Thanks for all the good ideas
I had concerns that the 2nd/3rd harmonics where abnormally strong
(overdrive.. Etc),But after looking at the output both of the K3 and
the amplifier on a spectrum analyser, I¹m certain Nothing is out of
the ordinary. I see that the K3 output is very clean And the raw
output of the amp is ok - 2nd is -23dBc and 3rd is -23dBc. But
Bear in mind that I used the directional coupler to measure the output
And the coupling ratio changes with higher frequency. (I assume)
I decided to buy a Low pass filter designed by F5CYS - uses PFTE spacers
To make capacitors and is rated well above the KW level. I gave up finding
the
Proper material here in Iceland.
Taking a better look at the failed LPF - it appears the heat was generated
in
The PCB material - not the coil as the underside of the copper was burned.
Other than that - the amplifier is performing very well and will be pushed
On JT65 when I get the LPF - and some antennas pointing to the moon!
73, Benni TF3CY
On 30/04/14 05:08, "John Lyles" <jtml@losalamos.com> wrote:
>Built a lambda/4 stub out of 9 inch diameter EIA hard line, for
>the big amps at work. It was build primarily to pass a tuning shaft
>through the output line to the variable capacitor paddle for output
>coupling to the plate/screen resonator cavity. Originally I thought it
>might need water cooling, so that too was planned to pass through the
>quarter wave stub. It took some modeling and measurement to get the stub
>length to be high Z at the operating frequency as when the diameter of
>the outer conductor approaches fractional wavelength then the geometric
>length isn't exactly what works. This was all explained in Robert
>Pound's chapter in the MIT Rad Lab textbook on coaxial line center
>conductor supports. He figured out how to broadband the stubs so they
>wouldn't have VSWR when the operating frequency shifted slightly.
>
>http://www.ieeeghn.org/wiki/index.php/Oral-History:Robert_Pound
>
>Anyway, my big stub worked very well on suppressing second harmonic.
>Better than -50 dBc right out of the amplifier. Wish it still reflected
>a short to the 4th harmonic, 805 Mhz, but due to being far overmoded (9
>inch coax that is), it doesn't work.
>
>Essentially it is forcing a short for the even harmonic at a particular
>distance from the tee to the plate region of the tube. It modifies the
>plate current or voltage waveform. This sort of modification is similar
>to what is done with class F amplification, using the technique of Tyler
>from Marconi, later used in RCA broadcast transmitters. In my amplifier,
>I probed inside the main cavity in a safe location, and discovered that
>there wasn't much second harmonic being generated, due to the quarter
>wave stub on the output.
>
>73
>John
>K5PRO
>
>
>
>> Date: Tue, 29 Apr 2014 02:17:38 -0700
>> From: "Jim Thomson" <jim.thom@telus.net>
>> To: <amps@contesting.com>
>>
>>
>> ## Do you really require a LP filter ? You could also use a .25 wave
>>long coax stub..with far end shorted. You can also add additional
>>stubs etc.
>> Paralleling caps is the real answer to your exploding cap problem. 2-4
>>caps in parallel would be the ideal ticket. Your typ 2m ant will only
>>be resonant on
>> its 3 and 5th harmonic. You can make stubs that will kill odd
>>harmonics if I remember correctly. W2VJN has a good book on stubs.
>>
>> ## Parallel some caps..and you wont have any more issues. I have used
>>the teflon sheeting sandwiched between 2 aluminum plates,,and that
>>works superb...
>> provided it is constructed correctly.
>>
>> ## I like the idea of a LP filter......... then it kills everything
>>beyond the cutoff freq. A bandpass filter would even be better. You
>>could also use a LP filter
>> for TX....and a low power bandpass filter...for RX only.
>>
>> Jim VE7RF
>>
>>
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