In the minimum I got 6db improvement, it depends also on the
output
impedance and layout of the components. I gained 10db more
at high output impedance.
Most are 1000-1500 ohms, 160m is 3 kohm.>>
Are you saying the mid-impedance of the network, or the
impedance across the loading capacitor, is 1000-1500 ohms?
Or 3000 ohms at that point on 160?
If so, I agree with you. You could get 6dB or more
additional harmonic suppression.
The problem is the loading capacitor has 3000 volts peak
voltage at 1500 watts using a 3000 ohm mid impedance, and
2100 volts peak using 1500 ohms at 1500 watts
So while you might gain 6dB or more harmonic suppression
into a 50 ohm resistor load with a good layout, you now have
to deal with 3000 volts where most amplifiers only have 700
volts peak. This is OK in a monobander but not easy to
handle with components in a production bandswitched
amplifier.
On the other hand I can get more than 20dB addition
suppression with a regular pi if I add a simple series tuned
trap to ground, and the added cost might very well be close
to nothing. Why not just use a shorted 1/4 wl stub?
I'd hate to have people think that a conventional pi-L with
a mid impedance of 200 ohms or so would do anything remotely
close to what a system with a mid impedance nearly the same
as the anode impedance would do. Wen should probably all be
careful to not generalize and just say "a pi-L does this"
because it really depends on the layout, the load impedance,
and the mid impedance of the pi-L.
In the case where we can use a normal size bandswitch, cover
160-10, and a reasonable voltage loading cap we are lucky to
gain 3dB for all that work....and very likely could make
things worse if we are not careful with the layout. And that
is into an ideal load...not an antenna where things can go
very bad and the pi-L actually increase the harmonic
levels..
It's pretty tough and very expensive to obtain the perceived
harmonic advantages of a pi-L in the real world of
multi-band amps. It's much cheaper to do it other ways.
73 Tom
Before I used simple Pi, but for SO2R M/M operation I
converted them all.
Vacuum output capacitors were used already except on the
160m amp, no padder
anymore.
Usually the Pi-coil is above the chassis, the L is below it
together with
the load cap.
In the 160m amp both coils are above chassis but 90 degrees
to each other
and there the difference is very remarable.
The middle impedance on the lower z amps is 600-800 ohms and
I am using 1200
ohms on the 160m amp.
Antennas are monoband as well.
M/M is possible while antennas are 3m apart on the same
mast.
I tried Pi-L on a new built multiband amp but gave up and
went to Pi again,
to many problems with arcs in the bandswitch.
73
Peter
-----Original Message-----
From: Tom W8JI [mailto:w8ji@w8ji.com]
Sent: Donnerstag, 29. März 2007 01:04
To: Peter Voelpel; amps@contesting.com
Subject: Re: [Amps] Pi-L Network
> Would be very interesting.
> And the Pi-L IS better then the Pi on harmonic
> suppression.
> I use it in all my monoband amps.
So how many dB better is it into your actual antennas?
What middle impedance do you use?
Do you partition or shield the L coil from the main tank
coil and tube
leads??
73 Tom
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