There are two sources of radio interference induced by antenna systems. One occurs from strong signals radiated by either the system’s own transmitter antenna or from a co-sited system operating on a close neighbouring frequency. The other source is intermodulation, ‘rusty bolt effect’, the mixing of two or more signals to produce the interfering frequency.
One solution to the problem of direct radiation is to space the antennas so that there is sufficient isolation between them. Because minimum radiation is present immediately below and above the ends of the elements of vertically polarized antennas, maximum isolation occurs when such antennas are mounted in a vertical line. Maximum radiation, and hence minimum isolation, occurs when antennas are mounted side-by-side. The degree of isolation depends on the spacing but a figure of 40–45 dB between a transmitter and a receiver antenna, and 20–25 dB between transmitting antennas, should be the target. To achieve 25 dB isolation requires a vertical separation of 0.9 wavelengths between the centres of vertically polarized dipoles; 45 dB requires a vertical separation of 3 wavelengths. The spacing for horizontal separation of vertically polarized antennas is 2.5 wavelengths for 25 dB isolation and 25 wavelengths for 45 dB. To preserve this isolation the feeder cables should also be separated over their routes.
A precise method of controlling the RF isolation between mutually sited systems where the frequencies are moderately spaced is for several systems to share one antenna by using multi-coupling techniques (see Section 19.9).
Any two or more RF signals applied to a non-linear device intermodulate, that is, they combine to form additional frequencies. Table 19.2 lists the combinations for two input frequencies. It must be remembered that the side frequencies produced by modulation of the original carriers will also be present.
Table 19.2 Low order intermodulation products2nd A+ B 5th 3A+ 2B 7th 4A+ 3B A− B 3B+ 2A 4B+ 3A 3A− 2B∗ 4A− 3B∗ 3rd 2A+ B 3B− 2A∗ 4B− 3A∗ 2A− B∗ 4B+ A 5A+ 2B 2B+ A 4A+ B 5B+ 2A 2B− A∗ 4A− B 5A− 2B 4B− A 5B− 2A 4th 2A+ 2B 6A+ B 2A− 2B 6th 5A+ B 6B+ A 2B+ 2A 5B+ A 6A− B 2B− 2A 5A− B 6B− A 3A+ B 5B− A
3 A− B 4B+ 2A
3B+ A 4B+ 2B
3B− A 4B− 2A
4B− 2B
3A+ 3B
3A− 3B
3B− 3A
∗ indicates in-band products. No eighth-order products fall in-band but ninth-order in-band products are produced by:
5A− 4B
5B− 4A
A clue to the source of interference will be obtained from examination of the relationship of all frequencies produced on site, and also those produced nearby. That the side frequencies produced by modulation must be considered was proved in an intermodulation situation where the interference was only received when a nearby band III television transmitter radiated a peak white signal.
Corroded joints in metalwork close to the site, receiver RF stages subjected to very large signals and transmitter output stages are all non-linear devices and can create intermodulation.
Regular maintenance of the metalwork around the site reduces the chances of intermodulation from this source, but it is difficult to see and eliminate all corrosion on an inspection. Should interference from this cause be suspected after elimination of the other sources, a successful method of locating the offending joint requires a receiver tuned to the interfering signal and fitted with a whip antenna. While receiving the interference, a very sharp null in signal strength occurs when the tip of the whip points directly at the source of the signal.
Receiver RF amplifiers are usually designed to cope with small signals. When a strong signal radiated from a nearby transmitter is received the stage may well be overloaded and driven into nonlinearity, or even into a blocked state where the receiver is effectively dead and may take some seconds to recover after the signal is removed. In the non-linear state the receiver is in an ideal condition to create intermodulation. If this situation is suspected a method of identifying it is to install a variable RF attenuator in the antenna feed to the suspect receiver. With zero attenuation the interference will be received but, if the receiver is the cause, increasing attenuation will produce little reduction of the interference until a point is reached where the interference suddenly disappears – the receiver at this point has reentered a linear state. If the interference reduces gradually to zero with increased attenuation, the source is elsewhere.
Most amplifiers are designed to be linear, that is, the output signal level will follow that of the input signal. However, with a sufficiently high input signal overloading occurs, the amplifier becomes non-linear and compression of the signal results (Figure 19.2). The point where 1 dB of compression occurs is a commonly referred to amplifier parameter. If the input level is increased further the gain of the amplifier is reduced until saturation is reached when the output level can no longer increase.
At the onset of non-linearity harmonics and intermodulation, if any other frequencies are present, are produced. The strength of these rises rapidly with an increase of input, and the point where an extension of the almost linear portion of their curve crosses an extension of the linear gain line of the amplifier is the third-order intercept – usually it is only the third-order products which are considered for this purpose.
Third order intercept point
SaturationThe class C operated output stages of transmitters are by design non-linear. Strong signals from a neighbouring transmitter or antenna applied via the antenna feed will mix with the transmitted frequency to create intermodulation products. Increased isolation between antennas and feeders may be the simplest remedy. Alternatively, a circulator or filters may be connected to the output of the transmitter,
Port 2and the possibility of direct radiation from transmitters must not be ignored.
A circulator is a uni-directional device with either three or four ports as shown in Figure 19.3(a). A signal entering at port 1 will leave at port 2 but not at ports 3 or 4. Similarly, a signal entering at port 2 will leave at port 3 but not ports 1 or 4. Circulators are used for combining the outputs of transmitters for application to a single antenna but if a three-port circulator is connected as in Figure 19.3(b) it forms an isolator. Typical isolation over a 1.5% bandwidth is 20–30 dB with an insertion loss of 0.7 dB.