Hi-Fi Stereo FM Transmitter

I moved into a new house (well, new to us) in January. After months of projects that had higher priorities, I now am working on a project that is more along the lines of comfort and convenience.
In the main living room in the home is where the entertainment center is. My surround sound system has a secondary audio output that allows me to route through to it any of the audio inputs. This way I can listen to any audio source I choose in another area of the house. Previously I just had it interconnected to the stereo in the master bedroom. Now in the new home I will be setting up stereo’s to run speakers in the garage, three different locations around the yard and in the master bedroom. I have NO DESIRE to run interconnects to all these locations. So, my solution is to build a low power stereo FM transmitter and just feed the aux. output from entertainment center to it. That way I can just tune in the signal at each receiver/amplifier and have the same audio throughout the entire home.

Here is the deal. Although I am an EE, I really have had no dealings with RF let alone broadcasting it. So, I purchased a small DIY kit. I will be powering it from a 5vdc wall wart and dropping the supplied voltage to +3vdc with a 333ohm resistor and a 3vdc Zener. That part is easy, I just want to make sure I am thinking this through correctly.

Will this transmitter be sufficient for 2000 sq ft home on a 1/4 acre lot. It will be centrally located. Or, will I need to boost the signal to between 500mw and 1w?

(Why do I have a feeling I’m gong to hear from Al and Alan?) :smiley:

Please use a linear regulator instead. No need to have this simple design decision bite you later.

I’ll have a more helpful response later.

Thanks Eric, that is the type of input I’m looking for!

No problem, linear regulator is now in the mix.

OK, just for my education, why would the resistor/zener bite me later. I made sure to choose values that allow plenty of overhead and a good safety margin.
Curious minds want to know.

There are many FM radio transmitters available for use in cars, typically to allow you to use an ipod or similar device to transmit the sound to the car’s radio. They have small antennas, and very low power output. But if you add a larger antenna, you can get a bit of range out of them. The problem is you might be violating FCC regulations if you do this. So we’ll wait to hear from Alan and Al about what you should really do :slight_smile:

As for being Hi-Fi…hmmm…

For the FM bands, you can probably get away with it. A hacker EE can get all sorts of ugly to work (for some duration of work). But, why bother when the “right” solution is just as easy / affordable?
The first thing my RF mentor ever told me was “RF is the easiest of the EE disciplines as long as you do it right and don’t screw up. Once you screw it up, thats when it becomes FM*”.
Basically, you don’t want to add sources of error that are avoidable. The potential problem with a resistor + zener instead of a linear regulator is output impedence / load transients. In other words, the actual output voltage is now a function of its load. This effect can wreck the linearity of a system, and cause distortion.
Imagine an output amplifier that is supposed to double the input. That is, out = in * 2. For most RF amps, what it actually is doing is out = in * constant * power_rail. If power_rail depends on out, you get something on the order of out = in * constant * power_rail + in^2 * other_constant. This usually isn’t a problem with slower amplifiers, but those will still have a power supply rejection ratio that isn’t perfect.
This is partially why RF circuits have way more bypass capacitance than usual and why so much effort is put into ensuring the integrity of power and especially ground.

*FM actually stands for “freaking magic”, not “frequency modulation”. This describes both how FM radios operate, and the occult chicken sacrifices necessary to make an EMI problem go away.

You have now reinforced my agreement to use a linear regulator. From everything I have read about this circuit, the transmission power is directly related to the Vcc. In addition, the frequency of the transmission is determined by the L/R/C circuit which is driven by Vcc. Without a doubt, ANY variation in Vcc will introduce problems. So…scrap the Zener.

My understanding of FCC regulations on low power FM is that you are allowed to transmit up to 200 meters without needing a license. Again, this is just my understanding from third party sources.

Mr. Bill,
The kit you bought is intended for very low output and so very short distances. It is a simple design but may be subject to input overload from your audio source. A couple of input pots would easily eliminate that problem. Since the output circuitry is very simple, it is unlikely there will be an efficient coupling to your antenna making the range even shorter. I think Bluetooth ranges and coverage is likely what you will find.
When you wish to upgrade, take a look here…
I recently built an FM100B for the radio station here. Although it is likely an overkill for you, I really was impressed with the construction and coverage. Ramsey has been around for a while and I have built some of their Ham radio projects over the years.
I don’t have a problem with zener diode circuitry if properly designed. However, in this application, the zener may introduce significant additional noise that will degrade the transmitted audio.

