Antennas
First rule of Antenna Club is, everything is a dipole unless it's a loop.
The second rule of Antenna Club is, you MUST talk about Antenna Club.
The first rule of Antenna Club is simple: everything is a dipole unless it's a loop. Everything has a positive and a negative side. When building antennas, everything is a dipole, whether it's a flat horizontal wire fed in the middle and trimmed to resonance, an inverted V, or a vertical. Everything is still a dipole. One side is positive. One side is negative. One side is the radiator. One side is the earth.
Once you understand that everything is a dipole, a vertical antenna is the perfect answer when space is limited and you cannot fit a dipole in horizontally. But, as with a dipole, there are rules around this. Some make things complicated. Some make things very, very easy. So let's look at a couple of these rules first. A dipole fed with 50 ohm coax is always going to be a mono-band antenna. Height plays a critical role in horizontal dipole antennas.
We can overcome the first problem by changing the feed system. A resonant 40m dipole fed with 50 ohm coax, around 33 feet per side, is normally a mono-band antenna. If we replace the coax with ladder line or open-wire feeder, commonly 300, 450, or 600 ohms, and tune it with a balanced tuner, the same centre-fed wire becomes far more flexible. It can usually be matched on higher frequencies as well. The key point is that the antenna does not need to be double the length just because it is fed with ladder line. For the lowest band you want to work, aim for roughly a half-wave total length on that band.
So everything is looking good in the garden. We have built a dipole and strung it from the corner of the house to a pole in the corner of the garden. If you want the main radiation to favour east and west, orientate the wire north and south. A horizontal dipole has a broadside pattern, with most of the signal off the sides of the wire and smaller nulls off the ends. But what about height? Most people can get the centre of a dipole up to around 10 metres, and the centre matters because the current maximum is around the feedpoint. That is the part you want as high as possible. For a good low angle of radiation, a horizontal dipole wants to be around half a wavelength above ground. On 10, 12, 15, 17, and 20 metres, a 10 metre support is generally enough to give useful low-angle radiation for DX, and a horizontal dipole will usually be quieter than a vertical. The problem starts on 40 metres, where half a wavelength is around 20 metres, or 66 feet. That is where dipoles become difficult in normal gardens. Below that height, and especially below a quarter wavelength, the radiation angle rises and the antenna starts to behave more like an NVIS antenna. That is excellent for UK contacts, UK activity contests, and near Europe, but it is not ideal for DX because much more of the signal goes upward rather than out at a low angle.
So if you want to use a dipole effectively, you need height, and quite a lot of it, especially if you want to work 40, 80, and 160 metres. On top band, a half-wave above ground is roughly 80 metres, or about 260 feet.
Unless you live somewhere with huge open space and serious support structures, the average UK garden is not going to accommodate that. My own available space is even smaller than I first thought. I originally thought it was 30 by 30 feet, but it is actually closer to 30 by 27 feet. That is a very small area in which to make a good DX antenna. Or is it?
There is, of course, one caveat: the inverted V dipole. Because the legs slope downwards from a single high centre point, it takes up less straight-line space than a flat horizontal dipole. You still want the feedpoint as high as you can get it, but by angling each leg downwards you immediately start to save room. The pattern also changes. An inverted V is usually less sharply broadside than a flat-top dipole and can include more vertical component, so in a small garden it becomes a practical way to keep dipole behaviour while using far less horizontal space.
So now, in our 30 by 30 plot, we have an inverted V dipole. We have some useful gain on 20 metres and above, but on 40 metres we are still not going to be working much DX. We have also divided the garden directly at 45 degrees, leaving only two small triangular spaces. There is still 80 and 160 metres to think about, we still do not have an effective DX antenna for 40 metres, and we have already cut our available space down. This is where we enter the vertical antenna.
