I'm still very puzzled, even after fixing errors in my experimental setup (principally the DIODE in the Heathkit cantenna!)? that I still appear to have harmonics that are above the limits.? ?Even when using a commercial high power "pad" and no heathkit at all.? ??
I was concerned that the low pass filters might not be robust enough.? ? To study that theory, I realized that I could lift the power output wire from the big transformer of the final stage....and inject spectrum analyzer tracking generator (through a 6dB pad) right there at the beginning of the filter system.? ?I could read the output of the low pass filters by connecting (through a 6dB pad) the RF input of the Siglent spectrum analyzer to the output.? ?I set the DRIVE at 1, and mic gain at 0, and lower sideband on all bands.? ?There was no apparent signal that the spectrum analyzer saw before turning on the tracking generator.??
A schematic of my setup:
There could be a mistake that I'm not recognizing, so here is a PHOTO of my setup:
When I measured the frequency response of the filters, they looked very very effective.? ?Here is a table of the -1dB point, the 2nd harmonic point, the -24 db point, and the 3rd harmonic point.? ? Frequently at the tails my measurements were "in the noise" with my settings (approx 10kHz VBW and probably 20dB of internal attentuator -- it didn't dawn on me to adjust that)
BAND? ? ? -1db point? ? ? ?2nd H. Freq? ? -24 dB point? ? ?3rd H. freq
80 M? ? ? ? ? 5.4 MHz? ? ? ? ? -22dB? ? ? ? ? ? ~ 7.06MHz? ? ?~-60 dB
40M? ? ? ? ? 11.4 MHz? ? ? ? ? -21 dB? ? ? ? ? ~ 14.6 MHz? ? ?-51 dB
20M? ? ? ? ? ?16.4 MHz? ? ? ? -44dB? ? ? ? ? ? ?21.6 MHz? ? ? ? -57 dB
15M? ? ? ? ? ~3dB@...? ? ?-21 dB? ? ? ? ? ? ?43 MHz? ? ? ? ? -57 dB? ?(I had to change the Siglent to go to much higher frequencies to get this one measured)
Those look (to me) like rather impressive filtering!!!? ?
When your 3rd harmonic frequency (as proxied by a tracking generator, tiny signal) is being attenuated by 50-60dB....how in the world can you end up with a signal stronger than -43dBc???It appears to me that there are a few possibilities:
A)? The Siglent is measuring improperly when measuring the power from the sBitx.? ?(However, it sees a used ICOM 718 as completely clean)
B)? With full transmittter power,? something in my MEASUREMENT system is generating harmonics -- clearly I had some issues there, but now I'm able to make measurements using ONLY commercial pads.... and the Siglent....-- and besides, this didn't seem to happen with the Icom 718??
C)? With full transmitter power, something in the SBITX is generating harmonics.
Does anyone see why it wouldn't be (C)??? ?Am I missing something?? ? ?Maybe I'm doing something stupid again???
Here are photos captured as I mapped out the various filters - they look very impressive to me.
Gordon Gibby KX4Z
