AERIALS AND RECEIVERS
For the purpose of this chapter we shall regard a receiver as that section of a TV or video recorder installation concerned with the selection, tuning, filtering, amplification and demodulation of transmitted TV signals, culminating in the deliverance of the standard 1 V video waveform, and a 0 dB (0.775 V r.m.s.) baseband audio signal.
AERIALS
The first and one of the most critical links in the receiving chain is the aerial. In effect it forms the first tuned circuit of many, and its performance is crucial to the recording and display of good pictures. The basic pick-up element is the dipole, consisting in practice of a metal rod, divided at its centre by an air gap of about 20 mm for connection to the transmission line. Its overall length is approximately half that of the wavelength on which optimum reception is required. The impedance at the centre is approximately 72 Ω, a reasonable match to the 75 Ω coaxial cable used to link the dipole to the r.f. input of the tuner, whose characteristic input impedance is likewise 75 Ω. Normally the centre conductor of the coaxial cable is connected to the upper half of the dipole and the outer (screening) braid to the lower half.
Parasitic elements
The basic half-wave dipole is omnidirectional, and this can be a disadvantage in terms of susceptibility to interference and the pick-up of unwanted signals. To overcome this, and to add some useful gain, further elements are usually fitted. The H-type aerial, much used on VHF band I, consists of a half-wave dipole and a reflector: a second and slightly longer metal rod. The reflector is mounted one-quarter or one-eighth of a wavelength behind the dipole. It has no electrical connection with the dipole, but influences the dipole impedance and its directivity. By reflecting an in-phase signal back to the dipole an improvement in gain of some 3 dB is made by the reflector for signals in the ‘forward’ direction, while signals arriving from the rear are attenuated; a front-to-back ratio of 9 dB is typical of an H-type aerial.
See Fig. 3.1 for an explanation of dB ratios, and Table 24.4 for conversions.
Further gain and directivity can be gained by adding directors in front of the dipole. They have the effect of concentrating the signal on the dipole element, and up to sixteen may be fitted to high-gain aerials in a Yagi configuration, Fig. 3.2. Here the reflector takes the form of a mesh or grid for high gain and good back-to-front ratio. The dipole is a folded type for greater bandwidth and better impedance matching to the coaxial feeder; the presence of parasitic elements tends to reduce dipole impedance. The polar diagram in Fig.
3.2 gives an idea of the directive properties of the multi-element Yagi aerial.
Grid and log-periodic aerials
For use in areas of high signal strength the grid aerial is neater and more compact, consisting of a grid or mesh reflector mounted behind one or more folded dipole collectors, typically in a bow-tie shape. It offers great discrimination against signals arriving from the rear.
A log-periodic aerial has a series of dipoles, graduated in length (and thus resonant frequency) mounted along a dual boom which also acts as a transmission line to carry the signal to the downlead feeder. Its characteristics are relatively low gain, great bandwidth and freedom from side-lobes in its polar response. It does not have the directivity of a Yagi type, and is the least common configuration in domestic reception.
Bandwidth
As a ‘tuned circuit’ a receiving aerial has a certain bandwidth, determined by its physical characteristics. For reception from UHF transmitting sites in the UK an aerial bandwidth sufficient to cover all four or five local channels is required; with few exceptions the signals from each BBC/ITV site fall within one of the aerial group- ings given in Table 2.7. Things can be far otherwise with Channel 5 as Fig. 2.8 and Table 2.8 show.
FEEDERS
The transmission line between aerial and tuner is an important component. For minimum loss, thick coaxial cable should be used: semi-air-spaced coaxial cable is best, though cellular-polythene spaced types are a good compromise between performance and cost in areas of good signals and where the cable run is not too long. The performance of the feeder (and other distribution components like amplifiers) is particularly critical where teletext receivers are in use. Short-term reflections due to poor cable routeing and mismatch at terminations and connections will upset text reception and lead to the display of blanks and errors in the characters and graphics.