W3BE'S BE Informed!
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BE Informed No. 1.18

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How Steady Must My Station’s Transmissions Be?

John B. Johnston W3BE

Q. How steady must my stations transmissions be?

A. As a minimum, your station’s transmissions should must be sufficiently steady such that all emissions resulting from modulation are confined to the frequency band or segment authorized to the station by Section 97.301. Section 97.101(a) good amateur practice, moreover, calls for a much higher degree of transmitter frequency stability. Most notably, for instance, just to gain access to a repeater usually requires the using station to transmit on a pre-announced input channel with some degree of frequency stability. 

   There are, nonetheless, six discrete frequency channels authorized specified for the 60-meter band. Perhaps our rules are acknowledging that there is no longer any need for our regulator to be concerned with amateur station transmitter stability; simply stating its expectancy for technical excellence is sufficient for regulatory purposes. After all, it was the pursuit of technology that overcame instability – issue-by-tortuous-issue – from our unsteady 20th Century apparatus:  temperature change intolerances, unregulated DC power effects, shock and vibration disturbances, etc.              

Q. What about spurious emissions?

A. Section 97.307(c) says: All spurious emissions from a station transmitter must be reduced to the greatest extent practicable. If any spurious emission, including chassis or power line radiation, causes harmful interference to the reception of another radio station, the licensee of the interfering amateur station is required to take steps to eliminate the interference, in accordance with good engineering practice.

Q. At a club meeting, we discussed at great length spurious emissions from VHF transmitters. What is Section 97.307(e) trying to regulate? It reads: For a transmitter having a mean power of 25 W or less, the mean power of any spurious emission supplied to the antenna transmission line must not exceed 25 µW and must be at least 40 dB below the mean power of the fundamental emission, but need not be reduced below the power of 10 µW.

   At least 40 dB below the mean power of the fundamental emission would make any spur +10 -40 = -30 dBm = 1.0 microwatt.  But, the rule also says: . . . need not be reduced below the power of 10 microwatts, which is -20 dBm, or 30 dB below the fundamental emission. There seems to be a conflict here. 

A. It seems to be an attempt to regulate the degree of interference to reception. It does, however, seem to be in a conflict with Section 97.307(c) which says that all spurious emissions from a station transmitter must be reduced to the greatest extent practicable. If any spurious emission, including chassis or power line radiation, causes harmful interference to the reception of another radio station, the licensee of the interfering amateur station is required to take steps to eliminate the interference, in accordance with good engineering practice. Then Sections 97.307(d) and (e) go on to codify possibly less stringent limits on those same spurious emissions. 

   Your concern is the second sentence of Section 97.307(e), which applies only to transmitters in the 30 – 225 MHz range having a mean power of no more than 25 watts. The mean power of any spurious emission from such a transmitter may be as much as 10 microwatts. Even that may be exceeded – to as much as 25 microwatts – as long as it is at least 40 dB below the mean power of the fundamental emission. It says, moreover, that a transmitter built before April 15, 1977, or first marketed before January 1, 1978, is exempt from this requirement. In which case, Section 97.307(c) stands alone.

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October 10, 2016

Supersedes all prior editions