Hi-Fi Stereo FM Transmitter

[Al Skierkiewicz]

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...
http://www.ramseyelectronics.com/hk/....asp?page=amfm
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.

[billbo911]

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!

Originally Posted by Al Skierkiewicz

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....
When you wish to upgrade, take a look here...
http://www.ramseyelectronics.com/hk/....asp?page=amfm
I recently built an FM100B for the radio station here....
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.

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.
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.

Originally Posted by Dave Flowerday

Bill, I strongly recommend you do your own research with the FCC who holds the authority for these types of devices....

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.

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.

Originally Posted by squirrel

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.

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

Originally Posted by Don Rotolo

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.

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.

[Al Skierkiewicz]

Bill,
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.

[DonRotolo]

Originally Posted by Al Skierkiewicz

tune it to an used channel in your area.

Typo: He meant UNused channel

Originally Posted by Al Skierkiewicz

The FCC does not have a sense of humor.

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.

[billbo911]

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.

[MrForbes]

Play with a Belkin Tunecast.....they don't cost much

[Al Skierkiewicz]

Bill,
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.

[DonRotolo]

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.

[billbo911]

Originally Posted by Al Skierkiewicz

Bill,
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..... It may indicate that the 38 kHz crystal is defective.

Originally Posted by Don Rotolo

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.