The vertical antenna is a great antenna for many reasons. It can be a single antenna in the form of a quarter-wave vertical: a quarter-wave of wire as the radiator, with three or four quarter-wave radials as the ground plane. It does not get much simpler. The feedpoint impedance is usually close enough that matching is straightforward, so extreme SWR should not be an issue. A quarter-wave vertical is also a very good receiving antenna. It receives low-angle DX well, and it transmits low-angle RF well. Perfect for DX.
But again, we face the same problem. As good as the quarter-wave vertical is, it is still a mono-band antenna. So while it performs well, it is limited. Bearing in mind that a quarter-wave on 40 metres is around 33 feet, let's look at the options available when using a vertical radiator of that height.
With 33 feet of wire vertically, we have a quarter wave on 40 metres and a half wave on 20 metres. The useful part is at the feed point: we can turn it into what is commonly known as an end-fed half-wave. End-fed basically refers to any antenna that is fed at the end rather than in the middle. It does not have to be vertical; an end-fed half-wave can be run completely horizontally. But where space is limited and we are using the vertical radiator, it becomes a very useful antenna. We now have a half-wave on 20 metres, giving more gain than the quarter wave, and with a 49:1 transformer we can cover 20 metres, 15 metres, and 10 metres, plus the WARC bands with a tuner.
So now we have 33 feet of wire and a 49:1 transformer, giving us a five-band vertical antenna covering 20 through 10 metres. The next logical step is to add another 33 feet, giving 66 feet in total. This is now a half-wave on 40 metres, so we have a six-band end-fed half-wave. Again, this can be fed horizontally, vertically, as an inverted V, as an inverted L, dog-legged, or any way you like. That is the great thing about an end-fed: it can be bent to fit. But can we go any further?
This is where things start to break down with the end-fed, because there are some odd decisions people make here, in my honest opinion. Let's look at some facts. We now have an end-fed half-wave for 40 metres. So on 40 metres we have some gain, that is true. There will be gain on 20 metres, and then as we go higher the lobes start to become quite frequent and point in many, many directions. Some are useful, and some are completely useless.
A lot of people will now add a small inductor and around 5 to 6 feet of wire. This will then add 80 metres. But let's just look at the sizes. We have a 66-foot end-fed half-wave. If we take the 49:1 off, we now have a quarter wave on 80 metres, which is a great antenna, be it vertical, in an inverted V, or as an inverted L. If we were to add the inductor and 6 feet of wire, we would now be looking at 72 feet of wire.
So why add a 49:1 transformer and work 80 to 10 metres, when you can add a 9:1 transformer, drop the inductor, and just add the extra 6 feet of wire for a total of 72 feet? This now gives us 160 metres to 10 metres, which seems a more logical solution. Admittedly, the performance on top band is not going to be fantastic by any stretch of the imagination, but it will give you a lot of radiation going upwards: NVIS. This is great for working the UK and into Europe, and on my 72-foot I have also worked into the States on FT8 and CW. So it is a useful antenna.
Right, let's have a look at some pictures of various transformers. There are 4:1, 9:1, 49:1, and 64:1 transformers. The 49:1 and 64:1 are doing much the same job; the difference is the turns ratio, with the 64:1 giving slightly better bandwidth at higher frequencies. So let's have a look at some RF plots of our end-fed antennas, both end-fed half-waves and what people often call random wire. But random wire is not exactly random.
