FM-110 Community Stereo FM Broadcast Transmitter

The FM-110 was designed for small Central American and Caribbean communities where a low cost, easy to use radio transmitter was needed. I am the former owner of Ramsey Electronics where we developed many such transmitters that were used worldwide. The lower cost hobby-grade transmitters that we used to produce were designed around a simple IC chip developed for the iPod and other such music players. These chips were very low cost and, as such, produced passable but nowhere near audiophile performance. When I was approached by a foreign communications official for a simple community transmitter, I knew that one of these low-end chips with a beefed up power amplifier just wouldn't get the job done! I know, because I designed with them and have seen the spectrum full of spurious signals, not to mention the grunge, hisses, swishes and whishing that these cheapy chips produce.

Back when I owned Ramsey Electronics, I often enjoyed designing simple, elegant, straight-forward circuits and this project was the ticket! I went back to basics where the stereo generation would be done all analog with no digital multiplexing, clocking or oversampling and the requisite low pass filters with their attendant phase shifts and coloring of the sound. Sure this would result in a lot of chips to do the same thing, but why not - isn't good sound the intention?

I had great success using a coax line as the resonant element in other high-end transmitters, so I did the same here, and then examined the great new LDMOS transistors now available - hey, this can do 10 watts easy!

Now, how to make it easy to use for untrained folks who are running government announcements, church programs or even live sports events. Thus the simple slide pots controls and easy to understand LCD display. Since power would be an issue, I decided to use an external power supply that runs on 100 - 240 volts AC - usable anywhere in the world, and if you need to run it on battery, that's not a problem either since 12 - 15 volts DC works just fine too! Simply add a microphone (sure, one of those simple computer ones will work fine) and CD player, computer with itunes or other such music source and you're on the air!

Remember, this transmitter was designed for international use and you must operate it within the legal requirements of your country.  The 10 watt, assembled unit is not for sale in the USA under any circumstances and this page serves only to provide information to potential users outside the USA.  This device has not been authorized as required by the rules of the Federal Communications Commission. This device is not, and may not be, offered for sale or lease, or sold or leased, until authorization is obtained.

We may offer a scaled down version (lower power) of this transmitter in an easy to build kit form where you may learn about transmitters and low power radio. In the USA, this would then be considered a low power Part 15 "Intentional Radiator" which would carry the following warning on the unit:

The radio-frequency "Intentional Radiator" device which may be constructed from this kit
is intended to conform to applicable provisions of Part 15 of FCC Rules.

The individual kit-builder and all users of this device assume responsibility for lawful uses
conforming to FCC Part 15 Rules. Operation is further subject to the following two conditions:

1.  This device may not cause harmful interference, and

2.  This device must accept any interference received, including interference that may cause undesired operation.

pdf file of the manual is here: click!

Now that you have the background on why this little transmitter was developed, follow along and see some of its features.

Display screens:

E-Z SETUP AND OPERATION INSTRUCTIONS

Be sure to have your whip antenna connected for low power operation, or a proper antenna or load if using the SO-239 high power output  configuration.

Insert the power cord into the appropriate rear panel connector.  Plug the AC power cord into an appropriate outlet.

Energize the unit by toggle in the power switch to the “ON” position. An indication of proper power up will be the display energizing and briefly displaying the startup screen showing software revision and power jumper setting. The display will then cycle to the Frequency/Locked indicator screen and then finally to the default Right Left audio level bar graph indicator screen as shown below.

(Dummy load shown in this picture not included!)

SETTING THE OUTPUT FREQUENCY OF YOUR TRANSMITTER

Rotate the DATA ENTRY knob on the front panel until you reach the edit frequency screen. Press ENTER to edit frequency (exactly what it said in the screen!).

Dial the DATA ENTRY knob to the desired MHz value.

Depress the ENTER key again, and the MHz place for the frequency will be stored, and the decimal place for the frequency will be displayed.

Dial the DATA ENTRY knob to the desired decimal value.

Depress the ENTER key again, the display will say “Saving data, please wait” then the screen will revert back to the Frequency/Locked screen and the transmitter will be operating on the new frequency.

Beefy heatsink with fan allows the FM-110 to run cool 24/7!

NAVIGATING THROUGH THE MENU SELECTIONS

There are several different display screens as you scroll through using the DATA ENTRY screens. Some of the additional screens monitor your output power, display a bar graph audio input, and display the transmit frequencies.

As a rule of thumb, any entry screen that allows user inputs will follow the same conventions as the frequency setup screen. Pressing the ENTER key allows you to access the sub-menu, and the DATA ENTRY knob allows you to select individual text.

A few examples of data type displays are as follows:

We recommend the use of the internal antenna and low power jumpering, however if your jurisdiction permits, using the unit on high power with an effective antenna will provide for best operating range. The external antenna jack is a standard SO-239 coax connector as found on most amateur and CB equipment, and this connector is readily available at most any electronic supplier. We make a high performance broadcast antenna ideal for this transmitter, for details, click here. Also, an effective and simple antenna called a “dipole” can be easily constructed from coax cable and TV twin lead. While it is beyond the scope of this manual, a Google search for “fm dipole antenna” will yield many hits!

 Correct dipole lengths for various parts of the FM band are:

88 MHz: each leg 2.7 feet - 5.4 feet total

98 MHz: each leg 2.4 feet - 4.8 feet total

108 MHz: each leg 2.2 feet - 4.4 feet total