The VHD turned out to be a great antenna. The low angle radiation really makes the difference sometimes. A mixed blessing is that the radiation pattern seen from above is omnidirectional. Hmmm… In the same way as two element beams have a driven element and a reflector (or director but that is not so common) what stops us from using a vertical reflector element on the VHD?
Nothing, it turns out. A reflector for the VHD is an easy add-on. Of course the system as a whole will only have directional patterns in one direction for one frequency. But the presence of the reflector does not destroy the original capabilities of the antenna when used on other bands.
Simulations in MMANA show that the gain over “average” soil is some 3–4 dB better than a canonicalVHD, but what is perhaps more important is that the back-lobe is suppressed about 10 dB. That’s about 2 S-units and not so bad considering the simplicity of the arrangement. Of course the circa 3 dB extra gain is also something to be desired as the plain VHD does not posess all that much gain in the first place. (Why does a vertical dipole have less gain than a horisontal one? The horisontal dipole’s pattern will consist of not only direct waves but also of ground reflected ones. All in all, they add to give an improved pattern.)
The addition of a reflector to the VHD gives a clear directional pattern in azimuth while remaining a low angle radiator. The F/B-ratio of some 12 dB ain’t bad! Picture from MMANA.
Directionality is OK if it is in the right direction! Of course on a field day, or other temporary installation, perhaps one direction will be enough, but for stationary use, that is not enough. So there must be a device to, preferably easily, change direction of the main lobe.
In the current installation the extra reflector is made very simply by a single wire hung a distance from the driven element, i.e. the VHD proper. An easy way to accomplish this is to use a piece of wood placed on top of the dipole as the carrier for the wire. The other end of the overbar is tied to the ground with polyester line. This arrangement allows one to rotate the overbar should so be wanted. Of course a rotor would be better but that would probably be more comlicated than the antenna itself.
The directional vertical dipole. From the right tip of overbar is hung 7.1 metres of 0.9 mm electrical wire which acts as a reflector. The wire itself is too thin to make it on the photo. What one sees here is a polyester line used for pulling the wire up and down during testing. The electrical wire is kept vertical by a small weight at the lower end. From the left tip is strung a polyester line so that I can turn the overbar in any direction. It can also be used for lifting/trimming the reflector position should that be called for. Due to the proximity of the reflector the driven element has to be made shorter, in this case it is 6.54 meters from end to end.
As my VHD is anchored by guy wires some azimuth angles are more difficult than others to achieve since the guy wires gets in the way for the reflector. If I anchor the VHD at the foot only, then there would be no problem rotating the overbar in any direction.
Does it work?
Yes, the directivity is there. The major benefit would be the reduction of interference “from behind” that will make me able to listen more and hear more stations. I also figure that the extra gain makes my signal go a bit longer, but that is more difficult to prove. Tests indicate that the antenna exhibits slightly lower rear rejection than the plain, vanilla, VHD. Simulations predict 6 dB, practice gives more like 1 S-unit (3 dB). I am not sure why there is such a discrepancy, although one source of confusion could be that the receiver’s S-meter is not calibrated. Despite that, considering the practically no cost and low complexity involved in building this add-on, it is definately a good thing to do with the VHD.