As long as you stay under 3.2 miles, your low power transmitter is completely (edit: not) legal. You should be fine with adding an antenna to any of the commercial iPod FM transmitters. A few of them have ports for antennas ready to go.

You used to be able to purchase everything you’d need to do this at Radio Shack, but I don’t think so now.

Be aware that FM sound quality will probably not be exactly what you’re looking for if you can tell the difference between an MP3 and a CD, or other similarly degraded audio. Running wires or using a Wi-Fi solution would get you better results.

Bill, I strongly recommend you do your own research with the FCC who holds the authority for these types of devices. Be wary of any advice you receive on a forum like this (including mine) as any of it can be misleading or wrong, and if you follow it and end up breaking the law by accident they will not accept that as an excuse. A prime example here is Chris’ assertion that “as long as you stay under 3.2 miles it’s legal” - this is flat-out wrong.

Here is the FCC website describing low-power FM stations, which is probably where Chris got his 3.2 mile limit from. Note that you would likely not qualify for a LPFM license (and yes, you still need a license):

Choice quotes from that document:

This page contains information about the Low Power FM (LPFM) radio service, which was created by the Commission in January 2000. These stations are authorized for noncommercial educational broadcasting only (no commercial operation) and operate with an effective radiated power (ERP) of 100 watts (0.1 kilowatts) or less, with maximum facilities of 100 watts ERP at 30 meters (100 feet) antenna height above average terrain (HAAT). The approximate service range of a 100 watt LPFM station is 5.6 kilometers (3.5 miles radius).

A construction permit is required before a LPFM station can be constructed or operated.

LPFM stations are available to noncommercial educational entities and public safety and transportation organizations, but are not available to individuals or for commercial operations.

Ramsey has some good real-world information on the use of low-power FM kits or homebuilt devices here: http://www.ramseyelectronics.com/resource/default.asp?page=fcc

One of the important limitation is probably this one:

The most specific FCC regulation of 88-108 MHz FM Broadcast band unlicensed operation is that the “field strength” of the signal must not exceed 250 microvolts/meter at a distance of 3 meters from the transmitter (FCC rule 15.239). If you have any concern about this emission limit, have your device checked by a technician with accurate measuring equipment. Remember that the “field strength” of a signal is determined as much by the antenna as by the RF output of the transmitter itself.

They also have a table which shows the field strength at various distances if you adhere to this rule. If you modify a kit in any way, though, you’d need to somehow verify compliance with this rule.

And, most importantly, you cannot interfere with any licensed broadcaster in the band. If you cause interference for any of your neighbors who are trying to listen to a licensed station, you are violating federal law. The FCC has been known to crack down on offenders too, so don’t just assume that no one will notice and you’ll get away with it.

I’m going to go ahead and fire my lawyer now…

Sorry about my post.

This is the crux of the matter…and whether or not you’ll be able to do this depends on how far away your neighbors are. I’m lucky, we live on 4 acres, and the nearest houses are about 500 ft away, so we could probably make an FM transmitter system for our house that would meet the interference requirements, even though it might exceed the allowable field strength. If you’re in an apartment or high density housing tract, it might be impossible.

The distance you’ll be able to go with an unmodified kit depends on what is between you and the antenna. A bunch of metal, not so far. Nothing but air, a lot further. Antenna in the basement, not so far. In the attic, or on the ground floor (in the linen closet?) better.

Build it, try it, and only if it’s not OK do you go further. 200 feet on a few microwatts is easy.

500 mW or 1 Watt is WAY in excess of legal limits. A typical NAL (Notice of Apparent Liability = a fine) for unlicensed FM broadcast exceeds $10,000. Oh, and the FCC can find such a transmitter in perhaps 10 minutes, if it goes looking.

I must admit, CD has done it again. I asked and I received the type of information that will really help. Most of this type of help comes from experience and wisdom directly related to the subject!

