A friend gave me some old mobile phones, one of them being a Nokia 6150 in good condition. As I wanted to have something unique, I thought about modifying it to work with an internal aerial.

You'll find some photos of my 6150 enclosed. They show how I did the modification. It works not that well as the original antenna according to "NetMonitor", the RX is about 5-10 dB weaker. But I don't mind - it looks cool and certainly is unique!

The wave-length "lambda" is 33 cm @ 900 MHz:

lambda = c / f
( c = 300.000 km/s ; f = frequency in Hz ; lambda in meters )

The aerial needs to be lambda / 4, hence it has to be about 8,25 cm. You might optimize it using "NetMonitor" - start with a 9 cm aerial and cut mm after mm until RX-level reaches a maximum. But it is not necessary in general, using a 8,25 cm peace of wire will do the job.

Use some kind of thin insulated wire to build the aerial, solder it to the antenna connection pad (where the original antenna gets contact to the PCB) and place it inside your 6150 like shown on my photos.

I have been to lazy so far to seal the hole where I cut off the original aerial. You might use some kind of glue or similar which is for example used when building model cars.

K.Choo reported that he has done this mod to his 7110 and it still works :-) , he used epoxy clay to seal up the hole (thanks for that information).

Here are some photos (click on them to enlarge):