The antenna system shown in Fig 10 consists of a group of center-fed dipoles, all connected in parallel at the point where the transmission line joins them. The dipole elements are stagger-tuned. That is, they are individually cut to be A/2 at different frequencies. Chapter 9 discusses the stagger tuning of dipole antennas to attain a low SWR across a broad range of frequencies. An extension of the stagger tuning idea is to construct multiwire dipoles cut for different bands.

A multiband antenna that does not require a lot of space, is simple to construct, and is low in cost is the G5RV. Designed in England by Louis Varney (G5RV) some years ago, it has become quite popular in the US. The G5RV design is shown in Fig 8. The antenna may be used from 3.5 through 30 MHz. Although some amateurs claim it may be fed directly with 50-Q coax on several amateur bands with a low SWR, Varney himself recommends the use of an antenna tuner on bands other than 14 MHz (see Bibliography). In fact, an analysis of the G5RV feed-point impedance shows there is no length of balanced line of any characteristic impedance that will transform the terminal impedance to the 50 to 75-Q range on all bands. (Low SWR indication with coax feed and no matching network on bands other than 14 MHz may indicate excessive losses in the coaxial line.)

The simplest multiband antenna is a random length of #12 or #14 wire. Power can be fed to the wire on practically any frequency using one or the other of the methods shown in Fig 1. If the wire is made either 67 or 135 feet long, it can also be fed through a tuned circuit, as in Fig 2. It is advantageous to use an SWR bridge or other indicator in the coax line at the point marked "X."

For operation in a number of bands, such as those between 3.5 and 30 MHz, it would be impractical for most amateurs to put up a separate antenna for each band. But this is not necessary—a dipole, cut for the lowest frequency band to be used, can be operated readily on higher frequencies. To do so, one must be willing to accept the fact that such harmonic-type operation leads to a change in the directional pattern of the antenna (see Chapter 2). The user must also be willing to use so-called tuned feeders. A center-fed single-wire antenna can be made to accept power and radiate it with high efficiency on any frequency higher than its fundamental resonant frequency and, with a reduction in efficiency and bandwidth, on frequencies as low as one half the fundamental.

The power gain and directive characteristics of electrically long wires (that is, wires that are long in terms of wavelength), as described in Chapter 2, make them useful for long-distance transmission and reception on the higher frequencies. Long wires can be combined to form antennas of various shapes that increase the gain and directivity over a single wire. The term long wire, as used in this chapter, means any such configuration, not just a straight-wire antenna.