The term “mast” is used in this site specifically to refer to a relatively light-weight structure which is held in position by “guy ropes” – these are properly known as “stay cables”. For broadcast sites with very tall masts the base normally tapers and rests on a bearing but Post Office practice seems to have varied. For microwave sites, due to the need to mount relatively large dish antennas, square-section masts were normally used. In many cases masts were used as an interim measure only – the capacity is limited to a relatively small number of dishes. One early mast which remains in use is at Thrumster.
Little or no “evolution” occurred with the design of masts in the PO/BT microwave network. The same cannot be said for towers where various factors influenced the design such as the introduction of horn antennas – and the subsequent return to exclusive use of dishes – the tendency for sites to require many antennas facing in the same direction, but not to use a structure which was higher than required, various attempts to adopt a universal design and improved knowledge of structural engineering.
A “tower” is self-supporting with no guys/stays. Some of the towers used in the PO/BT network are very tall, some are short and many are of very large cross-section. Whilst a significant proportion are either modular or based on a small number of design patterns there are also some unique examples. In the later years, from the 1970s onwards, three designs were favoured, known as the “Type 4A” – a lightweight tower used for the “Highlands and Islands” sites, “Type 5A” – used at most other 1970s sites and produced in “wide” rather than “tall” form, and “Type 8A” – used at most of the 1980s sites. The earliest structures tended to be of a specific pattern for a particular route: the London to Birmingham 900 MHz television link used relatively short towers with the equipment in a cabin at the top, the Manchester to Kirk o’Shotts 4 GHz link had a relatively light “pylon” design with provision for pairs of dishes and the route between Bristol and Plymouth used a square tower with tapered sides – effectively a tall and narrow “pyramid”.
The introduction of horn antennas created some difficulty – the waveguide needed to run vertically to ground below the horn and the antennas were large, heavy, and required a stable mounting. Various “bolt on” supports were used with pylon-style towers but the so-called “standard tower” emerged as a new design with different size sections and “steps” where the horn antennas could be mounted. In reality this was a standard set of components and almost all of the “standard towers” were one-off designs to suit the requirements of the individual site and routes.
A further response to the use of horns was the “daffodil tower” of modular form with the waveguide supports acting as the main support structure. One example was built, at Sibleys, though not without problems, and then the use of horn antennas was reviewed. Two minor examples of the “daffodil” were built at sites in Northern Ireland and the design was refined into the modular “Type 5A” tower.
A small number of sites were provided with concrete towers either “from new” or as a later replacement. Most were variations on the design used at Stokenchurch (known as the “Chilterns design”). The towers in London and Birmingam were individual designs as were those at Purdown (Bristol), Morborne Hill (Peterborough) and Tolsford Hill (Folkestone) all of which replaced existing steel towers.
In general, as a Government Department until 1969, the Post Office did not seem to be particularly subject to planning controls however there seems to have been a degree of sympathy to the environment when placing sites in what would now be considered “sensitive” locations – where a site was on particularly high ground the tower was often very short (in some cases making it unsuitable for expansion in later years) or “hidden” in woodland. There are, of course, a number of counter-examples.