Building an FM transmitter

Our van only has an FM receiver in it. The only way to get music in it (take it as a given that listening to FM radio is NOT acceptable) is to use one of those FM transmitter units.

Unfortunately, they suck. They particularly suck in a modest urban area where its hard to find a completely empty station (other stations bleed in and add noise). Further, when you do find an empty station, it’s not long before you drive into an area that does have some interference.

My assumption is that the transmitters are small and underpowered in order to keep the costs down and to comply with commercial regulations for the sale of transmission equipment.

I don’t want to overpower any commercial station or reach beyond 10’ of our van, but I do want to overpower the dead space around our van. I figure my budget must be at least $100, since it’ll cost twice that to replace the head unit in the van. And I have no problem space-wise going up to a shoebox or so in size. lastly, we have a 300W inverter to plug into the lighter when we drive, so I don’t think power is a problem.

Is there a Heathkit waiting for me? A Radioshack U-BuildIt? Something COTS? I’m finding a lot of kits, but don’t know what specs are important/relevant.


the noncommercial low end of the FM band is where most devices are tuned for. this is where you are likely to find an empty frequency. this is the area where educational and religious stations exist. religious stations are known to adjust their transmitter for most broadcast reach even if it isn’t legal. so pick a frequency not next to a religious station because they will overload your radio and bleed through.

knowing what frequencies are likely good for an area is the best plan. go to

Can’t you just disconnect/shorten your car’s FM antenna? Beyond that, what you’re asking for is illegal in the USA and thus probably against board rules.

Alternatively, have you checked to see if the head unit has rear inputs for a cassette/CD changer/MD deck? Often times they can be converted into an aux-compatible plug for your iPod or similar.

PS: If you can’t physically remove the antenna, often times you can at least disconnect it from the head unit. It’s either a separate (often coax) jack or a wire in a bundle that you can cut by looking at a radio pin-out from Crutchfield, Radioshack, or similar.

We’re in the northeast so we’re ‘blessed’ by the lack of religious stations but cursed by the lack of free space virtually anywhere. On an hour’s trip between cities you’re going to get a lot of change and crowding. Good planing tool though, thanks.

It’s a whip antenna so we can just squish it down I believe, but the occasional need/desire for NPR, traffic report or drive-in theater makes us want to keep it somewhat intact.

As for The Law, to be clear, I’m not asking how to set up a pirate station or how to overpower Jackshit FM. There are widely available, legal, commercial build-your-own kits that range from snap together parts to solder-here-and-there plans. There are commercial products that do all of it for me, too, also perfectly legal. The GQ is which are the specs that are relevant to the lack of clear signal, and is it possible to build a transmitter for less than a hundred bucks that will maximize that spec (as much as necessary to ensure a clear signal in the van but not so much as to run afoul of The Law)?

If you’re willing to build your own transmitter you probably have enough technical and mechanical know-how to manage installing this FM modulator without too much difficulty:

What it does is physically interrupt the antenna line and puts a an FM signal directly into the back of your factory head unit, the signal carries whatever you have plugged into its RCA inputs. You need antenna adapters for your particular vehicle (which would be easy to find on crutchfield). One to turn your factory antenna terminal into the normal one used by aftermarket radios, and one to turn the modulator’s antenna output terminal into the one used by your factory radio. Some factory radios use the same as aftermarket radios and thus you wouldn’t need any adapters. Putting them on is as simple as just plugging it in. The other thing is finding a place to ground the unit (pretty much any bare metal surface in the car, or a ground wire if you feel like finding one). The power wire can be trickier without a light tester and knowing what you’re doing. The simplest solution is to splice it into the hot wire of your cigarette lighter (if it’s not constant power, that is, powered even when the car is off, or else you run the risk of draining your battery), which if you do means you can just put the ground wire on the lighter’s ground. It works by completely overpowering the radio station it’s set to, but has a toggle switch so you can turn it off to hear the station. When it’s on you’re not going to get any bleed over no matter what station it’s set to nor how powerful it is. Let me know if you want to do that and have any questions!

With your $100 budget, you could buy a low end modern car unit with bluetooth and an aux port.

Take a look at the Crutchfield catalog.

I agree that it’s the best solution but he’ll probably need a dash kit (maybe not!) and wiring harness. You can replace a factory radio without a wiring harness but it’s an extreme pain in the ass. Both of which will drive up the cost. As far as the time and complexity of doing either a headswap or my option, that depends entirely on the year make and model of the vehicle.

