advice on mini FM transmitter
 

So, I have this MP3 player. I would listen to it in the car by using a cassette adapter plugged into the output jack of the player. No problem, right? Well, when we got our new minivan, we opted for the CD player. It seemed dumb to get another cassette player at this point in history. But, no more MP3's. Well, I thought, I've seen those little FM transmitters you can plug into a portable box and receive the audio on your car radio. So I got one.

No dice--the problem is, I can't find an open space on the radio dial in Philadelphia. If I'm sitting still, I can locate an area on the dial that I can deal with. But, as soon as I start driving, the signal goes in and out as I get "better" signal from whatever weak station is coming through from the outside.

So, is this a lost cause in Philadelphia due to the over-populated FM dial? Or do I need a better unit? (The one I got is a Sound Feeder 120 from Arkon.) Or is this kind of arrangment just not work, no matter what equipment you have and what radio market you're in?

I used to use a CD player that had a built-in transmitter of this nature. (Not exactly a top-notch brand-name product, but functional nonetheless.)

It worked... adequately, at best. FM station interference tended to pop in and out, finding a blank spot on the low end of the dial was problematic, and long trips guaranteed that you'd need to adjust the transmitter's target station at least two or three times.

If you don't have a better solution, it'll do, but there must be a better way. Either Best Buy or Circuit City was hawking a car CD player that handles MP3s in their weekend circular, but I know nothing about either the product in question or its quality.

Have you tried tying the transmitter antenna around your car antenna? :) It might just be crazy enough to work.

You can get boxes which go in between your antenna and your radio, rather than trying to broadcast wirelessly.

Or you can try to cobble something together directly to line-in on the head unit, if it has such a thing.

The effect you're seeing is called "FM capture", and it simply means an FM reciever will "capture" and reproduce the strongest signal near the frequency it's tuned to. As you drive around, signal strengths vary widely due to multipath reflections off of buildins, bridges, etc. Some FM receivers have "Local/DX" setting, which reduces this "capture" tendency somewhat; but these days that feature isn't seen as often; check the manual for your car stereo.

The only other thing you can do is increase the signal strength as heard by the receiver. Getting the sending antenna closer to (and parallel to) the receiving antenna is pretty much the key. Verbatim's suggestion has some merit, although I doubt you'll want to have wires hanging around the outside of the car. If the sending antenna is a piece of wire, wrapping it around the antenna cable *inside* the car (before it plugs into the stereo) may help, if you can access it behind the dash.

Also make sure the batteries in the sender are fresh (unless it's one of those that just plugs into the cigar lighter). It's also possible that a more expensive unit may have higher transmit power, but difficult to tell without trying it out. .

Correct me if I'm wrong, but AM does not do this, Maggie?

If a new radio is anywhere near an option, you'd prolly want to get one with preamp inputs, which are the RCA-style auxiliary inputs like you see in home stereos. Then you'd just get a headphone-to-dual-RCA adapter at radio shack.

Those type of car stereos tend to be expensive though, I think (at least a couple years ago they were; they may have come down in price recently). Did you check for this feature on your new factory stereo?

You're right, it doesn't. If two AM signals are within the passband of an AM receiver, the audio result is a heterodyne squeal that represents the sum of the two signals. You hear it on CB radio all the time when two transmitters key up on the same channel.

This is one reason VHF aviation radio is still conducted in AM mode; if two aircraft key-up at the same time (as in the case of two aircraft with similar callsigns acknowleging the same landing clearance) air traffic control wants to know that' s happened, rather than having the stronger signal conceal the weaker one.

Thanks for the comments! I'm just going to subsist on some cheap speakers for the MP3 player for the time being. Well, the radio came new with the van & so I'm not inclined to get a new one. My wife certainly won't be inclined to spend the $$$!

Maggie, I was a bit familiar with the capture effect from scanning the public safety bands. However, I expected it to work in my favor! I know the signal strength of these gizmos can't be very strong or the FCC would have a canary. But even so, I thought the fact that the transmitter would only be a couple feet from the radio's antenna would cause its signal to block out any marginal radio station signal that it was competing with.

1) Lower the antenna and put your FM transmitter near that antenna that is now inside the car.
2) Some frequencies are reserved for low power functions such as highway notification systems. You can visit the FCC database to identify those frequencies in your area. Some other frequencies now available are above 108 Mhz without too many broadcaster and morely low power 'competition'.

A great name for a rock band if I've ever heard one!