AM Transmitter Test

Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.

I put the transmitter from the previous post to the test. I changed the circuitry a bit though. I kept the oscillator (50 MHz) is now continuously powered. I programmed the ATTiny 2313 microcontroller (using PWM output) to send an oscillating signal to the base of a transistor (NPN). In this way the microcontroller PWM output didn’t supply power to the oscillator, but rather grounded it. I got a big boost in range this way. Yesterday I couldn’t even hear the signal in the parking lot of my apartment, whereas today I heard it loud and clear. I decided to take a drive with my scanner, laptop, and Argo to see how far away I could get and still detect the signal. With this bare bones transmitter setup (using a 2M J-pole antenna) I was able to detect it over 4,000 ft away. The receiving antenna was a 2m ~1ft high antenna magnet-mounted on top of my car.

This is the signal captured by Argo running on my laptop in my car as I drove away

In retrospect, I should have run Argo at my apartment and drove the transmitter farther and farther away. I presume that my transmitter is functioning decently, and that if I attached it to a proper antenna (and had a better receiving antenna) I might be able to get some cross-town distance? I’m still learning – this is the point though, right?

This is where I was when the signal died. The red marker (upper right) is my apartment where the transmitter was, and the signal began to die right as I traveled south on Chickasaw past Lake Underhill (~4000 ft away). This immediate loss may be due to the fact that I passed under power lines which parallel Lake Underhill which interrupted the line-of-sight path between my 3rd story apartment balcony and me. If this were the case, supposedly if I kept driving south the signal may have improved.