It's only taken me about 20 years
on LF to actually think about, and calculate, this...
We all know we're supposed to
attenuate harmonics and unwanted signals, so most LF
amplifiers, especially switching types, incorporate a low
pass filter - usually the traditional T or Pi design.
But I always felt the hi-Q antenna
assembly would do a pretty good job of filtering in its own
right, and didn't use any LPF on either the 700 Watt 137kHz
Tx which has a tuned tank output (Ql = 6) or the 400W class
E 475kHz unit.
However, thought it was time to try
to calculate the actual attenuation of harmonics. So,
using the inductance of the loading coil which goes up
proportional to harmonic number, N, Rrad of the antenna
that goes up with N squared and a crude approximation of
Rloss changing as 1/N, plugged all those into a spreadsheet
and found the following on my 7m high Tee antenna
At 137kHz the relative radiated
level of harmonics is between 50dB down at 2nd and 3rd,
dropping to 45db at 8-9. Considering the switcher PA plus
tank is about -30dB to start with that looks well within the
recommended -70dB for high power amps
At 475kHz low order are only 33dB
down and higher up -30dB. I don't have a figure for the
class-E starting harmonic level, but IIRC it was of the same
order as the 137 Tx, so again that looks near acceptable.
Of course, your antenna / loading
coil assembly will differ, and larger antennas / smaller
coils will not provide as much attenuation but it is still
enough to suggest the 5th order filters in use on some
designs are a bit OTT.
One thing I haven't considered is
how distributed capacitance in the loading coil might
compromise rejection of higher order harmonics, but that can
be mitigated by the Tx producing less of them in the first
place.
Did the original Decca transmitters
have any extra low -pass filtering ?
