Today, I repeated the remaining troublesome measurements, this time without the Radio Shack SWR/power meter (which includes a diode based circuit).
Sbits -->? ?15dB uHF pad? -->? 20dB pad --> 20dB Pad ->? Siglent spectrum analyzer, internal attentuator 10 dB, bandwidth 10kHz
Band? ? ?%power? ? measured dBm? ? ? ? Second Harmonic? ? ? ? ? ? ? ? ? ? ?Third Harmonic
Freq? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?freq dbm? dbc? ? ? ? ? ? ? ? ? ? ? ? ? ? freq dbm? dbc
20m? ? ? ? ?78%? ? ? ? ? ? -11.2dBm? ? ? ? ? ? ?28M? -47.21dBm -36dBc? ? ? ? ? ?42M? -52.7dbm -41.5 dBc
14.010
15m? ? ? ? 86%? ? ? ? ? ? ?-13.14dBm? ? ? ? ? ?42M? -35.1 dBm? ?-22dBc? ? ? ? ? 63M? -41.2dBm? -28 dBc
21.010 MHz
Unfortunately these aren't any better and just a tiny bit worse than previous measurements after I removed the diode from the Heathkit cantenna.??
On a whim, I decided to demonstrate what could be done with a little more filtering.? ?The only thing have I readily available is a 15m dual-section Butterworth filter that I have built as part of our Field Day antenna multiplexer work.? ?It has GREAT attenuation away from 15 meters.? ?So I threw THAT in series between the sBitx and the 15dB pad....and here is what happened:
21.010M? 86%? ? ? ? -13.57dBm? ? ? ? ? ? 42MHz? BELOW -88 dBm? ? ? 63MHz? BELOW -88dBm
Spurs -- were approximately -61 to -63 dBm....which means -48 dBc to -50 dBc --?
The noise floor of the analyzer was -88dBm.? ? The harmonics were therefore BELOW the observable limit and were at least 75 dBc below!? ?
Here is how beautiful the display looked:? ?This is a completely, totally, thoroughly legal spectrum.? ?With filtering, this is possible.? I'll admit, this requires volume for the parts....
I think this makes an interesting additional product possible.? ?A very simple filter system could be marketed as an add-on that would add even one section of additional filtering, or one more low pass section -- and reduce the harmonics quite significantly.? ? Alternatively when I use my vacuum tube amp, with its tuned filters.... it cleans everything up also.??
Now that my errors in measurement have been corrected, we have a much clearer picture of where Serial #141 falls well or not so well.
Gordon KX4Z
