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Antennas for use in the amateur bands are usually based on the fundamental antenna, ie the λ/2 dipole. Study of the usual textbooks on amateur radio reveals that many different forms of antenna have been developed. Basic information on the more common types only can be given within the scope of this manual.

Fig 7.11. λ/2 dipole antenna fed by coaxial cable. Length is approximately equal to 143/f metres

The dipole

The impedance of a λ/2 antenna at the centre point is roughly 70Ω (resistive). Thus this point could be coupled directly to the output of a transmitter by 70Ω coaxial cable, resulting in a good impedance match (Fig 7.11). This arrangement is known as a 'half-wave dipole' or simply as a 'dipole'. Lengths of half-wave dipoles are given in Table 7.1.

As mentioned earlier, the centre impedance of the dipole depends upon the height of the antenna and the proximity of buildings etc. The arrangement of Fig 7.10 may therefore be preferable.

However, if a 7MHz dipole was fed with power at, say, 14MHz or 28MHz, there would be an impedance mismatch, as the impedance at the centre would no longer be 70Ω. It would be much higher than this and moreover would not be resistive.

The exception to this is that the impedance at the third harmonic is around 90Ω. The only application of this in amateur radio is the use of a 7MHz dipole at its third harmonic, ie in the 21MHz band. The mismatch is then not excessive.

The dipole is a satisfactory antenna but, apart from the example quoted above, it is a single-band antenna. The common use of coaxial cable (unbalanced) to feed a dipole is convenient as the output of most transmitters is unbalanced, but consideration of the antenna itself shows that a dipole is balanced so that it should not be fed with an unbalanced cable.

Alternative and more correct arrangements are either the use of 75Ω twin cable between the antenna and the ATU, or a balance-unbalance transformer between the top end of the coaxial feeder and the antenna.

The balance-unbalance transformer, commonly called a 'balun', enables a balanced circuit to be coupled to an unbalanced circuit and vice versa. In one version, it consists of three tightly coupled windings on a small ferrite core, as shown in Fig 7.12.

The trap dipole

The trap dipole is a dipole having a parallel-tuned circuit or 'trap' inserted at a particular point in each leg. At resonance, the trap presents high impedance and therefore at the resonant frequency the length beyond the trap is virtually isolated from the centre portion. Below the resonant frequency the trap provides an inductive reactance which reduces the length of antenna required for resonance. The example shown in Fig 7.13 is known as the 'W3DZZ antenna' after the callsign of its originator.

Fig 7.13 gives the dimensions of the trap dipole; the traps are resonant at 7.1MHz. At 7MHz the system operates as a λ/2 dipole, the traps isolating the outer sections. At 3.5MHz it operates as a λ/2. the traps electrically lengthening the top. At frequencies above the resonance of the trap, the end sections are not isolated, but the traps do provide series capacitance. This enables the antenna top to resonate at odd harmonics of its fundamental and so at 14MHz, 21MHz and 28MHz the trap dipole functions as a 3λ/2, 5λ/2 and 7λ/2 antenna respectively. A reasonably satisfactory match to a 75Ohm feeder is obtained on each band. At 1.8MHz the feeders may be joined together at the transmitter and the system will operate satisfactorily as a top-loaded Marconi antenna against ground or a counterpoise.

The trap dipole has become very popular as a multi-band antenna in recent years because of the commercial availability of suitable traps. It must be appreciated that, as with all multi-band antennas, it is a compromise arrangement and as such will not give optimum results on every band.

The folded dipole

A dipole arranged as shown in Fig 7.14(a) has a conductor connected across a normal half-wave dipole and separated from it by a small distance. This configuration is known as a folded dipole. This is the simplest example of folding and it results in the centre impedance being multiplied by four.

Fig 7.14. (a) Folded dipole (b) Construction of a folded dipole from 300Ω ribbon feeder

This increase in impedance is the main advantage of folding. A folded dipole has an impedance of about 300Ω and so may be fed with 300Ω twin feeder.

The loss in 300Ω feeder is somewhat lower than that in 75Ω coaxial cable and so a folded dipole may be preferred if the feeder length is extremely long. In fact, as shown in Fig 7.14(b), it is possible to construct a folded dipole entirely of 300Ω feeder.

The vertical antenna

A vertical antenna offers the attraction of low-angle, omni directional radiation and is popular where space does not allow a long horizontal antenna.

The simplest form is a vertical radiator one quarter of a wavelength (λ/4) long (see Fig 7.15); the impedance at the bottom is 30-40Ω and so it can be fed by 50Ω coaxial cable.

The achievement of a satisfactory earth presents the major difficulty. A single earth rod, say 2m long, is unlikely to be satisfactory unless the soil has exceptional conductivity. Two or three such rods bonded together close to the bottom of the radiator may reduce the earth resistance.

Earthing problems with a vertical antenna may be virtually eliminated by erecting it over a perfectly-conducting surface, eg a large sheet of copper. This is known as a 'ground-plane antenna', but is only realisable at VHF (eg a λ/4 radiator mounted on a car roof).

In practice a satisfactory ground plane may be made by laying four to six radial wires about λ/4 long on the surface of the ground (they can be buried a few centimetres below the surface if more convenient). Alternatively, the radiator may be mounted at the top of a mast which uses the ground-plane radials as guy wires, insulators being introduced at the appropriate points as shown in Fig 7.16. A ground plane erected in this manner does in fact present a better match to 50Ω coaxial cable than does the conventional ground plane.

