These sources must be continuously calibrated, so their estimated contributions can be subtracted out, and the result multiplied by the correct, experimentally-determined conversion factor. I suspect that it picks up moisture as the building dries out, because deterioration seems to set in very quickly. In my opinion type D cable should not be installed behind the plaster in a new building. I suppose I should have set up some sort of comparative test in which the different types of cable were (a) subjected to a pretty violent installation by disgruntled electricians on piece rates and (b) installed by placid electricians keen on transcendental meditation. Class-A amplifiers generally use only one device, unless the power supply is set to provide both positive and negative voltages, in which case a dual device symmetrical design may be used. Remember that to the average electrician coax is coax, and if he has half a reel left over from a surveillance-camera installation he will use it for TV downleads.

Many and varied are the interference problems that can be cured by replacing cheap coax or flyleads with CT100 (Ed: WF100) cable. This isn't the place to go into cable impedance, standing waves and what have you but, take my word for it, impedance bumps are a Bad Thing! Tiny radial cracks appear in the conductive coating and, since the braid is very skimpy, impedance 'bumps' are likely. One such cable has no braid, just a few strands of wire that run longitudinally, and some sort of shiny (and allegedly conductive) coating that's attached to the inside of the outer sheath. Its one of the best indoor choices around, but its too small and light to take outdoors. If you come across an unfamiliar cable, I suggest that you take a sample away and test it. But I didn't. Take it as read: if coaxial cable is squashed, kinked, twisted, scorched or stretched, its performance will suffer.

The copper foil in type A and B cable can crack and, of course, the inner core of all cables will eventually snap. Visible evidence is presented by a yellowing of the dielectric and dark discoloration of the copper. Such cable is easy to spot, because it has a solid dielectric. Special flexible coaxial cables are available, with seven-strand inners, solid dielectric and a dense braid of fine wire. Additionally, optocouplers with internal base-to-pin 6 connection (there are also optocouplers without it), the base may be left unconnected. On the left hand side the circuit around T1 forms the 1000 V boost converter, assuming that in the end 500 V capacitors are used. Sometimes electricians will use oddments of coaxial cable left over from previous jobs. This is where you are likely to encounter cable that's intended for baseband video use. So, for this reason, the figures for cable types A and B are probably slightly worse than they would be in practice. Downleads inevitably pass alongside or at least near mains cables so, given digital terrestrial TV's susceptibility to impulse interference, type D cable is simply not suitable. The signal losses are much worse at higher frequencies so, if you are quoting for the conversion of a system from Group A analogue to wideband digital, beware!

Idea in the simple electromechanical relay operation is that the relay output contacts are normally open, typically a spring keeps them in this position. Two of them are Normally Open, so they are switched on when a control signal is applied. In transmit, the receive switch is turned off and the "transmit switches" are turned on. A SPDT (1P2T) ON-ON switch is a single pole, double throw switch with three terminals that connects one main terminal to one terminal in one position, and that main terminal to the other terminal in the other; a SPDT ON-OFF-ON switch has an added position in which none of the poles are connected (this is sometimes described as SPCO). In terms of power-handling capability, a ferrite switch is typically somewhere between a rotary electromechanical switch and a PIN diode switch, and has been used in applications such as fire-control radar systems that use multiple antennas for identification. It's very lossy at UHF, and utterly hopeless for satellite use. A TV dish with an offset angle of around 20° for example would have to always aim 20° below the satellite.