Why do pocket radios have small antennas

[prabhatkarpe4]

I need to know why do most pocket AM radios come with a small whip antenna of about 20 cm because that would work only with 80 cm bands. How it will receive SW from 2 MHz to 22 MHz with a 20 cm whip antenna?

[Phil1977]

A physically short antenna can be electronically prolonged e.g. by inductors. And even a non-matched antenna is not an ideal antenna, but it´s receiving anyhow - it´s just much less efficient.

For a receiver a bad antenna efficiency can partially be compensated by amplification. That´s why well matched antennas are most important for transmitters.

For much longer wavelengths like LW and MW you can also receive the magnetic part of the transmission e.g. with a ferrite rod antenna. Again that´s only pragmatic for receiving, not for transmission.

[tszaboo]

To state the obvious, they have small antennas because otherwise they wouldn't fit in your pocket, and then they wouldn't be pocket radios.

[radiolistener]

I need to know why do most pocket AM radios come with a small whip antenna of about 20 cm because that would work only with 80 cm bands. How it will receive SW from 2 MHz to 22 MHz with a 20 cm whip antenna?

Short antenna has worse efficiency. But it also has smaller Rayleigh near field region, which helps to reduce near field coupling when you use receiver near buildings or other objects that can have noise sources. Smaller efficiency can be compensated with low noise amplifier, so the main issue of antenna is noise floor which is very sensitive to near field coupling and smaller antenna size also helps to compensate its low efficiency for a little by reducing near field coupling.

And since portable radios are unable to use large antenna and usually are used near buildings, it's short antenna is a compromise which works not so bad.

[Infraviolet]

Does antenna size have some general relation to power levels (atleast for transmission) too? For anything pocket portable they'd design to keep the power requirements as low as they could, both to prolong battery life and make it weak enough not to be likely to interfere with other users of the same slice of spectrum.

[radiolistener]

Does antenna size have some general relation to power levels (atleast for transmission) too?

Almost no. But if you want to increase antenna efficiency, it needs to increase antenna size. And if you building high power transmitter, there is a sense to make it very efficient in order to pay less for electricity and as result it needs to be large. Small antenna can radiate the same power, but it will have more losses for heating due to higher voltage/current required to radiate the same power.