Hello,

I'm modifying a 28-50 MHz transverter to work the new (for us) 70 MHz
band, not having a spectrum analyzer I'm trying to get as close as
possible with substitutions. I have a doubt about the low pass filter
after the final amplifier:
on the arrl handbook I have found formulas for pi-filters:

----|----L------|--------
=C/2 =C/2 R
----|-----------|---------

(I hope the ascii art is readable)

L=R/(pi Fc) C=1/(pi Fc R)

Now the 50 MHz original filter is composed of three cascaded filters
as the one above, the capacitor are 47pF at the two ends and 100 pF
the two middle ones, which is consistent with the formula as the
middle ones can be actually seen as two parallel capacitor for the
cascaded pi-cells, the formula indicates an Fc of about 63 MHz with
these values, which seems correct.
The inductors are 6 turns, internal diameter 6 mm, enameled 0.8 mm
wire, no spaces between turns for the two "external" inductors and 7
turns same diameter for the internal one.
These inductors should have an inductance of 0.17uH the 6 turns ones
and 0.21 uH the 7 turns one and the formula don't agree with these
values, can anyone explain where I'm wrong?
Thanks in advance

Francesco IZ8DWF