While driving the family mini-van up I-5 over the Fourth of July holiday, with lots of time to think, I was contemplating the idea of a phased array for mobile operation. Sure, one could use a single whip, but it has a strange pattern (typically oriented towards the direction away from the whip, across the roof of the car) and some really noticeable nulls (like straight up). I started thinking that if you were to put 4 whips, at the corners of the roof, you could phase them to synthesize some useful patterns. You could use a compass (or GPS) to automatically keep the "beam" steered in the correct direction (or, for the truly amazing, use an adaptive antenna scheme).
On VHF or UHF, this would be a very powerful technique. On HF though, you get bitten by the fact that the cross ways axis isn't all that long (about 4 feet on a Dodge/Plymouth/Chrysler minivan, about a tenth of a wavelength on 10m, and even worse as you go lower) so you're going to have some problems with mutual coupling and high element currents. It's going to be a pain to feed and steer.
Then, I started thinking about what you really need in that HF antenna. You're not going to get the searchlight beam you'd get from a 50 foot yagi, because the aperture isn't there. However, what you can get, and what I think is actually useful, is the ability to null an interfering signal on receive. For this, you could just use two antennas, one of them an omni(ish) receive only. It occurred to me that multiple transmitting antennas might be nice (so you don't wind up transmitting into the null of your bumper or roof mounted element's pattern), but two antennas which you switch between might be sufficient.
This has been anticipated by a variety of folks who are researching the problem of getting NVIS propagation from military vehicles. Their solution is to actually drive the body of the vehicle at several points (like the windshield sides (the A pillars). The body would be RF hot, but it is anyway, when you have a whip antenna (unless you are trying to beat the CC&R's by mounting a 40 foot crank up tower in the bed of a pickup truck).
radio/hfmobile.htm - 11 July 2000 - Jim Lux
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