Amp for iTrip Transmitter? Boost Your Signal, But Also Boost Your Noise. Be Safe and Do It Right.

Boing Boing points to a design for a tiny amplifier to boost the signal of the iTrip personal FM transmitter, which is the older brother to the iTrip Mini, blogged about yesterday as hackable to do short-range “pirate” broadcasting.

One of the basic problems with boosting the signals of these mini FM transmitters is that they were never intended to produce strong signals in the first place. Thus, their output tends to be relatively “dirty” — producing harmonics of the broadcast frequency that can cause interference to other stations. This aspect is generally not a problem with the stock mini-transmitter, since harmonics are usually much lower in power than the broadcast frequency, which is already ultra-low.

But when you start boosting the output of these mini-transmitters, you definitely boost the interference, too.

I’m not a real electronics whiz, but I don’t see a low-pass filter of any kind, which means that the interference can go past the FM band into the aircraft band. Now, at the kind of power level this amp is likely to produce, this doesn’t reall pose much of a danger. The real concern is the “splatter” of interference that can screw up other radio stations on the dial in the vicinity of the transmitter.

My advice is, if you want to start broadcasting further than what the power of an iTrip or a similar mini-transmitter will do, then you should look into transmitters designed to go further. There are some nice 1/2 watt and 1 watt designs out there that you can get for $100 or so. They’re specifically designed to limit interference in the broadcast band and other bands, too. NRG Kits in the UK sells a nice 1 watter for £99.95 fully assembled (that’s about $181.00 US). DIYmedia.net has a good list of other vendors who might have cheaper models.

However, do note that once you’re boosting the output of an iTrip or using a transmitter that puts out much more than 10 milliwatts, then you’re likely exceeding the limits of FCC Part 15 regulations. That means that you are no longer broadcasting within the limits of the law — you are a pirate.

As anyone who reads this blog knows, I’m fine with that. But if you want to keep broadcasting with minimal harassment from the feds and other broadcasters, then you need to be a lot more careful, and try to be a good neighbor on the radio dial by not causing interference and not broadcasting on someone else’s frequency.

Free Radio Berkeley has a great primer that goes over all these issues, downloadable as a pdf: Micropower Broadcasting – A Technical Primer.