Friday, September 20, 2013

Multi-band Inverted Vee

Following on from my previous article I can now report that I have built, erected and tuned the planned multi-band inverted vee antenna on my house-bracketed support mast (Site B). There are parallel inverted vee antenna for 30, 20, 17 and 15 meters. To cut to the chase, yes, it works. In fact it works better than I expected.

The picture shows the antenna as viewed from the backyard, facing approximately northeast. As with the 1.5-band inverted vee experiment the wires are anchored to the north side of the house roof and to the top of the Site-C tower.

A close-framed picture like this is needed to provide a sense of how it's constructed. Now that you've got the finished product in front of you I will step back a few paces and describe how I got here.

In the previous article I compared models of multi-band dipoles (also inverted vee dipoles) using two configurations: radial and parallel wires. The predictability of tuning and performance was significantly better with parallel wires.

If I had only wanted to cover two bands I'd have been done. Because I wanted more bands there are mechanical and electrical challenges with scaling that simple model. Instead of parallel wires I chose a cylindrical model, with antenna legs on the perimeter of a virtual cylinder. There is nothing innovative about this design, which you may have already encountered elsewhere.

Rather than linear spreaders (wire spacers) I needed spreaders that centred the wires equidistant from a common (cylinder) axis. Since my design is for 4 bands I went with an "X" structure for the spreaders. This required fabrication in my workshop. It had to be simple and fast since my time is limited.

There are ample choices of plastic that are suitable for this application: good dielectric properties, strong, easy to work with and resistant to the weather and ultraviolet. For example, Lexan. Wood is not a good choice since it will deteriorate in the weather and can be heavy enough to weigh down the antenna, causing wire sag. For some reason almost all the plastic-supply outlets seem to be on the opposite side of town. To avoid a long drive I decided to first see what I could scrounge nearby.

First I surveyed the inside of the house for derelict plastic that would suit the bill. I struck out so I went browsing through a building materials store. I was going there anyway so this was not a special trip.

Hams are not the only tinkerers. You can almost always find a few middle-aged men casually strolling the aisles of these and similar stores, touching, weighing and even shaking all manner of raw materials. We all have a final picture in mind to which we are trying to fit the available materials. That is, we don't know what we're looking for, but when we find it we will know that it's exactly what we need.

That should explain the following picture. Hopefully you'll find the lettering to be legible.

This is printed on a 10-foot length of ½" PVC water pipe. It's rigid (strong), nonmetallic (dielectric) and sunlight resistant (UV protection). It only cost $3.39, which is guaranteed to be considerably cheaper than almost every alternative. All it needs is a little bit of labour to transform it into antenna gold.

My tools were a tape measure, felt-tipped marker, Workmate, drill, hacksaw and half-round bastard file.

I made the spreader so that no wire was closer than 10 cm to any other. This is not a strict criterion, just enough to ensure against over-coupling caused detuning, even when wind and wire sag lessen the separation.With 4 wires this requires spreader arm lengths of at least 15 cm. I made them 18 cm (7") since the wires penetrate into the arm about 1 cm on each end.

This was not a precision operation! I put the pipe lengthwise into the Workmate and made two cuts along the length of the pipe, dividing it into 4 quarter-round sections. Doing it this way results in wobbly lines and varying widths along each arm. The PVC is thick and strong enough to withstand a few weak points. I could have made them uniform and pretty, if I had taken twice as much time. Friction-fit notches at each end of every arm were made with a hacksaw and then reaming the cut with a drill bit slightly narrower than the wire width.

I used the file to clean the edges, and then to carve a mid-point recess in half the arms. Since the arms have the curvature of the pipe this allows one arm to comfortably nest within its partner. These were then drilled in the centre and screwed together. Since they could still move when forced -- the plastic is very... plastic -- I glued them with cyanoacrylate (super glue). Now they hold their shape.

I made 6 spreaders. There are 3 for each leg: one adjacent to the feed point (see picture below), one just inside the end of the 20 meters leg, and the third one positioned midway between them.

I next proceeded to cut the antenna wires. The 30 meters antenna was the easiest. Since the model showed that the higher-frequency wires would not affect its tuning I simply modeled it by height and interior angle and then cut it about 10 cm longer (5 cm on each side). I made sure to model the insulation covering the wire since it has a significant impact on the wire length. You should typically expect that the wire must be cut about 2% shorter than bare copper. I used #12 stranded wire for 30 meters since it was not only the longest of the four wires but would also be used to set the tension for the other three.

I reused the 20 meters antenna from my 1.5-vee experiment. This was easiest since it was already soldered to the center insulator. However, I knew that it would likely need to be lengthened since, unlike the previous antenna, 20 meters would not be the lowest band. Interaction with the 30 meters antenna would require lengthening the 20 meters antenna.

