1/4 wavelength later. This signal is delayed through a 3/4 wavelength coax cable resulting in a 1/2 wave phase shift (180 degrees). When this hits the T connector, it cancels with the original signal from Ant #1 and creates a deep null. Both antennas have to be identical for this to work. It's also a very narrow band system as any change in frequency requires a change in antenna spacing and coax cable length.
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