Antennas? what is the difference inside?

Question

Between a 900mhz, 1800mhz and 1900mhz antenna --they look very similar--?

Thanks

Answer 1

It depends on the antenna. The ones rated 2dbi are all just 1/4 wavelength stubs, from your 30" FM radio antenna to 1 1/4" 2.4ghz antenna. The higher frequency ones are shorter.  Higher gain antennas are either dish types, yagis (the ones that look like fish skeletons) or multiple antennas phased together. They can take dozens of forms.

The only real difference between frequencies is that the higher frequency antennas are smaller than the same type of lower frequency antenna.

Answer 2

You're probably talking about a cordless phone.  Those antennas are designed for convenience, appearance, and to survive careless use.  Inside is a coiled wire designed to be a mediocre antenna covering all the frequencies that phone uses.  That could be several bands.

The elements of a resonant antenna are proportional to the resonant wavelength, thus inversely proportional to the resonant frequency.

Answer 3

the length of the antenna decreases as the frequency increases. If they are vertical antennas, there is a coil of wire wound around inside them. the higher frequency antennas have shorter wire wound inside

the length of the antenna is normally 1/4 of the wavelength

wavelength =speed of light / frequency

for 900mhz  (3 * 10^8) /(900 * 10^6) = 0.33 metres

1/4 of 0.33 = 0.0833

for 1800mhz  (3 * 10^8) /(1800 * 10^6) = 0.166 metres

1/4 of 0.166 = 0.0416

for 1900mhz  (3 * 10^8) /(1900 * 10^6) = 0.1579 metres

1/4 of 0.1579 = 0.03947

Answer 4

The internal conducting structure will have different electrical lengths depending upon the required operating frequency.

More commonly these days are the dual resonant type antennas which have a low Q match to approximately a 1/2 wave at the midpoint between 1800 and 1900 MHz and approximately a 1/4 wave at 900MHz.  This is often achieved by some form of re-entrant helical structure.

Cheers.