Multiband Antennas

There is no reason that the ½ l length for dipoles needs to be applied as a rigid rule. The same is true for verticals. The only issue is that even multiples of the original length lead to high impedances, making it difficult to deliver power to the antenna. With larger sizes more lobes, with intervening nulls appear. Below is an example of what can be expected when using a doublet (essentially a dipole - a balanced antenna with two equal length, opposite direction horizontal arms) cut for a low frequency, on higher frequency bands. The plot is for free-space radiation, so for more realistic mountings, the nulls evident in the patterns will not be as deep on long-haul paths as they will contain some vertically polarized energy. It is also assumed that the antenna may require the use of low-loss open wire feedlines or a remote tuning unit to provide matching to the transmitter.

There are also some advantages in operating ground-mounted vertical antennas at lengths beyond the standard 1/4 l. As you can see from the example below, the longer lengths produce power at lower elevation angles, which is desireable for long-range contacts. This is also a desireable feature for the elevated vertical antennas used with VHF/UHF repeaters as the energy can typically be confined to the area where it does most good - at the angles that typically exist between mobile and base stations. There is still the issue of impedance matching - but for single band vertical antennas this can often exist as a fixed value L-C circuit at the base of the antenna.

Antennas can be designed to present a good impedance match for a number of frequency bands. The simplest of these is the multi-wire dipoles designed for the old commonly used HF bands of 80, 40, 20, 15, and 10 meters. Since at even harmonics each of these wires appears as a high impedance, it is possible to just parallel a number of wires from the common center feedpoint and have a reasonable match on any one of the bands. The 80 meter dipole is virtually “invisible” if operated at 40 meters, while a 40 meter dipole on 80 meters looks mostly like a capacitor, providing a “shortening” for the 80 meter wires. Thus we have the multiwire antenna. No wire is used for 15 meters, since it presents an odd harmonic response. Instead, the 40 meter dipole is operated on its third harmonic (see 1½ l dipole example above), usually giving a match with an SWR of less than 3:1. This can be improved by adding small "capacitance" wires to the 40 meter antenna to tune the 3rd harmonic resonance to the 15 meter amateur band.

Another approach is to use “traps”, or parallel LC resonant circuits to isolate outer parts of the antenna so that resonance is obtained for the remaining inner pieces. Off resonance, the trap acts as inductive loading, reducing the size of the adjacent piece needed for the next lower band. Traps need to be constructed with a high Q to minimize losses. This implies large (read heavy) components, which may not be suitable for antennas raised as dipoles, but may be acceptable for inverted vee configurations. The trap vertical has long been a mainstay in amateur antennas, as a rugged trap assembly can easily support the remaining tubing above it (see 10-40m vertical on right). Newer commercial designs take advantage of the longer lengths of tubing available to make some of the bands operate at something other than a 1/4 l element to reap the low elevation angle advantages pointed out earlier, and in these cases not all coils on the antenna may be acting as traps.