Digital Tv question; Aerials ?

[confus-ed]

A friend of mine is in some flats & has no outdoor ariel (the building has cable but he doesn't want to pay) - so I went around complete with my laptop & my digital tv add on, & we eventually concluded (after a bit of balancing on the sofa, & cries of 'left a bit' etc) yes he can get signal, but only just .. so methinks he wants a 'high gain' aerial.

But what consitutes one ? (a high gain aeriel, as they all 'gain' to some extent) & what do phrases like " Less than or equal to 2.0 Gain: 7.0dBi " actually mean ?

All this TV technobabble is making me confus-ed !

[CeeBee]

It's the gain of the given antenna compared to a simple dipole.
Gain(db) = 20lg(V/Vref)
Ex with a 6db gain you get 2x the signal level of a dipole, 10db~=3x, etc..
To have something decent you would probably need at least 10db gain. That would be a Yagi antenna with 5-7 elements.
However, the higher the gain is the narrower the bandwidth becomes. Such antennas are built for specific channels, so if someone tries to sell a high-gain wide bandwidth antenna stay away. It may have the gain but it is only efficient when the signal is quite strong already. You need first to obtain a signal level significantly higher than the noise, which is (almost) a constant for a given frequency, bandwidth and impedance. That is because you are amplifying the signal AND the noise...

[confus-ed]

Ta Ceebee .. I'm not so sure how much of that actually made sense to me completely, but at least I understand the bit about amplifying noise, so I reckon that's probably enough to help me make a sensible buying decsion for my mate.

Good man ! Thanks