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.
Frequency to wavelength formula
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.
As the feedpoint of the dipole is in the centre we need to divide this figure by two to find the length of each wire.
NOTE: don’t learn the hard way, always allow a few inches more than you actually calculate.
It’s a lot easier to shorten the wire than it is to lengthen it!
Constructing a dipole
As 14.175MHz is in the centre of the 20m band this is used as a base for this example calculation.
Using the formula above: 468/14.175 results in 33.01 feet total length or just over 16.5 feet per wire.
Allowing some extra wire for trimming initially cut each wire to 17 feet or slightly longer if you’re not feeling confident with the calculation/measuring.
Connect one end of each to the coax feed and seal this against moisture as this shouldn’t need to be touched again.
Once the dipole has been raised into position an antenna analyser can be used to find the resonant frequency. This can be achieved using a simple SWR meter but it takes considerably longer to do so.
If everything performs as expected, because the wires are initially cut slightly longer than required, the resonant frequency will be below the chosen center frequency.
Next, trim an inch from the free end of each wire (BOTH WIRES MUST be trimmed evenly to maintain balance) and the resonant frequency is re-checked.
This process has to be repeated until the SWR and resonant frequency are correct.
The final length after trimming can vary quite substantially from the formula.
There are a lot of variables which affect the final length of the wire. Things such as antenna height, other objects located nearby, ground soil composition and resistance, type of wire used etc.
Fan dipole construction
Simply repeat the calculation & cutting process as above for as many pairs as you require.
Tuning the dipoles really does require the use of an antenna analyser. Without one, you can spend many hours trying to get things right!
As the single dipole example above, connect your pairs to the centre and seal against moisture. Raise the pairs into position keeping the wires spaced from each other.
To do this you can use plastic or other insulated spacers to “hang” the shorter wires from the longest wire. Of course this wire must be strong enough to carry the weight of the spacers and other wires.
I find that making the dipoles into a shallow inverted V shape really helps keeping the wires separated as can be seen in the picture. The ends of the wires are attached to different points along my fence. Making adjustments to a pair means I only have to release that pair, rather than lower the complete assembly.
Some people recommend tuning one dipole at a time, then connecting the next and tuning again, etc.
I think my way works quite well.
Start with the longest pair for the lowest frequency and using the analyser tune it for approximately the center of the desired band you wish to use.
Carry on with the remaining pairs working from longest to shortest, or lowest to highest frequency.