At my QTH, I was using fan dipoles until very recently. During a bit of maintenance, I decided to try a doublet before completely rebuilding the fan dipoles and re-erecting them. I have to say that I’m quite happy with the results so far…
It should be said that a doublet may not work for everyone. Just like any other antenna, there are so many variables that can affect it’s efficiency and operating parameters, what may work at one place, may be no use somewhere else.
What is a doublet?
In it’s simplest form, a doublet is any dipole fed with balanced line feeder and cut to length for it’s lowest frequency of operation. In addition, a doublet must always be used with an antenna tuning unit (ATU) due to the impedance mismatches.
80m doublets appear to be the largest commonly used. Perhaps the reason for this, certainly in the UK, is due to the small areas most people have to erect a long antenna. Another possibility is that because of the size of a 160m doublet it becomes hard to tune on most amateur bands. I really don’t the reason, but those are my theories.
For my doublet I have used Flex-weave antenna wire although any wire will do. Flex-weave antenna wire has a high number of copper strands (either 168 or 259) and is constructed using a method that makes the finished product similar to rope in appearance. It is sometimes referred to as “wire rope”. Because of it’s method of manufacture, good quality flex-weave does not kink, tangle or stretch.
The length of the wire used to make the doublet does not really matter too much. There is no need to be precise when measuring it out except that both sides of the dipole must be equal. The quickest method of getting them equal is to take a single length of wire, fold it in half and cut it at the fold.
You can use these lengths as a starting point depending on what space you have available. In theory almost any length will work as long as it’s at least a half wavelength long for the lowest frequency. It doesn’t need to be straight, you can bend it around corners to fit it into the space you have available. If you can’t manage to get the length you want, just make it as long as possible and give it a go – you may be surprised at the results.
|Lowest Frequency||Overall Length||Each Side|
|3.500-3.800 MHz – 80m||130 Feet||65 Feet|
|7.000-7.200 MHz – 40m||66 Feet||33 Feet|
|14.000-14.350 MHz – 20m||34 Feet||17 Feet|
For the centre support I used a commercially available centrepiece. The support for the dipole centrepiece is provided by a 150mm long eye-bolt through the top of an aluminium pole. The heavy duty spring allows for movement in the wind etc. In the picture you will see a similar centrepiece for use with a coax feed. Instead of using the supplied crimp terminals, I have soldered the flex-weave directly to the ladder-line feeder to help prevent poor contact due to corrosion. The end supports have been fabricated in a similar fashion.
You must always use ladder-line feeder with a doublet. You cannot use coax.
Why? – as a result of the varying frequency transmitted into the antenna, there will always be different impedances at the feed point. Although your ATU may be capable of handling the extremes of impedance, the losses in the coax will be absorbed and transferred into heat. Ultimately, coax becomes very lossy when presented with a mismatch.
Ladder-line, on the other hand, has such a tiny loss, even when presented with a huge mismatch, that most of the RF power will be fed to the antenna instead of just warming up the cable. Ladder-line can be almost any pair of cables as long as they are kept parallel. In a pinch, it is possible to use ordinary twin core mains cable. I have heard of people experimenting, quite successfully, with twin flex!
The route of the ladder-line should be without sharp bends, at least twice it’s own width away from any metal objects running parallel to it and preferably suspended above ground. A shallow spiral twist, about one turn every metre or so, may help to keep the currents balanced in respect to the ground.
I have supported my ladder-line using a UV stable, weatherproof nylon cord threaded through the slots. As a result, this keeps the feeder above head height without placing undue strain on the conductors due to the span being in the region of 14m from the shack to the antenna support pole.