Introduction: Turn an FM Transmitter Into a Highspeed Packet Modem for Free.

Picture of Turn an FM Transmitter Into a Highspeed Packet Modem for Free.

Hello. Here I will show you how you can turn any transmitter into a high speed packet modem. With a Walmart type wireless transmitter. You can expect a max connection rate of about 60.4 kb/s. This may seem slow, But comparing to what is still used today by ham operators. (The standard in packet radio is x.25 with 9600 band rate even today. ) Now 60.4 kb/s is a good connection rate for using "junk" off the shelf dirt cheep equipment.
With this kind of a setup. You can serve a webpage. You can send some files. Or you can even stream in "ok" quality, media such as audio and video, maybe even a live camera. All the receiver needs is to be in range of the signal and a normal fm radio with a headphone jack. Most cell phones even have a built in tuner and is perfect for this. Just connect an audio cable to the headphone jack of the cell phone or radio. Tune to your transmitter frequency. Plug the other end of the audio cable into the line in of your computer or laptop soundcard. Fire up the software and your in business. For the advanced user. You can even setup a RS232 port for transmitting and receiving data for use with any external application that depend on RS232 for communications including virtual COM ports and modem drivers. Think of all the possibilities!

Step 1: Introduction - Grab the Files

Ok so start by downloading the QAM modulation software. It is called J-QAM. It runs on Windows and Linux.

QAM which stands for quadrature amplitude
modulation. compared to modulation schemes like
FSK (frequency shift keying), QAM in contrast is unbelievably
complicated. QAM's advantage though is that this is much more efficient
than FSK. QAM is used in satellite TV, digital TV, telephone modems etc.

So first you need to download the software, So head on to Google, Find and download this application. We will be using version 2. The file name is jqamv2_setup.exe

It took me weeks to figure out the proper setting combinations that worked well enough. This application can be very picky. A slight miscalculation could lead you to never being able get a connection established at any rate. I will save you the trouble and walk you trough the steps at tweaking the settings next.

Step 2: Putting It All Together

Picture of Putting It All Together

After the installation of the software. Start up J-QAM. If the icon is not on your desktop then look into your start menu. By default the application runs in demodulator mode. This is fine for receive but we need to configure J-QAM to work as a modulator

-Go under Options

-Bring up Settings

-This should bring you to the General tab. From here check Enable Modulator box and de check Enable Demodulator. This sets J-QAM to modulator mode.

-In the General tab. Make sure your sound device points to the proper sound card line out device you may be using. If not then select it from the dropdown list.

-Make sure your Sample Rate is set to 9600. This should be the default. Still won't hurt to take a quick look before moving on.

-Under Modulator General Settings decide what content you wish to stream. Check the Unreal Streaming box if you wish to stream Audio/Video content using UnReal Server. If not then keep it de checked, If you wish to transfer files "Packet Radio Style" Then make sure you check Enable WebServer box.

-Some settings to change to get the 60.4 kb/s rate.

At this point you should still be under Modular General Settings.

-Change Center Frequency to 7250. (7250x2=14500 KHz About the max bandwidth a radio can handle on FM)
-Change Gamma to 0.51 ( A lower Gamma results in faster speeds but harder to decode. Lower then this gives unpredictable results and will depend on equipment limits etc..)
-Change Alpha to 0.01 ( This figure is probably false but forces the software to use the smallest value possible. This setting needs to be very low to compensate for the lower Gamma )
-Change Mod FIR Size to 300.
-Change Frame Period to 300.
-Change RS Error Percent to 15%
-Change Interleaving Length to 600

Next step is to configure the receive end.

Step 3: Let's Tune In

Picture of Let's Tune In

Now it is time to configure your receiver end of the link. As in the first step. Install the J-QAM software on the receiver system. Since the default settings run as demodulator mode. You don't have to change any settings. Just make sure your sound card input settings are correctly selected. Then click on Start button within the J-QAM main window. Make sure your receiver radio has a good signal and that the audio cord is plugged into the correct line in port of your computer or laptop.
Now wait a few moments. Take a look at the main J-QAM window. The Carrier Frequency will fluctuate as the receiver gets a lock. The closer you are to a successful lock the less the numbers jump around and more stable it becomes and then shows you the proper carrier frequency in our case look for 7250 or a number that gets closer and closer to that one. You may need to carefully adjust the line in volume input for J-QAM to get a lock. Start at 1 % and with the keyboard. Hit then up arrow key to bring up the volume 1 percent. Wait a few seconds while monitoring the main J-QAM window as you do this until you get a lock. As you get closer to the proper volume you will begin to see the 7250 frequency appear as detected under Carrier Freq Display on the Main J-QAM window. Within a few moments you will get a successful lock and there you have it.


If you don't have a transmitter to use. You can just use an audio cable in between 2 soundcards. Playing around with settings in this direct configuration can expect speeds of about 400kb/s. This could also be possible with special or modified radio equipment.

What I Tried That Works:

-No problem getting the 60.4kb/s connection rate with Walmart FM transmitter anywhere in the house. Even the weak spots yield good results thanks to error correction.

-Streaming QAM modulated audio to virtual audio cable and streaming via mp3 over a network to a shoutcast server. The client side plays the stream within virtual audio cable with J-QAM in demodulator mode.

-Streaming 1kb/s AAC audio over HF via J-QAM. ( AM broadcast like voice quality )

-Streaming 2kb/s AAC stereo 44.1 kHz audio via J-QAM on AM broadcast band. ( Close to FM Stereo Broadcast quality )

-Streaming live near HD quality video and audio via J-QAM on FM broadcast band. 1 watt transmitter with antenna outdoors about 25 feet up equaled to about 2 miles of range.

-Packet modem using RS232 port features of J-QAM on 2meter ham band to transfer large files across a repeater 30 mile range.

-Ham 2meter fast scan television like quality live broadcasting webcam video stream with the help of J-QAM

What I Tried That Did Not Work:

I wanted to stream the QAM modulated audio to my Raspberry PI and transmit using the FM transmitter code for the PI. This resulted in a disaster. Could not get a lock from the transmitted signal. My guess is the PI's internal clock chip drifts to much to keep the modulated information accurate enough for a decode.


If you like this software please consider taking a look at the software developers website and purchase the recent now a paid for software version 3.0 that includes more features and bug fix.


kpataska (author)2014-12-01

A MODEM is a MOdulator/DEModulator - this is a modulator and a separate demodulator.... Half duplex not full duplex..... One way ONLY. Transmitters do not receive.

joel.lagace.14 (author)kpataska2014-12-01

It can be configured to send and receive via RS232. It is an advanced configuration and requires more transmitters but it can be done. The software needs to run in both modulator and demodulator mode.

About This Instructable




Bio: I live in Cochrane. I own and operate a radio station in our local community (Iroquois Falls). When I'm not working with the radio ... More »
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