The single 17 meters leg was similarly reused. The second 17 meters leg and the 15 meters antennas were newly cut. The 20, 17 and 15 meters antennas were made from #14 insulated stranded copper wire.
For modelling geeks only: I did not directly measure the insulation thickness. I looked up the diameters of #14 and #12 stranded copper wire, measured the width of the insulated wire, and subtracted the first number from the second, then divided by 2. The results are approximately 0.4 and 0.5 mm, respectively.
Close up of the fully-constructed feed point, including the first set of wire spreaders
I modelled each antenna as I did for 30 meters. I then mentally added additional length to each, and added several more centimeters for the end loop and wrapping of the wire on the centre insulator wire loop. I hoped this would give me enough wire to allow me to cut and trim each antenna to length. Doing a full model of this antenna would have taken time I do not have right now. I might do so later, long after the fact.

The end of each 30 meters leg is wrapped onto a large galvanized flat washer. Nylon ropes tie the wire-end loops of the other antennas to the same flat washer. Like the wires themselves, the end ropes for the shorter 15 and 17 meters wires attach to the remaining spreaders on the way to this 'terminal' washer.

After soldering the wires to the center insulator the spreaders are placed at their intended positions, with the wires pressed into the spreader notches. The centre insulator is then anchored to some handy fixture in the yard (I used the tower). The tie-down nylon ropes are then pulled to draw tension on the antenna half and tied so that the entire assembly is suspended in midair. The tension for 20, 17 and 15 meters legs is then adjusted so that the wires do not appreciably sag.

Thin nylon rope (I used ⅛") is very stretchy which allows some latitude in setting wire tensions. Just be sure there is no significant slack that wind and gravity can affect. After setting the tension adjust the spreaders so that they are orthogonal to the wires. One or several spreader arms may have moved out of position during this procedure due to the friction fit.

Repeat the above procedure for the other half of the antenna. If, unlike me, you properly machined the spreader notches the wires and rope will not spontaneously self-eject from the spreaders.

This contraption is not too fragile, which is good since it will inevitably need to be dragged along the ground and then bounce against, and tangle with, who know what as it's hoisted by pulley to the top of the mast. There is also the problem of twisting near the terminal washers since the mess of nylon ropes and ends of the 30 meters antenna will twist around each other. This is not really a problem even if it does look ugly. The spreaders keep this from happening to the main body of the antenna.

You can see this happening in the picture below. The flat washer is hidden within that group of knots. It isn't as bad as it looks!

View from the roof of the antenna on the mast, looking towards the southeast
When I was done the antenna worked but was not without faults.

The 20, 17 and 15 meters were still too short, even though I anticipated the problem and compensated for that on 17 and 15. The undershoot was 1.3% on 20 (13 cm), 2.5% on 17 (22 cm) and 1.5% on 15 (10 cm). Wire needed to be added, which I did by tying and soldering additional wire. I did this all one one side of the antenna. There is no need to make the antenna halves exactly equal, but do make sure you are using a coax choke or current balun at the feed point. I worked on the roof (north) side since it was easier to access than the end going to the tower.

When trying out the antenna after lengthening the wires I noticed that on 17 meters the SWR continuously swung back and forth over the range 1.2 and 2.0. It turned out that one leg of that antenna had insufficient tension, causing one section of it to swing close to the 20 meters wire in the breeze. That was adjusted during the next iteration. Unfortunately this was on the tower end so I had to climb the tower, twice, to remove and return the tie-down rope.

The Site-B mast is very flexible. It was challenging to judge the tension from each end of the antenna, particularly on the tower end, so that the mast was not pulled too much to one side. As you can see in the picture the antenna halves are not straight. To do so would require too much tension. It takes a surprising amount of tension to straighten even a light structure such as this antenna. It is about more than aesthetics since the interior angle is more acute due to sag. The performance impact is small, but there is an impact.

After this lengthy description you might not realize how quickly this antenna came together. On day 1 I bought the pipe and then spent no more than 2 hours that evening fabricating the spreaders. On day 2 the wires were cut, the antenna assembled and raised into the air. The on-air performance and SWR were checked that evening, and calculations were done to lengthen the antennas. The third and final day the antennas and rope tension were adjusted, and the antenna was raised into position.

At this point everything worked as planned. I worked stations on every band (to confirm that it did indeed transmit well) and the SWR was below 1.5 on the CW and low phone segments of all four bands. Out of curiosity I briefly tried it on 12, 10 and 6 meters. On 12 it received well but the SWR was very high. On 10 and 6 meters it not only heard well, the SWR was quite low. On 6 it is 1.4 between 50 and 50.2 MHz, and on 10 it is 1.6 at 28 MHz and dips to 1.1 at 28.4 MHz. This goes to prove that if you get enough wire in the air can be unexpected resonances showing up here and there.

I have yet to attempt any QSOs on 10 and 6. It will be interesting to see how it plays on these bonus bands. DX performance is expected to be erratic since dipoles longer than 1 wavelength have multiple smaller lobes, and nulls between those lobes. This is true for both azimuth and elevation, and their number increases as the antenna gets longer.

Once I have used the antenna long enough to assess its DX performance I'll report back. Perhaps within the next week. It should, and so far seems to, show the far field characteristics of an inverted vee on the 4 primary bands.

The apex is up 14 meters, the north end is 6 meters high and the south end is 8 meters high. Its orientation favours south Europe, Africa and Oceania. The current orientation of the TH1vn tri-band dipole fills the gaps in the inverted vee's azimuth pattern. It is currently up 9 meters, having been lowered from 10.5 meters so that I can work on it.

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