The key constraints on antenna performance are the height above the ground and the working space available in the attic. A survey of modern housing designs gave the following typical parameters:
|House size Ridge length |
Two bedroom Mews 3.6m (12ft)
Three bedroom Semi 4.5m (15ft)
Three bedroom Detached 5.4m (18ft)
|Storey Ceiling height |
Roofs usually slope at 25 to 30 degrees and are clad in concrete tiles or slate. A typical roof has trusses spaced 0.6m apart with the main struts angled at 60 degrees to the horizontal. The apex can be between 1.5 and 2.0m above the ceiling, access is normally through a ceiling hatch approximately 0.5m x 0.5m in size.
Modern houses have pressurised hot water and central heating systems. Older houses may be gravity fed with plastic water filled header tanks in the attic which take up some of the available room but these attics tend to be more spacious. There will be daisy chained wiring on the floor of the attic for the room lights immediately below, there may also be hot and cold water feed pipes to a shower unit if it was installed after the build. In the UK any power sockets will be supplied by a ring main in the floor of the room below together with the lighting circuits for the level below that.
Any plastic water tanks present will have a minimal effect on the antenna propagation because HF frequencies can penetrate through 1m of pure water with an attenuation of only 0.2dB (based on the formulae in section 3.6 of Ref.1).
Experiments carried out by the North Cheshire Radio Club have shown that HF attenuation through concrete or slate roofing tiles is less than the experimental error of 0.4dB, where 6dB corresponds to 1 S point on the signal strength scale.
The recommended antennas are largely immune to conductors in the attic but experimental methods of determining if there is interaction are given in the ‘practical’ section together with the techniques for dealing with the problem. The conductors and pipework in the floors of the rooms below are usually far enough away to be ignored.
Ref.1 – ARRL 1991, Antenna Book, by Hall,G. et al; Pub. The American radio Relay League, Newington, CT USA 06111.