Sounds like you’re not familiar with Crutchfield. SOP there is to supply wiring harnesses and adapter plates for the front end free.

Give up on the FM transmitter. You’re trying to polish a turd. The only thing you should do with an FM transmitter is to toss it into the nearest garbage can and forget about it. These days, even a basic car radio usually has an aux port on the front. That is, by far, your best option.

I had a similar issue and installed an FM modulator which uses a direct antenna connection. It’s a simple install to your antenna cable, it worked great, I also connected it to my aux power for charging.

Here it is.

I think there is also a Bluetooth model.

Your stereo might also have CD changer capabilities which can also be used to port in an auxiliary device.

Since he’s clearly willing to carry around some gear, dealing with the dash PITA can be avoided with a little custom work.

I have an antique, and I was not willing to cut the dash to accept a modern stereo.

Phase 1 was an $80 Sony stereo. I made some basic brackets that screwed to the mounting holes in the side of the unit. I put a small metal plate under the carpet on the transmission hump and screwed the brackets to that (small bolts actually). I also hung a couple of crappy 4" speakers on the brackets and then just ran a line to the fuse box and grounded to something convenient under the dash. It worked, but was pretty Rube Goldberg.

Phase 2: I built a plywood box shaped to fit snugly under the dasg right over the hump, with an arched bottom shaped to fit over the hump. The overall shape was trapezoidal, following the lines in the dash. I mounted the stereo and a couple of 6" speakers in the box and carpeted the whole thing. I was able to get matching carpet from a restoration shop. Modesty be damned, it looks like it came from the factory that way. And the best part is that the arch over the hump combined with almost no clearance at the dash means it stays secure without having to drill any holes.

My point is that for very little time & money you can make a standalone rig that gets the job done, and for very little money and more time you can even make it look good.

ETA: Lest anyone think it was fancy, my brackets were made from rolls of pipe hanger. I put lengths in the vice and folded them 90 degrees the long way, making them remarkably strong.

Car stereo I wanted ten years ago…
I’d looked into replacing the head unit, that’s not an option.
Boring details: there are actually two vehicles that need music in them, a 1995 Mazda MPV and a 2005 Infiniti (the Infiniti only has a CD player, no aux or other input). Both have installation problems. I’d talked with Crutchfield directly before posting, and they don’t have a dash-kit for the MPV. Plus, there’s no way to tell how a head unit will install/hang in the space without taking things apart. For the Infiniti, they (and others) have an install kit, but it’s about $200. Not wiring harnesses and whatnot, just the dash kit.

Hence I’m hoping to find a way to get music into the head units without replacing them. I figure there is some room power/broadcast-wise between the small wireless units they sell and what’s permissible/feasible–I’m probably willing to pay a higher premium for this than what the general market would support.

It looks like **Arthur See Clarity’s **gadget is actually the best way to go. Looking at it and similar units, a bit of soldering to patch into the 12v and ground is all that’s needed, and we won’t need to worry about interference~
(ETA: Wrote this before seeing Sparky182’s and OldOlds posts. Great suggestions, and I’m really glad for the aux ant. feedback)

I was not aware of that, but it looks like OP’s case is out of the norm. Thanks for the info, though.
OP, our distributor apparently has a dash kit for the MPV, but it’s just a Mazda universal kit. Which means it’d work, but no guarantees on how seamless it’ll appear. In the worst case scenario it’ll just stick out a little further than your current one, but no exposed metal or anything. We sell the kit for 24.99, but that’s without wiring harness. We’re also a local store, no online presence, so if you wanted one you’d have to PM me about it, but I’ll call the distributor tomorrow to see if they have a dealer in your area. I’d have to know a little more specifically where you are, though. But the MPV doesn’t require any antenna adapters so if you went with the FM modulator your only cost will be the unit and the 3.5mm to RCA cord, and they have that as a bundle for 29 dollars on Amazon. It’s hard to beat that unless you want integrated charging and bluetooth and all the other features that come so easily on new radios.

FM radio in the US starts at 87.9 MHz, but some tuners and some transmitters can go below this. A few years ago, I bought a (IIRC) Garmin transmitter that could put out 87.5, and the radio in my truck could be tuned to the same, so I never had any interference problems.

So, before any hardware hacking, check out how agile your radio is.

Ramsey Electronics has some pretty nifty things along these lines, including a $50 FM stereo transmitter kit. It’s kind of a radio-oriented version of the old HeathKit.