08/09/2024
The Clansman Dipole
Below is a Clansman dipole. It comprises 2 antenna (Kevlar wire) elements graduated in meters, by adding markers at various spacings from 2 to 30 Mhz. The wire is wrapped onto double reels. The markers are designed to make it easy to set up in the dark with different markers at 1, 5 and 10M intervals. The reels also hold about 100 M of very nice string*. There is a dipole centre junction, (not a balun). The kit also comes with 30m of 50 Ohm Coax and a four wire counterpoise for improving ground reflection when operating the antenna against the earth.
This type of kit has traditionally been used by the military to create a number of balanced and unbalanced antenna for portable use. It accompanies the Clansman HF radio sets as standard and the radios come with a laminated chart detailing the required length for end fed and dipole antenna.
The chart simplifies the job of calculating the length of wire needed for any particular frequency.
In use, to erect the antenna, the operator reels out enough string to throw the whole wire element over an object such as a tree, and then ties the end of the string off. The required length of wire is then reeled off, fastening the appropriate length marker to the reel. This is repeated for the other side of the antenna and then each side is connected to the centre junction, and the coax before hauling it taught to form a horizontal, vee shaped or inverted vee dipole.
The inverted vee antenna is a simple and effective method of achieving high angle radiation for omni directional use and good in hilly countryside or woods. This is because the Signal is bounced between the f2 layer of the ionosphere and earth , in all directions, for between 0 and 900 Km.
For longer distances, mounting the Clansman dipole antenna horizontally will produce a bidirectional radiation pattern and if the height is set at a quarter wavelength it will achieve distances in excess of 1000km. The military user is supplied with additional equipment to aid construction.
There are a number of electrical issues that have been designed out of the clansman radios, to create this simple effective tool kit. The surplus wire affects the electrical length and the antenna lacks a balun, although to prevent interference from nearby transmitters the set is provided with a tunable filter instead. In amateur radio such an antenna would be cut to length and trimmed, then a reel added to make it portable and reusable. Several antennas would be needed to cover various bands and traps or switches could be added.
Amateur radio enthusiasts have different needs and might find difficulty in practice to bring the Standing Wave Ratio to an acceptable level. Using the published chart produces an antenna that is too long and therefore unmatched to the 50 Ohm output of the set. One way to overcome this is to use an Antenna Matching Unit (AMU or ATU) to Tune the antenna. As I don't yet have an ATU in my mobile setup, this time out I used an antenna analyser before hooking the antenna up to the radio, simply reeling in the surplus antenna wire until the analyser showed the lowest SWR.
Using this setup, I worked Peter, G3RE in Poole, Derek G3XWD in Staffs and Mike G4ICC in Leicester using a FT891 using 100Watts. Signals were conversation quality.
* very nice because it has been specially selected to slip smoothly across most surfaces, which decomplicates setting up and taking down the antenna.
The antenna reels come up on Ebay from time to time.
