Understanding Spurious Emissions


Howard Fidel
 

There have been a few threads discussing how the uBitx has spurs that may exceed the spur amplitude allowed by law. Ham radio is regulated by Part 97 of the FCC regulations. The allowed spur amplitude for frequencies below 30 MHz is in section 307. I have reproduced paragraph D below:

(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission. For transmitters installed on or before January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must not exceed 50 mW and must be at least 40 dB below the mean power of the fundamental emission. For a transmitter of mean power less than 5 W installed on or before January 1, 2003, the attenuation must be at least 30 dB. A transmitter built before April 15, 1977, or first marketed before January 1, 1978, is exempt from this requirement. 

The first interesting thing about these regulations is that they are not consistent across time. If you have a transmitter that was installed before April 15, 1977 it is not even regulated for spurious emissions. Before 2003, the spurs were limited to 50 mW max and must be at least 40 dB below the carrier power. Today, the constraint is -43 dB below the carrier power. So if you have a 1 KW transmitter, you are allowed to have spurs that do not exceed 50 mW, hence the earlier 50 mW limit. To keep things in perspective, if your uBitx puts out 5 watts of power on 15 meters, 50 mW of energy would be only 20 dB down from your carrier. 
So if you use your uBitx barefoot (i.e without a linear amp) although you may exceed the allowed spur amplitude on the 15 and 10 meter bands, your radiated power level will be so low that it is virtually impossible for it to interfere with other services, and that your radiated emissions maybe in line with what other ham operators are radiating legally. Also, I might add that these spurs are not consistent in amplitude  from unit to unit. I measured mine as being in compliance, but right at the -43 dB limit. Furthermore, the testing is done into a dummy load. When connected to an antenna, your SWR at the spur frequency is probably high, so you are radiating even less energy. 
My personal take away from this is that although the uBitx may at times not be in technical compliance with the regulations, operating at frequencies above 21 MHz, it is in compliance with the intent of the law, which is to prevent interference with other services. You can help your uBitx stay in-compliance by not over driving your audio, and by only operating it barefoot above 21 MHz. 
So, the bottom line is you should not use the uBitx above 21 MHz if you are concerned with the letter of the law, but you may use it if you are concerned with complying with the intent of the law.

Howard

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