# Need help designing an Antenna!

Discussion in 'Electronic Design' started by zoti, Apr 22, 2006.

1. ### zotiGuest

Hi,

I'm trying to design a replacement antenna for a GSM phone. One that
will cover 850MHZ, 900MHZ, 1800MHZ and 1900MHZ (Quad band GSM).

Doing a bit of reading on the internet I came up with an antenna that
is made out of a piece of wire at 1/4 length of the average frequency
(1350MHZ). The antenna is just over 5CM (2 inches) long.

It seem to work fine and I get good reception with the phone but here
are my questions.

- Do I need to take anything else into consideration? Like the
material the antenna is made of?

- Did I even do my calculations right?

Thanks,
zoti

2. ### SunkistGuest

Your best bet is to search the Ham Radio Websites, there are programs
designed to calculate the exact length. The problem is that if the
Antenna is optimum for one Freq it wont be for another, but, the
antenna at 1800Mhz if it's a 1/4 wave is 1/2 wave at 900Mhz (inversely
Proportional, higher freq = shorter wave). Because you cant have a full
wave antenna the best option is 1/4 wave at 1800Mhz.

1/4" Pipe Copper is best, the thicker the conductor the wider the
bandwidth. The length is critical because of "standing wave ratio", and
RF that cant get out of the Antenna is fed back into the Transmitter
and will burn it up. You also need a Corona Discharge Ball on top to
dissipate static. If there is a reflector that reflects wave away from

If you want the best reception and transmission, your best bet is to
keep the Antenna vertical at all times as the Cell transmits on a
Verticle Axis.

Maybe be better off to switch to a service provider that has more cells

Regards, Joe

3. ### Ken TaylorGuest

Of course we all know you meant to say that a 1/4 wave at 900MHz is 1/2
wave at 1800MHz, not the other way around.
One thing the OP needs to keep in mind is that the antenna needs to be
matched correctly for the output stage of the phone in question - it may
not like to transmit into the antenna you make for it. Chances are all
will be okay, but.....

Cheers.

Ken