The future requirement for 50dB of harmonic suppression in the HF range is
pretty difficult to achieve with a pi-L - the second harmonic component in
the plate current is only 6dB down in Class AB1 - unless the Q is higher
than desirable. The Q of the L network is dependent upon the intermediate
impedance ( assuming a fixed load of 50 ohms), and the usual considerations
apply to the Q of the pi part. A complication in the real world is that the
load impedance at the harmonics is not usually 50ohms, so the attenuation
can be greater or less than the 50ohm dc - to - daylight load condition.
>From replies received, it seems that the disadvantage of too high an
intermediate impedance is that voltage can be enough to cause a problem.
It's doubtful that an intermediate impedance high enough to cause excessive
Q is likely - it needs to be 5050ohms to give a Q of 10.
My plan is to use a 3kV vacuum variable for C2, so I don't expect a
problem.....
The future requirement is from ITU-R Recommendation SM329. Which countries
will implement this Rec isn't yet known, but it will probably be accepted
within CEPT. I'm told the US proposed the 50dB limit (actually 43dB + 10 log
P, not exceeding 50dB) but whether the FCC will put it into Part 97 is
another matter.
It seems that if a pi-L won't give sufficient harmonic rejection, it may be
easier to use a straight pi, and simple low pass filters for those bands
below 18MHz where transmatches aren't used.
I haven't looked at the ARRL handbook - I use a Smith chart program to
produce the values, so I couldn't say if there was an error or not.
73
Peter G3RZP
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