A Multi-Dipole, also called a multi-band fan dipole, is the antenna consisting of a number of dipoles with a common feed point. In some versions, the arms of different dipoles spread (what explains the term "fan") but sometimes they are kept parallel. I am focusing here on the latter version. That's why I prefer the name Multi-Dipole rather than the Fan Dipole. I am discussing here a 5 band version for 20 through 10 m bands.
When you design a multi-band dipole, you can quickly notice that its bandwidth depends on the distance between the parallel dipoles. If the distance is small, the bandwidth (especially on the higher bands) is also small. Satisfactory results can be achieved if the distance is set to 0.3 m. But it means that the distances between the 20 m band dipole and 10 m band dipole needs to be 4 x 0.3 = 1.2 m. Such an antenna would look rather awkward, and most designs available on the Internet show much smaller distances.
Fortunately, there is a relatively simple trick you can use in the Multi-Dipole with closely spaced wires to increase its bandwidth. It will grow if you use split feed point (SFP). SFP features are explained in the SFP Cubical/Diamond Quad page.
Let's compare the Multi-dipole with a single feed point and the wires 0,1 m apart from one another as shown below...
... with the SFP Multi-Dipole, shown below.
The individual dipoles are fed from the bottom and from the top, as shown in the picture. A transmission line of characteristic impedance 100 ohms is needed to connect the bottom and the top feed points. Such line can be easily created with two 50 ohms coax cables, as shown below.
The shields of parallel coax cables are connected together, and their inner conductors form a 100-ohm symmetrical line. A coax cable with VF=0.66 was assumed in the antenna simulations.
The plot below presents the SWR of the classic Multi-Dipole and SFT Multi-Dipole.
The difference in the 10 m band is striking!
These simulations were done in free space. But I also simulated SFT 10 m above ground level and compared the SWR with the one in free space. See below.
The difference is not significant.
The SFT Multi-Dipole is a little longer than the half-wave dipole, so its gain is a split dB better. SFT Multi-Dipole feed point impedance is close to 450 ohms, so a 9:1 balun is needed between the antenna and the coax. Alternatively, you can use a 450-ohm window line as the feeder and add a 9:1 balun near your transmitter.
Click here to download this antenna models. Please note that I used NEC-5 to get the SWR plots and because of that the SFT_multi-dipole_NEC-5.inp file is included in the model package. Additionally, I created models suitable for NEC-2 based simulators: 4nec2, EZNEC and AutoEZ+EZNEC. I did not create a file for the MMANA-GAL because this simulator does not support transmission lines.