> Hi Joe, I started it when I asked the question about the
> use of a 4to1
> torroid step down transformer in place of a coil and
> switch assembly for
> the Pi-L. Many of the commercial ham amps use this method.
Jim,
These threads seem to take off into all kinds of tangents,
but here is what I was saying before we got lost on talking
about what we think happens in commercial amps.
1.) I've prototyped and measured harmonics to see that the
amp meets TA for at least two dozen amps, probably more.
2.) There was a statement an amp couldn't or likely wouldn't
meet TA because it takes a pi-L to get enough harmonic
suppression.
3.) I know that is wrong, because I have measured dozens of
amps that pass by a wide margin that don't use pi-L's. I
have no doubt at all that Commander and QRO can exceed TA
limits by a very comfortable margin without a pi-L.
Let me put some hard numbers on this people can look at,
rather than all the wild speculation. We had a claim an HF
AB linear could barely, if at all, pass FCC TA with a pi
network.
Look at QST's review of the AL800H. It uses a conventional
Pi network on 40 and up and a pi-L on 160 and 80. The ARRL
measured 2nd harmonics and worse case harmonics of:
160m -61 2nd and -56 worse case on the 8th harmonic (this
is with pi-L)
80m -50 2nd (which is also worse case) and this is a pi-L
40m -59 2nd and worse case also is the 2nd....and here it
is a pi. No L section!
So we can all see by looking at published data we can run on
and on about what happens but when we actually look at an
actual amplifier we suddenly learn layout and stray
impedances have much more to do with harmonics than throwing
an extra coil in the circuit.
If we look at a number of amplifiers that do NOT use a pi-L
we can find 60dB of harmonic attenuation or more. I have an
AB2 class triode homebrew amp running next to me right now
using vacuum caps that does much better than -60dB for any
harmonic on any band. So the claim we need a pi-L is easily
proven wrong, and not just by what I guess is happening.
4.) In at least a half dozen amps when I needed more
suppression, adding a L section and using a reasonable
center impedance (say 200 ohms) didn't help any significant
amount and actually increased some higher order harmonics.
Now it is true I could have shielded the tube from the tank
and the pi from the L and probably not increased the higher
harmonics, but that wasn't a reasonable solution when a
change of some lead lengths caused everything to fall into
spec for no cost increase at all!
I can show data from a dozen amps or more where a pi easily
meets all requirements, and I can probably show data (if I
still have the engineering test files) where adding the L
section made things worse on some harmonics.
On the other side of the coin we seem to have people who
"believe" through some blind faith that a pi-L always makes
things better and always is required.
So from actually having measured many different amps in all
configurations.... my position is a pi-L might help. It
also, if the layout doesn't fully isolate or shield the L
section from sources of harmonics as most amateur layouts do
not, it might make some harmonics significantly worse. It
also is a fact that what we do on a dummy load is actually
less repeatable with a real antenna when we use a pi-L. All
it takes is the antenna system go high impedance and
capacitive and by adding the L section, if center impedance
is the traditional few hundred ohms, the second harmonic can
go way up in level.
73 Tom
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