Farhan,
Just to double check, I disconnected the parallel cap and ran the drive level down well below the point where power started to drop, and the second harmonic was still only -40 DBC.
The two section half wave output filter was pretty standard, especially for QRP rigs, before the last rule change (2003) where all TXes installed after that time were required to have suppression of all spurs to better than 43 dB below the fundamental.
Since that time more effective output filters have been needed for transmitters with single ended power amps.? Push pull amps suppress their even harmonics when properly balanced so that is why you may still see the two section half wave filters used with them as the second harmonic is suppressed in the amp.
When the rules changed in 2003, I went back and added parallel caps to one section of every half wave output filter in my homebrew rigs and simply selected whatever value was needed to closely resonate the inductor in use to the second harmonic.? This worked very well in every case and made no difference in power output on the fundamental frequency.
For info, if the value of L7 is indeed 1.1 uH, then a 110 pF cap paralleling it would resonate at approximately 14.4 MHz.? The 100 pF cap resonates at some 15 MHz, but is still close enough.? In my tests it dropped the second harmonic down to -56 dBC at the low end of the band and about -60 dBC at the top end.
And, for info, the addition of the parallel cap will not harm or otherwise change normal operation of the rig.
In terms of the voltage rating of the C0G (NP0) cap, you typically see 100 volt caps in rigs of 5 to 10 Watt output or so, 200 Volt caps in rigs of 25 Watts or so, and 500 volt or higher rated caps in amps of 100 Watts or more.? In this case a 100 Volt rated cap will be just fine and perhaps even a 50 volt rated unit as long as the PA is operated with a 12 to 13.8 Volt supply.?
It is simply good amateur practice to ensure that our transmitters meet current standards.
Whether or not you have the means to test your rig for second harmonic suppression after adding the cap, if you parallel L7 (or L4) with a 100? or 110 pF NP0 100 Volt cap you can rest assured that your second harmonic will no longer be a problem.
73,
Wayne
