The local FM transmitter
Serendipity. I choose this from a long list on ebay. Just so happens, I am very happy with my choice. Ten bucks.
This is a Model CY-688 flashlight, clock, thermometer, and of course, MP3 player FM stereo transmitter.
On this board, there are three devices that amplify the FM signal from the left of A to A. Above the device at A there was a chip capacitor that coupled the signal to the MP3 player cord which is the antenna for this product. Iíve removed that capacitor and added a capacitor to the output to couple the signal to the coax cable. By this time Iíve removed the USB port and battery, B, and have it running on a lab supply. Neat battery for an MSP430 project. It is a hefty, rechargeable, Lithium Ion. You can't see it well but where the coax passes off the board I've twisted a piece of #22 around it and soldered it to a pad. Use a strain relief like this so you don't stress the connection to your capacitor at A.
The device on my proto board is an old MWA-130. There is no point in wasting time matching up some discrete device here if you can find a hybrid like this. I'm measuring 19DBm of signal at a scope load output into 50 ohms. If you just want to broadcast a hundred yards, this is as far as you would need to go, almost. You will have to add the appropriate power supplies and possibly put it in a box.
But I'm looking for a watt! The next device I found in my junk box was a 2N4427. A one watt VHF transistor. It is not a great device but it works.
Upper left, the MWA-130. Upper right, an LM317 regulator to power the MWA-130. Down the left is the matching section for the 2N4427 input. The short piece of coax goes directly to the base of the transistor. This keeps the input well away from the output. The transistor emitter is soldered to a little piece of upright PC board. Once all the legs are connected it is very stiff and rugged. As the case of the device is the collector, you can't solder it down to the board like the MWA-130. On the lower right is plenty of room to work out the output matching section. Now it was time to break out the bird.
I had a 5B slug for this so that is 9/10ths of a watt. By the time I got done tuning up I had a watt flat across the band. The best I could do with efficiency with that old transistor was 40%. Now that I have the bird on line I can work out a quick and dirty collinear antenna. That's coming later. So, back to the FM synthesizer.
The back light for the display times out a few seconds after a button is pressed. I wanted it to stay on. The engineer of this board must have been planning. By moving the LED- lead to the solder dot above the label it stays permanently grounded. BTW, this is a very interesting board. There are a set of pads marked as ports. Some with signals and some as inputs. Of course, without documentation, there is not much you can do with them.
By now I've removed the flashlight LED and the MP3 input plug. They would be in the way of mounting this to a panel.
Well, that's where I'm at this morning. I have to shelve this for a couple of days but when I get it finished I'll update this page.
Oct 17, 2008
I whipped up a cheap antenna. This is known as a 'Slim Jim'. I'll never measure the gain but it is said to be as good as a collinear two to three times longer.
The grey thing is our crazy cat. So, I propped this up in the lab and got in my truck with my real dog to make the post office run.
Almost two miles. By then, I could hear some whooshes as I drove. But there is a rise to the half way point so once the antenna is above my roof, I should be able to get a very good signal to everyone in the 'Home' area with just one watt.
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Page created: 14 Oct 2008 rev: 17 Oct† 2008