Continued from Part Two.
When I put the kit on the test bench, I found that it didn’t meet FCC Part 97 requirements on any of the six bands: 10, 15, 20, 30, 40, or 80 meters.
Part 97.307 requires that “. . . 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.”
Here are the results of my testing:
| Band | Frequency | Power (dBm) | Harmonic spur (relative) | VHF spur (relative) | Test Image |
|---|---|---|---|---|---|
| 10m | 28,126,075 | 22.86 | N/A | -41.75 | Image |
| 15m | 21,096,075 | 23.24 | N/A | -42.20 | Image |
| 20m | 14,097,075 | 24.49 | N/A | -42.50 | Image |
| 30m | 10,140,175 | 21.17 | N/A | -42.95 | Image |
| 40m | 7,040,075 | 25.28 | -42.66 | N/A | Image |
| 80m | 3,594,075 | 25.27 | -39.89 | N/A | Image |
Note that except for 80m the kit just barely doesn’t meet the -43 dB spec.
What does -43 dB really mean? for a fundamental output power of 23 dBm, which is about 200 milliWatts, the maximum allowed power of the spurious emissions is -20 dBm (23-43), which is 10 microWatts. That’s really a tiny amount of power. Here’s a handy table showing the dBm to Watts conversion.
Does it really matter whether it meets part 97 if the output level is so low?
Aside of the fact that it’s not legal, I think that the answer is yes. As amateur radio operators know, we live with an ever increasing amount of radio frequency noise. Consumer products are the source of most of this. Compact fluorescent bulbs, LED bulb power supplies, TVs, and many other devices are distributed sources of noise. Even if each of these meets applicable regulations, they still contribute. As amateur radio operators, I think we have an obligation to set an example, and do the best we can.
If you add the 5 Watt amplifier option to the kit, or use it with another amplifier, you will make the problem even worse.
In this case of spurious emissions, most of them fell into a range of VHF frequencies with allocations for radio amateurs, aviation, public safety, and TV stations. It’s just good practice to avoid interference there, especially when it’s a problem that can be solved.
Even though it wasn’t far out of compliance, I was surprised that the kit wasn’t well within specs. My expectation was that it would meet part 97 with plenty of ‘headroom’, in order to account for natural variations in the way it’s built by different people.
According to the QRP Labs web site, the kit “Produces 250mW RF output on 30m (lower output on higher frequency bands)” (250mW is about 24 dBm), and in the manual it states that the prototype tested at 23 dBm (200 mW).
My measured power output ranges from about 21 – 25 dBm, very close to what was stated but slightly higher, which turns out to be a significant fact later. My output as I first set it up was “250 mW on 10m, (higher power on lower frequency bands)”. Comparing that with the spec for the kit in the previous paragraph, there’s a clue to one of my problems.
A search turned up previous issues with this kit and VHF emissions, which were discovered by the ARRL testing lab. The linked report is good and a lot of work was put into the investigation. It’s worth reading. These issues should have been fixed in my version, because it shipped in May of 2017.
One of the fixes they found was replacing the nylon spacers between the main and filter boards with metal ones. This provided an improved rf ground path between the boards and decreased the spurious VHF emissions. Even though I had a revised board design, I decided to try changing out the spacers to see how that might improve things.
Illruminations 