The basic design of the antenna is a fairly simple folded dipole.
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.
So what do we need, and how do we put it together?
Every radio frequency has a given length (wavelength). The early MW radio receivers used to have the wavelength on the tuning dial rather than the frequency. For example Radio Luxembourg was marked as 208m which equates to 1439 KHz, therefore every antenna has a given length at which it operates at it’s most efficient.
To calculate the length of a dipole the standard formula of 468/f is used, where f is the frequency in MHz. For example, a 14.100 MHz radio wave in free space is approximately 68.6 feet long. It therefore follows that half a wavelength is 34.3 feet in length. Because the RF energy travels about 5% slower in copper wire our formula gives us 33.19 feet for the total length of our dipole.