More Improvements for My Small Station
Joe Chester M1MWD experiments with a fan vertical as a possible solution to his antenna constraints.
Allow me to introduce you to my nemesis. He hasn’t got a name but he is a right pest. I dream of DX hunting, of putting out a strong signal on every available band, of working the World from South Wales. But when I wake up and look out the window, my problem stares back at me. You see, I live in a bungalow with a very small rear garden. So, thoughts of putting up a vertical for 160 or 80m stumble at the first hurdle. A full-sized dipole on 40m won’t fit either, even if I could get it up high enough. And as for the higher bands – well, a 10m dish for EME is just a dream (more a nightmare really!).
“Stop moaning”, says M, owner of a mountain somewhere up country, on which he grows antenna poles, mast and pylons, all topped with the latest in Yagi and log periodic beams for every conceivable occasion. You could probably see it from space! “You think you’re the only one with these issues?”, he asks. Just for reference, the ‘rear garden’ in question is a triangle 9m x 7m, with veg grow boxes and a stone-tiled patio, and with the pointy end taken off for the tool shed. M once suggested sending my XYL (and Accountant!) out shopping, so that we could dig up the patio to lay a field of radials for an 80m vertical. He was barred for a month for that one!
Many of us operate from similar conditions. The rash of books and articles on stealth antennas, for example, is evidence that many others are equally challenged by the lack of real estate. Some are brave enough to relocate. Herself caught me one afternoon reading a section of the ARRL Antenna Handbook entitled “Choosing a QTH for DX” (Section 14.3.1 if you’re interested). It was cold shoulder for supper for a month after that mistake! So moving house is not on the agenda.
But it’s not all bad news. Even those with acres of space have issues – not always too different from those of us small garden dwellers. Lately, I’ve been making a list of all the possible things I would like to be able to do, working from my small piece of real estate. I call this blue skies thinking. But when I look at this list my brain freezes
– all that construction work, and the mechanical engineering!
Enter the FanVertical
But fortune sometimes favours the desperate. I produced a bit of a rabbit out of a smartphone-shaped hat over dinner a while ago, and received a very nice, heartwarming shrug of indifference in reply. You see, much research has uncovered the fan vertical. This is a multiband antenna, which if properly erected, does not even require an ATU! What I’m talking about is a set of approximately quarter-wave vertical radiators, mounted in parallel up that telescopic pole. My pole has been up for a year now (on and off, the latter when it’s very windy). I changed it recently for something more sturdy. This supports my inverted-L (see PW April). In effect, it’s achieved invisibility status! So, a few more lengths of invisible wire running up that invisible pole will not contravene the ‘local’ by-laws!
But will it work? Well it’s a quarter-wave vertical. So, it should. Put another one parallel to it, and it should work too. But what about switching between them? None needed! Think of a fan dipole, where all the radiating elements are connected together and use the same feeder. It’s the same for the fan vertical – a single feeder is connected to all the active elements.
Interaction between the elements, such as induced currents, will have some impact on precise tuning, and I’m told that it takes a bit of effort to get the tuning of the various elements right. The general idea is that the feedpoint impedance of the radiators far from resonance will be so high that the majority of the signal power goes to the resonant radiator.
Any examples? The ARRL Antenna Book mentions it (Section 10.4.5, page 10.23). And look on QRZ.com at AJ6O (wow!) and PA1M. Further research establishes that there was a QST article way back, and I’m sure there were other papers written even earlier than this. And then there is Callum M0MCX, who has championed this type of antenna for years. He is such a fan (pun intended!) that he has designed and manufactured aluminium base plates, and PVC spreaders to make assembly easy for the mechanically challenged (like me!).
Checking the Maths
To check the maths, I decided to model a two-element version, with wires for 20m and 40m. You can see the output in the diagrams – one for a single 20m vertical and one with the two radiators, 20 and 40m, Figs. 1 and 2. The 20m on its own is the standard pattern for a ground mounted vertical, with a gain of 1.83dBi at elevation 23°. When the two elements are fed at a
single feedpoint, as in the second diagram, the 20m has roughly the same shape, but is down about 1dB. The angle of maximum gain has dropped a few degrees while the 40m element gives a slightly deformed pattern, but still gives a 4.55dB gain at 15°. So, in principle, the vertical should give me a lower take-off than either the dipole or inverted-L. There is naturally some coupling between the elements, very much like the resonance you’d get from plucking a single string on a multi-stringed instrument (such as a guitar). With the fan vertical, this coupling creates parasitic induced currents in some of the elements at certain frequencies, and this reduces the absolute performance by, maybe, 1-2dB at times. But this is not even half an S-point. I don’t think that in practice you’d notice this. And as always, practical use of the antenna will differ from the theoretical predictions anyway, almost always giving better performance than the mathematical model.
One of the main benefits of the fan vertical is its small footprint, which is especially useful where space is at a premium (like in my small garden). It’s basically just a pole, with wire radiators. Calum says he has succeeded in tuning the verticals to avoid having to use an ATU.
Radials
Which brings me to the issue of radials. The performance of any vertical depends crucially on the quality of the ground over which it is mounted. In a later article, I will discuss the question of radials in more depth. It’s been the subject of lots of theoretical work and practical experiments. And there is also the issue of using elevated and tuned counterpoise wires. It’s a complicated question but I am going to settle on using as many ground radials as I can on my small patch. And this is where the local management enters the project again. Remember what happened to M! So, it was important to put the radial issue on the agenda for the weekly meeting. I was well prepared, but the decision was taken that the work I suggested would be an ‘all hands on deck’ day – in other words, I could supervise, but other, ‘more careful’, workers would be purloined into doing the actual work. This consists of laying down some 30 or so wires, mostly 2.5m long, but a few 4m ones as well, in as close to a radial pattern as we can achieve, under the gravel in the back garden. “I suppose we could also order an extra load of gravel to increase the coverage if we need it”, I was told. I agreed. But what I didn’t say was that these wires needed to be as near the surface as I can get away with! More than an inch or two down, and the ground losses start to accumulate again.
Giving it a Go
So, I’m convinced that I want to give this a go, but I had no idea that it would lead to a major redesign of my whole station. To start with, I moved the end of the inverted-L out of the tree and put it on a second pole. The constant movement of the tree, pulling on the horizontal wire, kept causing the vertical pole to collapse. This move also has the benefit of rotating the axis of maximum gain of the inverted-L more into G-land. The next step is to put quarter-wave lengths of wire for 20m and 40m up the first pole, and create a feedpoint for them. The outer of the coax at this point will go to an elevated counterpoise temporarily (laying down radials is hardly what might be called ‘essential work’ these days – nor are garden parties allowed).
And I have another tweak in mind. Last time I played with verticals was for the Footsteps project. With the antennas I built then, I used a large tapped inductance coil (from Buddipole) to bring the system close to resonance. I had an old SG-237 ATU lying unused in the attic junk box. I’ve dug this out to use at the feedpoint.
I know Callum says his design doesn’t need an ATU, but I have one, so might as well see what happens. And rather than drive this with 12V from the shack end, I have a healthy unused 12V car battery lying nearby, which I will tap for a 12V feed. I put it all together one day last week (in the sense that I got some spare wires together – not finished by any means). It worked − the ATU started clicking when low power RF was applied to it. So, it’s now time to finish the mechanical engineering work. More next month.