Any ear cord that is used as FM antenna heavily interact with human body. Resulting antenna function depends mostly on this body and how the wire is bent.
As it not exist a detailed description on how the wire is allowed to be bent or recommended distance between a standard body and lose hanging wire, can the antenna performance be anything for any phone brand.
Best antenna function is probably if someone else is holding the phone and as this improves the dipole effect. Lets see, if the wire is held straight and is limited to 80 cm length and both person are standing bare feet in a muddy ground is best listening height then likely to be at 80 cm height as resulting antenna shape will be a full wave loop from the FM receiver view. Two persons sinking in quick sand comes on my mind... One says to the other:
- Can I borrow your headset?
- Calling for help?
- No, I want to listen at FM radio as we now have good reception.
Sorry, but as you can see, antenna parameters for a phone equipped with FM cord antenna is depending on a lot of external factors but almost nothing on the phone model.
There are few phones with embedded FM antenna and for them are the situation a bit different. I have seen simple and relative poor performing internal coil-shortened monopole antennas but also more advanced and real good working antennas from manufactures such as Ethertronics and Laird.
But still, also these internal antennas are very depending on external factors due to the long wave length compared to the phone internal ground size which never can be considered as a good working ground plane at these frequencies.