Microwave fundamentals

The electromagnetic spectrum is sometimes referred to as “DC to light” – low frequency radio waves can cover a vast area from a single transmitter – for example the Radio 4 Long Wave transmitter on 198 kHz at Droitwich can be received in most of the UK. As the frequency increases the effective range reduces and the behaviour becomes similar to light – any obstruction blocks communication and antennas operate much like a mirror, allowing the signal to be concentrated in a narrow beam.

There is some ambiguity around the terms “microwave” and “SHF” (super high frequency) and this site abuses the terms for simplicity. Some sources regard the range 300 MHz to 300 GHz as “microwave” – the wavelength ranges from 1 m to 1 mm between these frequencies. The strict definition of “SHF” is between 3 GHz and 30 GHz (10 cm to 1 cm) and most of the bands used for Post Office/BT links were within this range. Some early links were at 2 GHz, with the first operational television link at 900 MHz – these bands are properly “UHF”.

Many of the devices and techniques used in microwave links were developed originally for radar and improvements in technology have allowed higher frequencies to be used. The 1950s to 1970s PO network operated mostly at 4 GHz and two bands known as “Lower” and “Upper” 6 GHz – the exact frequencies are here – but some use was made of 11 GHz for short-range links and this band was then used in the 1980s to provide digital links.

A characteristic feature of modern microwave links is the “dish” antenna – much as the reflector in a torch or a car headlamp focuses light in a relatively narrow beam a dish consists of a metal surface, normally in the form of a parabola, with the radio signal introduced at the focal point. Modern dish antennas provide a gain in the order of 40 dB and the signal is concentrated in a beam only a few degrees wide, with virtually no radiation to the rear and very little at the sides. Point-to-point linking is therefore possible with the same frequency used on multiple paths, even from the same site.

For a period in the 1960s “horn” antennas were favoured since they could operate simultaneously over more than one band. The Post Office network developed with horn antennas operating at 4 GHz and 6 GHz and with the expectation that 11 GHz could be introduced later. Although the horn antenna had better performance than dishes at the time of introduction and the use of a single antenna for multiple bands was a benefit it was found in practice that the “large” horn in particular created challenges for the designers of the support towers and their use at 11 GHz was found to be troublesome. By the late 1960s improvements in dish design mean that there was little justification for using horns and many were replaced by dishes in the 1970s to 1980s. (In some cases they remained in position due to the physical difficulty in their removal.)

At lower frequencies coaxial cable is used to connect antennas with transmitters and receivers but the loss increases dramatically with frequency and above about 3 GHz “waveguide” is used. This is essentially a hollow tube, the dimensions are related to the wavelength(s) to be carried, and the signal effectively reflects from the sides of the waveguide in the direction of travel. Early waveguide was rigid with special couplers to change direction and the design of towers and antenna mountings allowed for straight vertical and horizontal runs as far as possible. This was particularly a factor when horn antennas were used – the waveguide was circular and needed to pass vertically almost to ground level.