[sdiy] re: generators/antennas

[Trevor Page]

> Lincoln Fong wrote:
> 
> > Whilst I am familiar with the idea of 'back emf' in a generator there is
> > something I have never understood which Im sure someone can clear up.
> >
> > I once heard that Radio and TV stations can judge audience figures by the
> > amount of power 'absorbed' ie the Watts or MWatts they have to pump out. On
> > one level this makes sense. A popular station would need only the weakest of
> > transmitters if this were not so. But can the resonance of millions of tuned
> > circuits really indicate power usage that accurately? And if not then how do
> > they judge audience figures? I don't really buy the argument that grid power
> > usage can accurately reflect these things when so many programmes start at
> > exactly the same time.

> > I once heard that Radio and TV stations can judge audience figures by the
> > amount of power 'absorbed' ie the Watts or MWatts they have to pump out. On
> > one level this makes sense. A popular station would need only the weakest of
> > transmitters if this were not so. But can the resonance of millions of tuned
> > circuits really indicate power usage that accurately? And if not then how do
> > they judge audience figures? I don't really buy the argument that grid power
> > usage can accurately reflect these things when so many programmes start at
> > exactly the same time.

No, this is nonsense. Receivers do not 'pull' RF from the transmitter, nor is RF 'attracted'. RF is launched from the transmitting element and is absorbed (and in some proportion reflected) by whatever it eventually bumps into. Imagine the RF radiating away from the antenna uniformly in all directions (which it doesn't in practice, and we wouldn't want it to, from a real antenna, but hey). The signal continues to radiate outwards in all directions getting weaker as it does so, and gets mainly absorbed by the earth and other objects eg buildings, and into the ether, and a tiny, tiny proportion of it will be absorbed into antenna systems or indeed any bit of metal the RF might encounter. 

Someone near to the transmitter 'stealing' power is a different story though I guess, if whatever they implement to actually absorb the power is elaborate and near enough to cause a significant mismatch in the antenna system or something... especially if they physically attach something to the radiating element. I'm guessing 'ere.

Audience figures for television are measured by taking data from a small sample of the population (i.e people who are paid some money to log what they view!) and then taking that figure as representative of the entire population. Normally special set-top boxes are used for this I think. 

Trev 









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