Hi Amp'ers
The harmonics from my homebrew amplifier, on some bands, are much lower
than expected, and I've been wondering why.
The output Pi tank coil is made up of 3 orthogonal coils for 10-12m,
15-80m & 160m, mounted at 90 degrees to each other, see
http://granta.digital-crocus.com/linear/Mvc-078x.jpg
The 15-80m coil is tapped in 3 places and turns are shorted out by the
band switch. The other 2 coils, 10-12m & 160m, are not tapped.
Now a Pi-tank, transforming 1400R to 50R with a working Q of 12, has a
2nd harmonic attenuation of about 30dB and an amplifier running in class
B has an anode current that is a half sine. So the resultant 2nd
harmonic level at the output can reasonably expected to be about 35-45
dB down.
However there's more, on the bands where there are shorted
mutually-coupled turns, the harmonics are attenuated much more.
The level of the second harmonic relative to 1kW on each band is shown
below.
160m -45.3 dBc no shorted mutual turns
80m -47.0 dBc no shorted mutual turns
40m -58.5 dBc 7 shorted mutual turns
30m -56.3 dBc 7 shorted mutual turns
20m -62.9 dBc 10 shorted mutual turns
17m -70.0 dBc 13 shorted mutual turns
15m < noise 13 shorted mutual turns
12m -45.3 dBc no shorted mutual turns
10m -54.2 dBc no shorted mutual turns
See http://granta.digital-crocus.com/Linear_design_notes.php3
It seems that the shorted turns are absorbing harmonic energy. Has
anyone seen this effect before?
73, Mark, G4AXX
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