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If by "ridiculously tricky" you mean "requires specialized equipment and domain knowledge". Like say, skiing. With a cheapish Rigol spectrum analyzer and home

If by "ridiculously tricky" you mean "requires specialized equipment and domain knowledge". Like say, skiing. With a cheapish Rigol spectrum analyzer and homebrew receive antennas and DIY wooden/pvc tripods you could probably do 3m open-air measurements to within 3dB of what you would measure in an anechoic chamber at a test laboratory if you were very careful. I can 100% assure you that is possible with a commercial dipole kit and horn antenna, neither of which is that expensive if you're getting paid. This in in the FM band so you could use dipoles and a low-end analyzer to the tenth harmonic.

myself248 2y ago
You can absolutely hook an antenna to a spec-an, but how do you calibrate the result? I can get a power measurement of mV or dBm at the instrument input, but that's not how the regs are written. They're written as field strength in free space (microvolts per meter), which as I understand it, either requires a calibrated field probe, or a calibrated antenna. I'm sure there's a way to perform such calibrations from first principles, but that's where I'm invoking "tricky". If you've got a trick to it, please share. In practice, Part 15.23 is pretty generous about home-built equipment, and I think since the "200 foot" thing came from the FCC's own mouth, if you can show that you walked the perimeter with a portable radio and confirmed that you're good, that probably shows good faith on the power issue. ... Absolutely yes a spectrum analyzer is helpful to make sure your harmonic filtering is working. I would put that forth as "good engineering practice" under 15.5, and it's far more than any hobbyist could be reasonably expected to do even just a few years ago; this equipment has gotten mindbogglingly affordable and accessible.
- PaulHoule 2y ago
  I have to admit that having a ham radio license that would be easy to take away is one reason why I wouldn't mess with anything questionable radio-wise. I wonder though if you could trust the published specs of a commercial FM transmitter to calibrate a power meter on it from a distance of a mile or so...
  - bityard 2y ago
    The FCC doesn't revoke ham radio licenses except in egregious circumstances. Most of the time, it's someone who is deliberately interfering with public safety or commercial frequencies, and is told to stop, and doesn't. Even the douche canoe that intentionally interfered with firefighting efforts recently only got a hefty fine. He still has his license. Ham radio licenses are granted by the FCC with the understanding that the the license-holders intend to experiment and explore radio technology. And further understanding that while licensees are expected to adhere to all of the rules, mistakes will sometimes be made. Hams are therefore given wide latitude in terms of self-policing and forgiveness for accidentally or unknowingly breaking them from time to time.
- buescher 2y ago
  You get a field strength the same way the pros do - from the antenna factors for your antenna and the cable loss. Look at a public FCC test report for an exposition of the method. DIY dipoles and even horns are not going to be very much different from ideal. Calibrated antennas are typically not more than a dB or two off ideal. Dipoles not even that unless they're broken. Horns and log-periodic antennas, enough that it can matter - but fine for hobbyist practice and even most pre-compliance work. I once discovered an RF engineer was using ideal gain numbers for a horn antenna instead of the calibration data for pre-compliance - it fortunately did not make a practical difference. The Roberts balun for measurement dipoles is standardized and I have instructions for building one around here somewhere. That is literally the reference for calibrating other antennas. The primitive method for getting an antenna calibration is the three-antenna method. If you are particularly motivated, you can build three Roberts dipoles and check them. For a number of reasons, you might want to have three dipole kits anyway if you are doing a lot of these measurements. "Tricky" in the sense that skiing is tricky. You will struggle learning from books and the internet. Starting from scratch with DIY equipment would be quite challenging. But it's all in standards and the open literature.
  - myself248 2y ago
    > The Roberts balun for measurement dipoles is ... literally the reference for calibrating other antennas. Solid gold search term right there, thank you! > The primitive method for getting an antenna calibration is the three-antenna method. More good stuff. Seriously, I hadn't gotten here and this is suddenly demystifying a lot of fundamentals. Wave if you see me on the bunny hill! ;)

myself248 2y ago

You can absolutely hook an antenna to a spec-an, but how do you calibrate the result? I can get a power measurement of mV or dBm at the instrument input, but that's not how the regs are written. They're written as field strength in free space (microvolts per meter), which as I understand it, either requires a calibrated field probe, or a calibrated antenna. I'm sure there's a way to perform such calibrations from first principles, but that's where I'm invoking "tricky". If you've got a trick to it, please share. In practice, Part 15.23 is pretty generous about home-built equipment, and I think since the "200 foot" thing came from the FCC's own mouth, if you can show that you walked the perimeter with a portable radio and confirmed that you're good, that probably shows good faith on the power issue. ... Absolutely yes a spectrum analyzer is helpful to make sure your harmonic filtering is working. I would put that forth as "good engineering practice" under 15.5, and it's far more than any hobbyist could be reasonably expected to do even just a few years ago; this equipment has gotten mindbogglingly affordable and accessible.

PaulHoule 2y ago
I have to admit that having a ham radio license that would be easy to take away is one reason why I wouldn't mess with anything questionable radio-wise. I wonder though if you could trust the published specs of a commercial FM transmitter to calibrate a power meter on it from a distance of a mile or so...
- bityard 2y ago
  The FCC doesn't revoke ham radio licenses except in egregious circumstances. Most of the time, it's someone who is deliberately interfering with public safety or commercial frequencies, and is told to stop, and doesn't. Even the douche canoe that intentionally interfered with firefighting efforts recently only got a hefty fine. He still has his license. Ham radio licenses are granted by the FCC with the understanding that the the license-holders intend to experiment and explore radio technology. And further understanding that while licensees are expected to adhere to all of the rules, mistakes will sometimes be made. Hams are therefore given wide latitude in terms of self-policing and forgiveness for accidentally or unknowingly breaking them from time to time.
buescher 2y ago
You get a field strength the same way the pros do - from the antenna factors for your antenna and the cable loss. Look at a public FCC test report for an exposition of the method. DIY dipoles and even horns are not going to be very much different from ideal. Calibrated antennas are typically not more than a dB or two off ideal. Dipoles not even that unless they're broken. Horns and log-periodic antennas, enough that it can matter - but fine for hobbyist practice and even most pre-compliance work. I once discovered an RF engineer was using ideal gain numbers for a horn antenna instead of the calibration data for pre-compliance - it fortunately did not make a practical difference. The Roberts balun for measurement dipoles is standardized and I have instructions for building one around here somewhere. That is literally the reference for calibrating other antennas. The primitive method for getting an antenna calibration is the three-antenna method. If you are particularly motivated, you can build three Roberts dipoles and check them. For a number of reasons, you might want to have three dipole kits anyway if you are doing a lot of these measurements. "Tricky" in the sense that skiing is tricky. You will struggle learning from books and the internet. Starting from scratch with DIY equipment would be quite challenging. But it's all in standards and the open literature.
- myself248 2y ago
  > The Roberts balun for measurement dipoles is ... literally the reference for calibrating other antennas. Solid gold search term right there, thank you! > The primitive method for getting an antenna calibration is the three-antenna method. More good stuff. Seriously, I hadn't gotten here and this is suddenly demystifying a lot of fundamentals. Wave if you see me on the bunny hill! ;)