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Output Filter Issue?

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Zazzle:
Hey redhat,


--- Quote from: redhat on April 19, 2015, 2247 UTC ---Keep in mind too that the online calculators for filter design rely on ideal component values.

--- End quote ---

Jep. :) The C's I've caculated (the old school way with formulas from books and with a cualtaor) ended up with C-values way off from the standard values. But well, paralleling and adding a variable C helped here. I verified my values by using online calcs. Teh values are about the same. +/+ some percent. So I think I got that right.

Sadly, I don't call such equipment my own. :)

Greetings,
~Zazzle

ff:

--- Quote from: Zazzle on April 20, 2015, 0730 UTC ---
Nono, like written, the lowpass ALSO takes care about the impedance matching. It's possible to combine both. There isn't much to match anyway. I'm aiming for 1,5W, which means an PA impedance of approx 52R.

--- End quote ---

We all have our own ways of slaying the RF dragons, Zazzle!   :D  Over the years I have found that my highest chances of success are had when I'm not getting too ambitious.  Absolutely you can roll an LPF that will do the impedance match but I have found it much less frustrating to use a separate transformer.  As Redhat said, different ways of winding coils and capacitance variations within their tolerances will skew a whole lot of good math (is there any such thing as GOOD math?)  Yes, if you're shooting for 1.5 watts then you should need no matching - IF - the feedpoint of your "fuchskreis" is approximately 50 ohms.  I know nothing about that antenna design so I leave that to you.  Other things I would check:

RF choke - do you have shorted windings?  Also bad solder joints here will make the choke look like a resistor and will cause heating of the PA.  A compromised RF choke will also mess up the stage loading.

Do you have a series coupling cap in the PA output circuit?  A lot of low power designs have one.  If you have one that is starting to go bad it can cause similar problems as yours.

Since your operating frequency is around 10 MHz and your LPF is calculated well above that frequency, my guess is that is not really your problem.  I have had troubles when I try to calculate the filter cutoff too close to the operating frequency (see Redhat's admonishment above).

One more thing - I don't trust SWR meters to give me more than a RELATIVE reading of the situation unless the meter is connected right to the antenna feedpoint.  But that's just my opinion of course.  Okay Zazzle, good hunting!

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