[Lowfer] The real filter deal!

Thu, 5 Sep 2002 16:52:41 -0500

Throw the filter as far as you can. How about a series resonant circuit
inline with the feedline like Jim suggested??? Or a parallel resonant tank
at the output of the driver and tap down to match the feedline impedance???
I'm still working on the basics of running PSpice so I can't be of much help
yet.

Mike>WE0H
http://www.we0h.us/lf.html

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Bill Ashlock
Sent: Thursday, September 05, 2002 4:19 PM
To: [email protected]
Subject: [Lowfer] The real filter deal!

Filter friends,

I think I finally figured out what is happening here! The filter must loaded
with it's design load resistance over a wide range of frequencies or the
first L and the shunt C to ground form a series resonant circuit that
effectively short-circuits the PA. Try this:

1) Set up a simulation, or bench circuit using the famous 63uh-18nf-63uh
filter with a 50 ohm load and a generator with a 1 ohm source resistance.
The frequency plot of voltages and currents will look reasonable.

2) Remove the 50 ohm load and watch the input current increase to the same
numeric value as the source voltage at the resonant frequency of the filter,
IE: 7A if the input is 7V!  (i = v/1ohm).

3) Re-attach the 50 ohm resistor but in series with the resistor add a
series resonant circuit composed of an L of 5mh and a C of 140pf (This
represents either a vertical antenna or a reflected-back loop load via the
stepdown transformer). The current again goes out of sight. Why? The filter
is not seeing a 50 ohm load over it's range of input frequencies.

In general, if the resonant frequency of the 63uh/18nf pair is close to the
resonant frequency of the 5mh/140pf pair some real strange things happen to
the voltages and currents and I'll guarentee this is what is causing all my
bench testing problems! If the resonant frequency of the filter is changed
to at least twice the resonant frequency of the load things begin looking
fairly good (IE: descent filtering of the upper harmonics and normal current
into the antenna load), but there is still a nasty peak at the resonant
frequency of the filter that could cause some undesirable radiation if this
falls on a harmonic of the square wave out of the PA.

So where do I go from here?

Bill

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