MOBILE ANTENNA for FM, GSM

Hi everyone,

I've some doubts regarding mobile antenna sizes, shapes for various purposes like FM, GSM etc.

i. Firstly, consider FM. It is common knowledge that the earphone wires act as radio receiver. But, the radio signals are transferred from mobile to earpiece along the same wire. I believe, the FM frequency(88-108 Mhz) different from the signal frequency of the demodulated base-band signal sent to ear-piece and so they don't interfere. Is it true?

ii. In FM, what if we use wireless earphones. Does it mean that we'll not be able to listen to FM?(The wire is not present to act as antenna)

iii. Consider GSM signals. In many places, 900Mhz is used. Then quarter wave antenna size would be 8.3 cm. How does this fit into small size mobiles(esp the small sized feature phones )?

Please reply soon...

What do you mean: "earphone wires act as radio receiver"?
Yes, in portable devices in FM mode the antenna is actually the wire of the earphone. Audio frequencies (from few Hz up to about 20kHz) doesn't interfere with RF signals (88-108MHz). To prevent this happen, the circuits use RF chokes, or low-pass/high-pass filters to separate those signals.
In GSM 900MHz (actually in all cellular bands) there are other types of antennas inside of the mobile phones. The most used antenna type is PIFA (Planar Inverted F Antenna). Search the net for more information.

i mean earphones act as the receiving antenna... Thank You very much for your response...

Can you answer my second question?

"ii.what if we use wireless earphones. Does it mean that we'll not be able to listen to FM?(The wire is not present to act as antenna) "

For FM broadcast reception, without using relative long external antenna (as the wire of the earphones) the reception will be poor.

There was a few cellular phones with embedded FM antenna on the market a few years ago.
Cost and required space in combination with assumed low consumer interest due to cheap cost of streamed media, did make the antenna less interesting.
A few examples: https://www.nab.org/xert/scitech/2010/Radio_TechCheck/rd092710.asp
Reception performance can be just as good as using headset wires for antenna, if nearfield EMI can be kept at a low level, at least for Ethertronics active tuned version.
I spent more then a year to develop/invent this antenna type.
Same antenna type was also implemented for DVB-h TV reception in a few phone models, but also DVB-h died due to cheap cost of streamed media.

Nokia has built few models with embedded FM broadcast antenna, that works very poorly. On some models they disable from software the FM application, even the hardware was available inside. The same did Apple with one of the very first iPhone models.
The problem is that when the signal is strong (mobile near the broadcast transmitter) things might be OK, but when the signal starts dropping just a little bit, or multipath fading appears, embedded antenna cannot make the desired signal to be above the internal noise of the mobile phone. Noise that is pretty strong at about 100 MHz in most of the smart-phones.
Another big problem that I found when using embedded antenna for FM broadcast was, when the RDS (Radio Data System) was ON. This situation can damage the receive SNR by 20dB.

Nokia 5030 had an wideband internal FM antenna, a kind of loop that did cover a big part of internals of the phone and did reuse more or less whole ground plane in main PCB as an antenna part. Internal noise can then result in saturation and intermodulation in following HF stages.
As my antenna design is very small, is it best placed in a corner of main PCB, in hope giving some distance from local EMI sources, and it is it also possible to rotate the antenna to make it a bit deaf for the worst local noise sources. As it is a very narrow band antenna design, 300KHz/3dB, does it reduce wide band noise problem in following RF stages. The antenna have a natural very high Q, which also reduce need of LNA gain. Same antenna tuned as VHF-UHF antenna did work real good as internal noise level normally was much less.
Main drawback with this antenna is that it need to be active frequency tuned.
Internals of a phone is not the best antenna location for an FM antenna, but if alternative is no antenna at all, for example when wireless headset is used, is it a very good alternative, if well designed.

Have never seen problem due to RDS but guess it increased local noise somehow. If it was Nokia 5030, had it a rather noisy LCD driver placed in the middle of the ribbon cable that connected display with main board. It did maybe get extra activation due to that RDS caused constant updated display.