Message-Id: <3.0.6.32.20040609105139.007f5ad0@pop.netspace.net.au>
Date: Wed, 09 Jun 2004 10:51:39 +1000
To: rsgb_lf_group@blacksheep.org
From: Richard Rogers <vk7ro@netspace.net.au>
In-Reply-To: <6.1.0.6.2.20040608100555.02801340@POP3.freeler.nl>
Mime-Version: 1.0
Subject: Re: LF: Effect of LP-filter om efficiency
Content-Type: text/plain; charset=windows-1252; format=flowed
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Content-transfer-encoding: 8bit

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