Why causes interference on my iPod FM attachment?

  • 2 Replies

0 Members and 1 Guest are viewing this topic.


Wayne Inglis

  • Guest
Wayne Inglis  asked the Naked Scientists:
Dear Naked Scientists.

I often listen to your podcast in the car on my iPod through the car radio some FM transmitter attachment.

At times the programme will be temporarily interrupted by interference that results in some radio station music blaring through the speakers.
Often I am in heavy traffic but sometimes it happens when no cars are about.

Can you explain what is happening?

South Australia

What do you think?



  • Guest
Why causes interference on my iPod FM attachment?
« Reply #1 on: 12/06/2009 22:32:28 »
Presumably your little ipod FM transmitter has a frequency / channel setting control. You need to find a frequency for it to operate on which is not subject to interference. If you are on the move, then it is possible that setting the device up at home may not be good enough to avoid interference from transmitters which you can receive elsewhere.

Other vehicles should have no effect - they should not be transmitting on broadcast frequencies.


Offline techmind

  • Hero Member
  • *****
  • 934
  • Un-obfuscated
    • View Profile
    • techmind.org
Why causes interference on my iPod FM attachment?
« Reply #2 on: 14/06/2009 11:48:54 »
Your iTrip or other FM transmitter is just a low power FM transmitter with a range of a few metres (or maybe 10-20 metres depending on conditions). It operates at a specific pre-set frequency just like any other radio station.

Of course while you're listening to the mp3 player via your transmitter, the aerial on the car roof is potentially able to receiving anything from the general airwaves and broadcast transmissions in the area.

If you happen to drive into a location where there's another (broadcast) transmitter on the same frequency (or very nearby frequency) as your transmitter, and that signal becomes comparable or greater in strength than the fairly weak signal from your transmitter then you will get breakthrough (ie you will hear snippets of the other station).

The strength of other transmissions can vary not only say as you reach the top of a hill, but also can change over very short distances especially in built-up areas as FM radio has frequencies around 100MHz, so wavelengths of around 3metres, and 'reflections' of the signals off buildings or other cars can interfere constructively or destructively (ie reinforce or cancel each other) as you or the other cars move by a metre or so.

The solution is to re-tune your transmitter and the car radio to a different frequency which is not used by any broadcast transmitters in the area you drive through.
« Last Edit: 14/06/2009 11:51:17 by techmind »
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"