Gadget blog Engadget posted that they were able to extend the range of their iTrip mini FM transmitter for the iPod Mini, along with instructions. Essentially what they did is open up the iTrip to get to the internal antenna and get it outside the case.

I’ve often told people that these little personal-type FM transmitters could be used to do “pirate radio” within an apartment building or dorm with a little bit of modification like this, though you can probably get more by extending the antenna.

The ability of an antenna to radiate a radio signal is directly related to the length of the antenna relative to the size of the radio wave you’re broadcasting. Various fractions of a FM broadcast wave work well for broadcasting, like 1/4 wave, 1/2 wave or even 5/8 wave. Since the iTrip puts out a miniscule amount of power, maybe around 1 milliwatt, antenna efficiency isn’t going to buy you much, but it will buy you something.

The wavelength in the middle of the broadcast band at 100 Mhz is about 118 inches. Since the iTrip transmitter is no more than 3 inches in any dimension, it’s unlikely there’s anything even close to 1/4 of that wavelength’s (29.5″) worth of antenna wire inside. You could coil up that amount of wire, but that won’t work for an FM broadcast antenna.

In their mod, Engadget only pulled out the little wire, which looks to be a few inches long — this simply gives it a little less plastic to radiate through. I suspect that adding another length of wire to it would extend range a tad, but probably anything more than 30″ would be overkill.

In the end any transmitter like the iTrip is going to be limited in how far it can broadcast because it is produced under FCC Part 15 rules, which governs unlicensed signal radiators (which includes PCs, since they generate electromagnetic interference). Part 15 rules put a firm cap on how far these little FM transmitters may broadcast — anything extending past this is technically unpermitted.

The iTrip looks like a pretty well designed mini-transmitter, however if broadcasting more than 10 feet is your goal, you might check out the venerable FM-10C from Ramsey Electronics. It’s a kit, so it requires a bit of soldering (but is easy enough that a beginner could do it), but is easily modified and easily accepts an external antenna that will greatly extend range.

The Ramsey used to be very popular with newbie would-be pirate broadcasters because it is so cheap and modifyable. There’s a treasure trove of FM-10 hacking tips and discussion (and flame wars) in the Google Groups Usenet archives from the mid-90s. The canonical FM-10 FAQ is there, or you can find it in the archives of the original FAQ author’s website.

There’s a whole underground of folks using Part 15-compliant ultra-low-power transmitters to create legal (or near-legal) unlicensed radio stations. The restrictions on the AM band are less stringent than FM, so many more people are working in that area. But, of course, FM is much higher fidelity, can be stereo, and most people have better FM radios.

And, so what, if you have to push the limits of Part 15 restrictions for FM broadcasting? What’s the real difference between broadcasting across the house and down the block? Is it illegal if nobody complains and nobody catches you?

For more on ultra-low-power broadcasting, see the Community Radio USA message board, which tends to focus more on AM. Or check out some of these older items on mediageek: