How does an iPod FM Transmitter work?

This is probably mis-posted but I'm not certain which NG to ask.
I recently bought an iPod and a DLO TransPod, which is supposed to be
used in the vehicle to play whatever is on the iPod. You select an un-used frequency on the radio and then match the same frequency on the Transpod. The thing is supposed to transmit over your vehicle radio.
But it didn't. Well, sort of, but I had to have the iPod and the radio both cranked to max, volume, so all I got was static.
I went to see the dealer, and he told me to try another transmitter by a different manufacturer. I did, and this thing works like a charm. Beautiful transmission. He told me this happens all the time - some radios like one manufacturer's transmitter, some like another. There doesn't seem to be a way to predict what works with which radio.
It all sounded like so much applesauce to me, until the second transmitter worked so well. So:
How do these things work? I was assuming they would have a mini-FM Transmitter built into them, with maybe a range of a couple of meters, but if that is so, why wouldn't one radio pick up transmissions from the TransPod, but would from the other device? (In both cases there is line of sight to the vehicle antenna; in both cases about 0.6 meters as the radio wave travels. And it is a real antenna, not one of those things built into a windshield, either.)
Any wisdom? Thanks, folks. (And if this is the wrong N/G, please point me and watch me go.)
HR.
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If that were strictly true there would be only one brand of broadcast transmitter (there are many).

Automobile electronics can generat low levels of RF. usully the biggest problem is in the AM band. I have had vehicles that produced noise on 146.52 MHz much to my annoyance.

yep
These transmitters in the USA are governed by "part 15" of the FCC rules and regulations. It must not generate a field that exeeds 250 uV/meter measured at 3 meters from the antenna. These devices will output much less then that so as not to interfere with other car radios nearby, minimize current draw from the source, and minimize cost.
Most likely the first unit you tried was; A: defective or B: poorly designed or C: both or D: not on the frequency your radio was tuned to.
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Rowbotth wrote:

These are (obviously) very low power transmitters, and inexpensive too. Some manufacturers may not pay attention to producing FM modulation with the proper specifications such as deviation, stereo subcarrier frequency, etc. Some receivers may be less forgiving about out of spec. FM signals than others.
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The kicker is that the one that didn't work runs about $120.00, and the other one runs at about $40.00. About the same size as the joint on the end of your thumb (with the nail), so a LOT smaller and more compact.
Go figure.
HR
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