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Re: [Amps] Low Pass Filters

To: "Robert B. Bonner" <rbonner@qro.com>, <tjjapha@earthlink.net>,<amps@contesting.com>
Subject: Re: [Amps] Low Pass Filters
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Mon, 4 Dec 2006 06:02:34 -0500
List-post: <mailto:amps@contesting.com>
The impedance of the filter is going to be 50 ohms.  That 
alone will keep
the voltage under tow.  The current through the filter 
however will be
substantial.  If the actual power is 600 watts average and 
average = PEP
then your antenna current should be 3.46 amps and the 
voltage should stay
below 200 volts.  (175)>>>

Voltage is NOT a heating problem, so you have to use peak. 
It would be square root of (600*50) or 173 volts RMS. The 
peak is 245 volts.
Still that number is meaningless by itself except for the 
input and output capacitors (assuming a capacitor input and 
output lowpass).


Current through the capacitors is a function of the 
capacitors reactance and voltage across the capacitor, and 
is not the same as current from the transmitter. It is 
always higher.

The sharper the rolloff of the filter the higher the current 
will be compared to current from the rig or to the load.

The voltage is also often higher in the middle of the filter 
than at the input or output.

The same thing applies to inductors and tank circuits, and 
the components in those. Like the iron core in an L network.

Knowing the actual current and voltage is important. Rating 
filter and tank components by "power level" is not a good 
way to do things, because it all depends on the application.

73 Tom 


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