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Re: LF: Effect of LP-filter om efficiency

To: [email protected]
Subject: Re: LF: Effect of LP-filter om efficiency
From: Richard Rogers <[email protected]>
Date: Wed, 09 Jun 2004 10:51:39 +1000
In-reply-to: <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
G'day.

Robert, AX2TAR, tried transmitting without a lowpass filter.   I live about
15Km from him and could barely hear the 3rd harmonic, but his 19th harmonic
at about 3.4MHz was very strong. Presumably that was where his antenna wire was a half wave long and coupled
through the stray capacitance across his loading coil.

At 10:06 AM 6/8/04 +0200, you wrote:
  To All from PA0SE

The following subject may have been discussed on the reflector before but
I can't remember it.

  drain of the FETs is high, current is zero; when current flows voltage
is almost zero.
But what happens if the transmitter is followed by a low pass filter with
a shunt capacitor at the input, as is often the case?
Looking at it in the frequency domain one could say that the shunt
capacitor causes the square wave to start becoming a sine wave.

 a very high kick-back voltage at the drain of a single-ended amplifier,
almost certain killing the transistor.
 amplifier this is prevented by the other transistor that starts to
conduct at the same moment.
Provided of course coupling between the two halves of the primary winding
is very tight.

So it seems to me that class D and E amplifiers should preferably be
followed by a low pass filter with a series inductor at the transmitter
side.
Please correct me if my conclusion is not valid.

An interesting question is whether a low pass filter is really necessary
when the transmitter feeds the aerial via a series tuning coil.
Harry Grimbergen, PA0LQ, has given this some thought and he says the
following (my translation).
Quote: The third harmonic of a square wave is 9.5 dB weaker than the basic
frequency. On the other hand radiation resistance increases with frequency
squared; 9 dB for d3. Nevertheless harmonics will be suppressed almost 50 dB.
About 16% of the power in a square wave is in the harmonics and this power
is reflected back into the final amplifier.
I have been able to show this also using simulation by MICROCAP. Unquote.
 The Netherlands would be that the radio inspector does not measure
harmonics as field strength but as power in the output of the transmitter
(or LP-filter, when present). So selectivity of the aerial system does not
help.

Any comments will be very welcome.

73, Dick, PA0SE


73, Ric, VK7RO

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