is that at which any increase in height brings no increase in gain. This is usually between six and twelve feet above the ridge of the average house. In strong signal areas this can often be below ridge height or even in the loft.
The cause of multipath, as the name implies, is the arrival of signals at the aerial from more than one signal path. All signals arrive simultaneously, some are absorbed by the building, others pass over and are then reflected back to the aerial, arriving later because of the delay. These signals combine with the direct one to form your total signal but, because of the delay, the reflections give a false level and are unused by the receiver. This shows as lack of base, too sharp treble, imprecise sound stage and general lack of lustre to the sound of your tuner. These symptoms show the presence of multipath. In extreme cases a "birdies" effect or "twitter"may be heard in the background, often accompanied by large variations in day to day signal levels. Especially suspicious are large variations in signal level coinciding with tree movements. The only two ways to cure this multipath are, to move away from the reflecting surfaces, or reject the offending reflections. In practise, once optimum height is achieved, the only way out of the problem is to use a well designed aerial, with good back and side rejection and high forward gain.
Cable quality can drastically affect the signal between the aerial and its arrival at the tuner. Super low loss double screened CT100 loses only 0.6 dB per 10 metres @ 100MHz. Cheaper cables with no solid copper screen can lose up to 30% more, and are prone to pick up stray signals and ignition or electrical interference.
Diplexers and Triplexers
These devices combine signals, FM, TV, and DAB, into a single downlead which can be separated on arrival, but are lossy. Separate lines are less lossy and avoid inter unit problems.