Laptop, transmitter and coupler box
My Procaster system finally came in this week! It's a small AM transmitter made to abide by the rules laid down by "Part 15", part of a document that allows low power radio use without a license, including broadcasting. I got the system used from a guy who ran it from a red caboose as a radio station for the shopping center attraction. Click on any picture for a bigger one
The transmitter's case is a little smaller than I expected it to be, mostly due to the surface mounted components used on the board. Both power and audio go through a 4 wire cable from the black remote box. This allows the transmitter to be located outdoors in the clear for best range. 12 volts comes from a power cube, and audio is though a 1/8 inch (3.5MM) stereo plug, the channels mixed down to mono, and fed balanced to the transmitter. The remote box contains a simple compressor/limiter, and this can be turned off by a jumper if you like to run your audio 'barefoot'. The laptop came with it, and the former owner had it set up with Zara Radio as automation in its former location at the caboose.
The circuit is straightforward and uses commonly available components, a great design move in an electronics world where special purpose integrated circuits offer great features, but are often discontinued in a few years. For the simple purpose of an AM transmitter, simplicity is a gain. The case is made from aluminum, and the cover has a foam seal to keep it water tight. One thing you can see is that the blue transformer has some rust on its frame. I found a small gap between two of the foam strips in the cover, probably not wide enough to let water droplets into the case, but enough for humid air to get in and cause some corrosion. In addition to the transformer rust, there's white crusting on solder sections too, likely due to moisture.
The broadcast frequency is generated by 3 CMOS chips: 4060 oscillator/divider, a 40103 divider and a 4046 phase locked loop, in a version of a well known circuit in transmitters of this type. These ICs have been around for years and should be available for decades yet. The audio chip is an LM386, and the output transistor a FQT7N10L 100 volt mosfet that feeds the big air wound coil to the left. The top side of the coil is tuned with a trimmer capacitor to ground, adjusting for a peak on the meter for maximum signal output to the antenna. The frequency is set by a bank of small switches, and covers 1290 to 1710 khz., as detailed in the manual.
I set the transmitter up on my patio, keeping the default frequency of 1610, since that's clear in my area during the day. I found a 3 foot piece of galvanized fence post and stuck it in a hole in the patio that had been used for a clothes line pole at one time. The transmitter mounts with two U-bolts that just managed to fit around the pipe and hold it snugly. AM radio needs a good ground for the most range, and the wires underneath are an attempt at a simple ground, connecting the transmitter's ground bolt to anything metal in the area. The antenna is 3 pieces of aluminum that fit inside each other, think of a portable radio with a rod antenna that pulls up, just in giant size. Once extended, tightening the hose clamps keeps the sections from sliding back down again. This setup is temporary, I just wanted to get something on the air for testing. The location is bad for getting a signal out with the short antenna, it's surrounded with lots of things like my house, a small hillside, bushes, a tree, and a neighbor's tree and house farther away. I plan to mount it on a side of my house that's more in the clear and with a better ground, including a ground rod.
So far the transmitter has been working well, and it sounds good, just connecting a computer's output directly into it with no EQ, compression or other processing. The tune up was easy, just turn it on adjust the variable capacitor for a peak on the meter, which with my setup wasn't too touchy. I started the audio and went for a walk with two portable radios, a Sony with a handle and a pocket model from Radio Shack. I went to the next block and was still getting the signal, but since it had just gotten dark, CHHA from Toronto was coming in on 1610 and jamming the signal. The Procaster was tight on frequency too, only about 1 hz off from CHHA, according to the signal beat heard.
Procaster setup and grounding manual 2010