[Lowfer] Re: Amplifiers for 136k

[Bill Ashlock]

John A,

I checked out the frequency response using Pspice of the output stage of 
your posted transmitter schematic with a transformer and loop attached and 
found the following characteristics: (BTW the filter is very close to Lyle's 
Simple Transmitter's output stage)

1. The filter adds a good degree of harmonic suppression at the loop over my 
filterless stage.

2. The current in the loop is somewhat sensitive to the filter's cutoff 
frequency as would be expected, so the filter needs to be tuned to maintain 
a flat loop current over the full Lowfer band - but no tuning is required to 
cover the 2k wide 136k band (assuming the filter is re-tuned for this band).

3. The output impedance of the stage, in spite of the filter, is close to 0 
ohms, and hence, makes a poor match for a coax line (take notice Mike).

4. When used to drive a loop with the typical stepdown transformer the 
second inductor can be removed without effecting the filter's response. I'm 
still trying figure one out. If the transformer is removed, and the output 
of the filter is loaded with a resistance equal to the reflected resistance 
of the loop, the second inductor is definitely needed.

This all leads back to my original thoughts last week on the simplicity of 
using a two transistor switching PA out in the woods at the base of the loop 
for the 136k, 100w application. Adding a core with four or five turns and a 
mica cap is no burden at all!

Bill

>From: "John Andrews" <w1tag@charter.net>
>Reply-To: lowfer@mailman.qth.net
>To: <lowfer@mailman.qth.net>
>Subject: Re: [Lowfer] Re: Amplifiers for 136k
>Date: Fri, 23 Aug 2002 11:08:41 -0400
>
>As Bill so cheerfully pointed out, there was an error in the Lowfer
>transmitter PA schematic that I posted yesterday. An updated version has
>been uploaded, and can be found at the same address:
>
>http://webpages.charter.net/w1tag/files/TAG-XMTR.jpg
>
>Sorry for the error.
>
>John Andrews
>


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