A standard rule of thumb in antenna design is: an antenna can be made more broadband by increasing the volume it occupies. Hence, a dipole antenna can be made more broadband by increasing the radius A of the dipole.
As an example, method of moment simulations will be performed on dipoles of length 1.5 meters. At this length, the dipole is a half-wavelength long at 100 MHz. Three cases are considered:
- A=0.001 m = (1/3000th) of a wavelength at 100 MHz
- A=0.015 m = (1/100th) of a wavelength at 100 MHz
- A=0.05 m = (1/30th) of a wavelength at 100 MHz
The resulting S11 for each of these three cases is plotted versus frequency in Figure 1 (assuming matched to a 50 Ohm load).
Figure 1. Magnitude of S11 for Dipoles of Varying Radii.
The first thing apparent from Figure 1 is that the fatter the dipole is made, the larger the bandwidth becomes. For instance, if the bandwidth is measured as the frequency range over which |S11|<-9 dB, then the bandwidths are 6.5 MHz, 14 MHz, and 24 MHz, for the blue, green and red curves, respectively.
Secondly, the fatter the dipole gets the lower the resonant frequency becomes. In other words, if an antenna is to resonate at 100 MHz, the resonant length decreases as the dipole gets fatter.