[amsat-bb] External Amplifier Legality Questions

Zach Leffke zleffke at vt.edu
Wed Mar 19 11:18:38 PDT 2014

Hello everyone,
     I have a question about FCC Rules regarding external amplifiers.  I 
am considering re-purposing a wideband commercial amplifier for amateur 
use as a final stage for my ground station. The amplifier is capable of 
producing about 100W of power between 20 MHz and 500 MHz.  I want to use 
this amp in conjunction with coax relays to switch between the 2m band 
and the 70cm band depending upon which spacecraft I'm operating.

My questions are as follows:
Is it legal to use an amplifier designed for commercial applications in 
the Amateur Bands?  This amplifier does not meet spurious signal 
suppression requirements, so I plan to use band specific filters after 
the amp to make sure I knock any spurs down below the required limits.  
Does the amplifier have to be type certified for use in the amateur bands?

As far as I can tell reading Part 97.315(subsection 3), The amplifier 
itself does not need to be type certified if:
"(3)The amplifier is sold to an amateur radio operator or to a dealer, 
the amplifier is purchased in used condition by a dealer, /or the 
amplifier is sold to an amateur radio operator for use at that 
operator's station/. [emphasis added]"

I'm planning to use it in my amateur station so I don't think it has to 
be type certified.  Am I missing something?

Also, The amplifier's 2nd Harmonic is about -30dBc down at 145 MHz. 
According to 97.307e:
"The mean power of any spurious emission from a station transmitter or 
external RF power amplifier transmitting on a frequency between 30--225 
MHz must be at least 60 dB below the mean power of the fundamental."

I'm planning to use a filter with about 40 dB of suppression at 290 MHz 
(2nd harmonic), so I get about -70 dB of suppression at the second 
harmonic, meeting the emission requirements of 97.307e.

I see no specific spurious signal suppression requirement for the 70cm 
band.  The amplifier has similar 2nd Harmonic characteristics and I'm 
planning to use a similar filter for the 70cm band so the spurs should 
be down by about 70 dB of the fundamental mean power as well.  The only 
wording I see that might apply is 97.307c:
"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. "

I'm taking steps to eliminate interference by suppressing the 2nd 
Harmonic down about 70dB from the fundamental "in good engineering 
practice" so I think I meet all the requirements.  Again, what am I missing?

I've heard of lots of hams re-purposing amplifiers for amateur use 
without legal concern, and I think this might be the case for my 
situation.   I'd like some advice from those that may have gone down 
this road in the past.  Any and all feedback from the group on this 
would be greatly appreciated.  If I'm way off, please let me know.

Thanks in advance.

Zach, KJ4QLP

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