I’ve never tried this type of aerial but it sounds interesting. It’s called a fan or multi band dipole and consists of several wire elements, each cut to a specific length to resonate on a different frequency. Basically, several dipoles fan out from the common feed point in the centre. Because each wire element is cut to a specific length, the antenna can be made to operate on specific bands. The dipole is often used on 80, 40, 20 and 10 metres.
One disadvantage of the fan dipole is that it’s not easy to setup on each band. Because each dipole is cut to a specific length, you must carefully adjust the length of each wire to achieve resonance on the desired frequencies. Once done, you have a cheap multi band antenna fed with one piece of coax.
At a particular frequency, the dipole tuned for that band generally presents a relatively low feedpoint impedance and takes most of the transmitter's power. The other dipoles generally present higher impedances, although interaction between elements still occurs. This means one antenna can work efficiently on several bands without traps, switching relays or separate feeders.
The following are approximate starting lengths for standard half-wave dipoles, calculated for frequencies commonly used by UK radio amateurs.
| Band | Frequency | Total wire length | Each leg |
|---|---|---|---|
| 80m | 3.65 MHz | 39.1m | 19.55m |
| 40m | 7.10 MHz | 20.1m | 10.05m |
| 30m | 10.12 MHz | 14.1m | 7.05m |
| 20m | 14.20 MHz | 10.1m | 5.05m |
| 17m | 18.10 MHz | 7.9m | 3.95m |
| 15m | 21.20 MHz | 6.75m | 3.38m |
| 12m | 24.94 MHz | 5.74m | 2.87m |
| 10m | 28.50 MHz | 5.03m | 2.52m |
These are individual dipole calculations, not guaranteed final fan-dipole dimensions. Nearby elements, insulation, installation height and ground conditions can change resonance.