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Sometimes ham radio (amateur) operators in your area like to listen to very distant signals, from all over the world, this is called dxing.
It is possible that even a properly adjusted RangeMaster can interfere with some of the frequencies these hobbyists use, even though the equipment is within legal specifications.
The rules require that equipment's harmonics be 20db down (1/100 less) then the fundamental frequency.
Here is the Part 15 rule: (see section c)
Section 15.219 Operation in the band 510 - 1705 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater
power) shall not exceed 100 milliwatts.
(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3
(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the
level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification
may be based on measurements at the intentional radiator's antenna output terminal unless the intentional
radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by
measuring the radiated emissions.
We recommend cooperating with any hams whenever possible, perhaps you could offer to shut down if they are chasing a particular signal. A filter at the output of the transmitter is not a good idea, the antenna is a high impedance radiator, not low impedance, often hams will ask if they can add a filter to your transmitter to further reduce emissions.
Our unit is one of the quietest on the market with virtually no radiation above 70Mhz, it was approved for use in the Federal quiet zone near the NRAO (radio telescope) in West Virginia. see link