| Wire Length Feet (Meter) | 1.8 MHz | 3.7 MHz | 5.3 MHz | 7.1 MHz | 10.1 MHz | 14.2 MHz | 18.1 MHz | 21.2 MHz | 24.9 MHz | 28.5 MHz | 50.1 MHz |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 175 (53.34) | 1.2 | 1.6 | 1.1 | 1.1 | 1.1 | 1.8 | 1.3 | 1.6 | 1.7 | 1.2 | 1.5 |
| 169 (51.51) | 1.4 | 1.2 | 1.2 | 1.2 | 1.2 | 2.1 | 1.4 | 1.4 | 1.5 | 1.2 | 1.1 |
| 162 (49.38) | 1.4 | 1.5 | 1.7 | 1.3 | 1.6 | 1.8 | 1.9 | 1.1 | 1.5 | 1.7 | 1.5 |
| 146 (44.50) | 1.7 | 1.5 | 1.4 | 1.4 | 2.4 | 1.5 | 1.3 | 1.2 | 1.4 | 1.5 | 1.5 |
| 135 (41.14) | 2.0 | 1.5 | 1.3 | 1.8 | 1.6 | 2.0 | 1.7 | 1.7 | 1.5 | 1.6 | 1.3 |
| 124.5 (37.95) | 1.3 | 1.3 | 1.2 | 1.3 | 1.7 | 1.6 | 1.8 | 1.6 | 1.4 | 1.1 | 1.4 |
| 98.5 (30.02) | 1.8 | 1.7 | 1.4 | 1.7 | 2.3 | 1.9 | 1.4 | 1.2 | 1.7 | 1.2 | 1.2 |
| 88.5 (26.97) | 1.8 | 2.2 | 1.7 | 2.3 | 1.9 | 1.3 | 2.0 | 1.8 | 1.4 | 1.5 | 1.5 |
| 72 (21.95) | 2.0 | 2.0 | 1.4 | 1.2 | 1.2 | 1.9 | 1.9 | 1.5 | 1.1 | 1.5 | 1.1 |
| 59 (17.98) | 1.6 | 1.6 | 1.3 | 1.5 | 2.0 | 1.5 | 2.0 | 1.1 | 1.7 | 1.2 | 1.5 |
| 53 (16.15) | 1.6 | 1.4 | 1.2 | 1.1 | 1.5 | 1.1 | 1.9 | 1.2 | 1.1 | 1.7 | 1.1 |
| 49 (14.94) | 1.5 | 1.3 | 1.4 | 2.4 | 2.4 | 1.3 | 1.6 | 1.6 | 1.4 | 1.7 | 1.5 |
| 44 (13.41) | 1.2 | 1.5 | 2.1 | 2.1 | 1.7 | 1.3 | 1.7 | 1.6 | 1.1 | 1.2 | |
| 36 (10.97) | 1.2 | 1.3 | 1.3 | 1.3 | 2.0 | 1.6 | 1.2 | 1.7 | 1.6 | 1.5 | |
| 29.5 (8.99) | 1.2 | 1.2 | 2.1 | 2.0 | 1.3 | 1.2 | 1.6 | 1.3 | |||
| 24.5 (7.47) | 1.6 | 1.6 | 1.4 | 2.1 | 1.8 | 1.3 | 1.2 | 1.4 |
Caveat: The diagrams below show high SWR because they are intended only to illustrate the far-field patterns. No feedpoint transformer was included in these models. For a 9:1 unun, use the wire-length chart above. A correctly installed 66-foot wire with a 49:1 unun should generally show an SWR below approximately 2:1 on its intended resonant bands, because the transformer converts the antenna's high feedpoint impedance to a value closer to 50 ohms.
Above are the plots for an inverted L, 10 metres high with a 12-metre horizontal section, fed with a 9:1 unun. This gives us a 72-foot random wire. As you can see, the plots are very interesting. This is also something that puzzles me. For an extra six feet of wire, why would you use a 66-foot wire with a 49:1 unun? Built in the same inverted-L configuration, both give the same NVIS pattern on 40 metres and useful low-angle radiation on the higher bands. The 72-foot wire with a 9:1 unun also gives useful low-angle radiation on top band, although the losses are huge. Do not expect to work the world with it. Having said that, I have worked the United States, Japan, and South America on FT8 on top band with this antenna. The radiation pattern is also very similar on 80 metres. Therefore, what is the point of the 49:1 unun with 66 feet of wire?
Here are two far-field plots for 40 metres. The plot on the left is for a 66-foot wire with a 49:1 unun, while the plot on the right is for a 72-foot wire with a 9:1 unun. There is very little difference between the far-field plots. Both antennas exhibit the same NVIS properties.