brainwagon

Before toddling off to bed last night, I did a bit more tinkering, and a bit of thinking, and then a bit of research.

The YouTube video I made showed that the spurious radiation from just attaching a clip lead from the oscillator to my oscilloscope gave enough signal to inject itself into my RFSpace SDRIQ software defined radio, even without any antenna attached. I hypothesized the signal should be stronger with my regular 40M dipole antenna, so I tested that, and sure enough, my FT-817 attached to the 40m dipole easily is S7, even without doing any antenna tuning. This suggests that for any application on my small property, I could reduce the power significantly (and use little/no antenna) and still have very legible signals.

I used a 3.68Mhz crystal because I have something like 200 of them (I bought them for something like $3 for a big bag) but that’s kind of a ham unfriendly frequency. While it’s not very likely for such a weak signal to propagate significantly at the power levels we are talking about, but it’s perhaps possible that someone within a radius of a mile or so might be able to hear it, and be annoyed. Steve Weber used 4Mhz crystals, which are at the edge of the 80m band, and probably less well trafficked. A good, friendly idea.

Last, I was trying to understand what the regulations actually say about legal Part 15 operation. The regulations allow for experimenters to build five transmitters of this type for experimentation. On the 80m, you are suppose to keep the average field strength at just 15 microvolts per meter at a distance of 30m. Working through the approximations presented in Understanding the FCC Regulations for Low-Power, Non-Licensed Transmitters, this suggests that the product of power and antenna gain should be less than 6.6E-8 or so. While I know the input power, I don’t know how to measure (or estimate) either the output power or the antenna gain. Can anyone point me at a reference which shows how I might calculate the properties of a small 6″ long stub antenna at 4Mhz or so? I know it’s mostly an academic exercise, but it’d be nice to know the rough limits are, and the FCC regulations for Part 15 do say that we are supposed to “employ good engineering practices in order to ensure compliance with Part 15 standards”.

One final thing: as you can see from the oscilloscope, the output is far from a clean sine wave. It’s not horrible, but I was able to detect it at 2x and 3x the carrier frequency. The obvious thing would be to design a filter to clean it up, but most filter designs assume a 50 ohm (or thereabouts) output load, which this clearly doesn’t have. Is that a problem?