The SVHD — A Switchable Vertical Halfwave Dipole

Having experienced my vertical dipole on the bands and having improved it with a parasitic reflector I decided to make the reflector a director also. I knew this meant making the parasitic element shorter than it was. My plan was to use a switch or clamp at the bottom of the reflector to shorten it to act as a director instead.

After having made several models in the MMANA antenna simulator and an extensive range of parametric runs I found that the problem was not easy. First out I wanted an antenna with a well behaved impedance, preferably 50 ohms resistive. Then I wanted this impedance to hold for both directions and finally I also, of course, wanted gain in the forward direction and decent front-to-back ratio. If I could manage without a hairpin match or some such matching device that would be a plus too.

I started off with finding a good driver-reflector configuration. That was the easy part. With a gain of some 4 dBi and F/B of 11 dB I settled for that combination. The troubles began when I wanted to change directions and use the reflector as a director. Even with this antenna’s low complexity finding the two lengths of the parasitic element was a problem. I ran many simulations with different lengths of the driven element, the parasitic element and the distance betwwen the two and come up with antennas with good impedance but lousy F/B-ratio or low gain. Or any other conceivable combination of the target parameters…

After a while I started to get a feeling for the dimensions and what effect they had on the final result. To cut a long story short I ended up with a compromise that I can live with but since it entails shorting clamps on both elements it is not the ideal solution. The impedance, gain and F/B are ok for the driver-reflector combination:

 Resulting patterns for the vertical driver-reflector combo. The ground is of “average soil”-type so the gain figure will be better or worse depending on the ground conditions at the antenna site. I always use the same model however so the gain figures can be compared with each other. The F/B-ratio is a function of the geometry of the antenna and largely independent of the soil and height above ground.

From the picture above we see that a F/B ratio of slightly more than 10 dB (that’s circa 3 S-units) can be expected. The impedance is perfect too. I modelled the antenna at a height of 0.5 metres.

So with a driver-reflector combination how do you turn it into a good driver-director antenna? You can’t. Well, I couldn’t anyway. I settled for maximum gain and close to 50 ohm match and had to let go of the good F/B-ratio. Still the pattern i definately directional:

Resulting patterns for the vertical driver-director combo. The F/B-ratio is not too good but gain is only half a dB down from the other direction and the impedance close to perfect. Fair enough!

Building one

I built one of these out of aluminium tubing, nylon isolators and rope to keep it in place. Unfortunately I did not bring along a camera so there are no pictures. The antenna is however stored in a garage and I plan to use later. So pictures might very well appear evetually.

The final dimensions can be seen in this figure. The antenna is cut for 21.050 MHz, the CW-portion of the 15 metre band but scales nicely to other frequencies as well. Some pruning might be necessary if you are really picky. Note the movable clamp: There is only one and it has to be used always otherwise the SWR will jump to 3:1.

Final dimensions as used during the live tests. The dipole is centre-fed when 640 long and a bit off-centre when the clamp is put in place. It seems to do very little (anything?) to the pattern.

The tubing used was about 12 mm (0.5″) in diameter, which was fine to work with. But impossible to erect! I was lucky to have some long pieces of wood at hand that I could attach to the tubing with sellotape. Otherwise the tubing would bend so much that one end almost never left the ground. A word of caution here: I’d go for at least 30 mm tubing next time.

The distance between the elments was kept with some rope of the right length at the fastening point of the two poles. That proved to be a good idea. It was never necessary to measure the distance between the poles, one could easily see when they were vertical. Or move the bottom part of a pole until they were. While the model used anticipated a 50 cm standoff from the ground I used only a 2 inch piece of wood with a nail in it sticking out a centimetre or so. The nail was necessary to keep the poles in place while raising the poles.

NB. I only ran a couple of tens of watts into the antenna. The ends are hi-voltage points so they should be well isolated. Had I used this setup with hundreds of watt the pieces of wood would most likely be charred. Use a plastic rod to lift the bases up some 50 cm if you use much output power.

Final setup of the two elements. There is no reason to let the lines that are holding the poles in place start from the top. Halfway down or so is enough. Note how the distance is kept constant with the 225 cm line. This can be setup when the poles are on the ground to ease the lifting of the elements.

When the poles are set in a vertical position very little force is necessary to keep them in place. It’s getting them there that is the snag. That surprised me. The lines are nailed into the ground fairly close to the poles.

What I ended up with

The vertical dipole’s great virtue — the low radiation angle is still present in these configurations. This is good since the DX-potential is still there, and that is the main reason I stick to verticals in the first place. Added to that low angle, there is now a 3 dB gain in one of the directions and some not to too bad F/B-ratio. A higher F/B-ratio would require more elements.

On-the-air checks were promising. Set in a north-south direction I would either point towards southern Europe where there were (sometimes strong) signals or over the pole where there were few signals that day.

The test was very crude. I found stations that were coming in at around S-9. Then I switched the antenna’s direction. To switch direction I had to leap up from the radio, run to the base of the antenna and move the clamp from one of the posts to the other, then I would scurry back to the radio again and take a new reading off the S-meter. There were inevitably some fading during this time so each result is not but a mere indication of the antenna’s directionality. And few stations were perfectly south either. But since I did many such tests I can claim that the theoretical analysis seems to hold.

Stations from the south-south-east made a 6 S-unit (18 dB) jump in signal strength when switching directions. Since the F/B-ratio of the model only accounts for 10–12 of these, the added gain of about 4 dB when changing direction nicely covers up the rest, or thereabout. Of course the S-meter is not calibrated and the scale of the rig used (a Yaesu FT-7B) is small so I could have erred +/- 0.5 S-unit.

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