Agreed, low end kit for first time attempt. I have a feeling I will learn more this way. Additionally, I do have some experience to draw from, I am a EE, that has to count for something.:rolleyes:
I looked at the FM10C kit offered on that site Al, it looks as though the circuit is very close to the one in my kit. We will see how it turns out.
I had already planned on adding a couple trim pots on the input if the need arose, so at least we are on the same page there.

Excellent advice and something I have followed up on. Although, I don’t have the proper test equipment to measure the field strength of the signal. So, I will try a couple other tests, such as, if I can’t pick up the signal beyond my home, I should be in good shape. If Al’s assumption of “Bluetooth range” is true, I will be in good shape.

Middle of town, .25 acre lot, nearest neighbor 50 ft. from where the transmitter will be. Hmmm, that’s three strikes.

Don, you and I are on the same page. Honestly, the only changes I plan on making involve audio quality. If the range is good enough to make it to the garage, I’ll be happy.

The kit is frequency agile so that the intent is to tune it to an used channel in your area. You may have to experiment to insure that your transmitter is not interfered with by an adjacent broadcaster. Because the distance is so small, make sure that the transmit antenna and receive antenna are parallel for best reception. I thought the Ramsey kit looked similar but did not see a schematic when I called up the manual. The IT department has been playing with my computer here at work and I am not sure where any of the apps are in terms of latest greatest revisions. I writing this in Safe mode since I cannot get the box to turn on in normal. “The Helpdesk will respond in one business hour of your request.” If they are here…
For those of you lurking out there, the $10K Don refers to is per day per violation. The FCC does not have a sense of humor. However, the majority of retail kits available meet FCC minimums if not modified or coupled to high gain antennas.

Typo: He meant UNused channel

Sure they do, the gave ME a radio license!

If you unwittingly violate the rules and seem really shocked and sorry and cooperative, they’ll probably just make you sweat for a while.

OK, status update time. Short and sweet, it’s finished and I’m going to run cables instead.

The circuit works, barely. I guess for a 10mW transmitter I shouldn’t expect too much. No matter how accurately I tune it, all I can get is a mono signal. The “stereo” indicator never lights on any receiver, and I’ve tried several. I can get good “tuned” indication, just not stereo.

Here are a couple pictures of the project and it’s modifications.

This is the circuit. The blue 50K pot replaces two resisters and allows adjustment of the MPX Mix. Originally there were two 27K resisters in place.
The yellow wire going off to the left is a half wave length antenna.

Here is a picture of the entire circuit.

The small board with the two pots is both a 2.7 vdc. zener regulator and level adjustment circuit. (Thanks for the suggestion Al.)
As you can see, this can be powered by Micro USB +5 vdc. source.

All in all, it was good practice, but I am not satisfied with the result. So, either I drop a couple hundred $$ and by a better grade 25mW transmitter or I run cables. That is, unless someone here can tell me what the problem with this little beastie might be.

Play with a Belkin Tunecast…they don’t cost much

Does the transmitter light a stereo light on a receiver very close to the transmitter? If not, this is usually an indication that the 19 kHz pilot carrier is not functioning. If the receiver does not see the pilot it does not try to decode subcarrier. If the close in receiver does come on in stereo, the pilot injection may be too low. All of this is to prevent unwanted noise in receive when the signal to noise on the subcarrier is low. Thoughts were, a quiet mono signal was better than a noisy stereo. It may indicate that the 38 kHz crystal is defective.

Try a half-wave counterpoise: If your antenna goes 38" in one direction, connect a wire to Ground (right where it first hits the board) and run it 38" in the the other direction. The best signal will be at a right angle to that “dipole”.

It’s cheap and quick, and could double your signal strength or better.

But try what Al said: If you can’t get stereo from 6" away, there’s a fault. I have built $25 FM kits which worked at 150 feet, no problem.

It looks like I will be dragging the scope out of storage and actually using it for the first time in over a year. I know the calibration is suspect on it, but that won’t matter for this troubleshooting exercise. If the signal from the crystal isn’t there, then I will know the cause.

Don, I was thinking the same thing. In fact, I have a dipole antenna from a stereo receiver I was going to try just for kicks.

I’ll let you know what I figure out.