Traps may be inserted in a vertical antenna to enable it to be used on more than one band.

The end-fed antenna

This is probably the simplest antenna of all as it consists of a length of wire brought from the highest point available direct to the transmitter output. The wire can be straight but good results are often obtained with quite sharp bends in the run of the wire.

Optimum results are obtained with resonant lengths, a 40m long end-fed antenna operates on bands from 3.5MHz to 28MHz, while an 80m length enables 18MHz to be used. This arrangement, although often used, is liable to create breakthrough problems (Chapter 9) as the end of the antenna (ie a high-voltage point) is brought into the house; if the transmitter is not at ground level there may be significant unwanted radiation from the long earth lead. The use of an antenna tuning unit as shown in Fig 7.17 is advisable. The ATU should be preceded by a low-pass filter and SWR meter as shown in Fig 7.10.

The three-element beam

The Yagi antenna (or Yagi-Uda array, named after its Japanese inventors) consisting of a radiator and two parasitic elements (a reflector and one director) mounted on a suitable tower with provision for rotating it is known as a 'three-element beam'. The radiator is a dipole which must be about l0m in overall length for operation at 14MHz; thus physical size normally dictates that 14MHz is the lowest frequency for which the rotating Yagi is used.

The addition of a director and a reflector to the normal dipole has the effect of reducing the centre impedance to the order of 20Ω. Therefore in order to feed it with 70Ω coaxial cable the impedance must be increased. This can be achieved by folding the radiator which increases the impedance to about 80Ω or using some form of impedance-matching transformer between the feeder and the antenna.

The actual spacing of the reflector and director relative to the radiator have a significant effect on the characteristics of the system, such as the gain compared with a simple dipole, the front-to-back ratio and the amount by which the impedance is reduced.

Commercial three-element beams are widely used. These use traps for operation on 28MHz, 21MHz and 14MHz. As a result of the different spacing in terms of operating wavelength at these three frequencies, the change in impedance is not excessive, and feeding with 50Ω coaxial cable is an acceptable compromise.

The quad antenna

The quad antenna consists of a square loop of wire as shown in Fig 7.18. The side of the loop is approximately λ/4 in length and it is normally fed with 75Ω coaxial cable at the point shown. The loop can be mounted with a diagonal vertical and fed at the bottom corner; in either case the feed point is a current maximum and the performance is identical. In the configuration shown, the polarisation is horizontal. If fed from the side of the loop, the polarisation will be vertical.

Fig 7.18. A three-band nest of two-element quads (radiator and reflector) maintaining optimum spacing for each band

Parasitic elements may be added to form a beam antenna. The most popular is a radiator plus reflector (the two-element quad), but one or more directors may be added, depending on the frequency. Spacing between the radiator and parasitic elements are similar to the Yagi. Commonly, quads for 28MHz, 21MHz and 14MHz are assembled on the same mounting and rotating system and fed by 50Ω cable. In order to maintain the optimum spacing between the elements, the radiators, reflectors and directors for each band cannot be in the same vertical plane (see Fig 7.18).

The quad is made up of wire supported on light-weight spreaders of bamboo or glass fibre.

Front-to-back ratio and SWR are optimised on each band by adjustment of the tuning stub on the reflectors, but alternatively a reasonable compromise is often obtained by eliminating the stub and making the reflector about 3% greater than the radiator in length.

Commercial data suggest that the quad may give a slightly higher gain than the Yagi and a noticeably better front-to-back ratio. Its smaller turning radius is also often advantageous.

Low-frequency antennas

An effective resonant antenna for the LF bands, particularly 1.8MHz, requires a large space. Shorter lengths are often used tuned against earth in what is known as the 'Marconi-type antenna', as shown in Fig 7.19; the slightly different tuning arrangement should be noted. A common arrangement is shown in Fig 7.19. The earth should be a short connection to an earth spike and the use of the mains earth or the water-pipe system should be avoided.

Fig 7.19. Marconi antenna for use at 1.8MHz; additional loading at outer end is useful if length is less than λ/4

VHF and UHF antennas

Resonant antennas, as discussed in this chapter, are in general applicable to all amateur bands up to 1300MHz.

The Yagi, and to a lesser extent, the quad, are widely used at VHF and UHF because the much shorter physical length of a half-wavelength means that more elements occupy less space and so antennas of appreciably higher gain than at HF are possible. Several such antennas may be stacked, provided they are correctly matched and phased, to provide even higher gain.

Photo 7.6. A 12-element "ZL Special" beam antenna for 144MHz. It provides around 14dBd forward gain. Total length is only 10 feet

At even higher frequencies, parasitic elements are replaced by parabolic-shaped reflectors or 'dishes'; however, these are beyond the scope of the RAE.

Loading of an antenna

Shortness in length of a resonant antenna may be compensated to a certain extent by the addition of a small amount of inductance, for instance as shown in Fig 7.19. Another example is the effect of the trap inductance when the trap dipole (Fig 7.13) is used on the 3.5MHz band. This artifice is commonly used on whip antennas for mobile use and may allow a beam antenna to be made to fit into a smaller than normal space. The process of loading an antenna may degrade its properties.