I'm on it. I only have a basic ceramic cap marked 101 will this do? Or does it need to be monolithic or something different?

We shouldn't have any noticeable differences in our daily operating styles, shall we?

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In his note it is stated to use a cap with a suitable voltage rating. What would that rating be? Thanks Steve kb1chu

Or this one: http://bitxhacks.blogspot.com/2017/01/nd6ts-suppression-of-pesky-2nd-harmonic.html?? de ND6T

ND6T had a nice informative post about a month before: ??

I have been advancing the silver trimmer to the point of achieving a amp meter reading of 1 Amp plus a little bit while speaking up into the mic. Is this overdriving the BITX 40? I have found that it gives me the expected amp meter and watt meter readings from the wire up instructions that I not had before. I understand that increasing it and the bias beyond the recommended settings increases risk of damage and radiating wild harmonics.. Am I doing this right? I can take instruction if not. And, does the cap change anything if I am not overloading the PA?

Saturday night we had the magic band conditions, and I heard my call signal really well on three WebSDR stations on each side of the U.S.

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

Farhan,

This may be a duplicate post, I replied to the group a bit earlier but haven't seen it come through.

Just to double check, I disconnected the parallel cap, turned the drive down well below the point where power started to drop and retested the output.? At the bottom of the band the second harmonic was still only -40 dBC.

The two section half wave output filter was pretty standard, especially for QRP rigs, before the rule change in 2003 that required transmitters installed after that time to have all spurs suppressed to more than 43 dB below the fundamental.?

After that time more effective output filters were needed, especially in transmitters with single ended PAs.? Push pull amps suppress their even harmonics when properly balanced so you may still see the half wave filters used with them.

When the rules changed in 2003 I went back and installed a parallel cap across one of the inductors in every one of my homebrew rigs that used the half wave filters, and simply selected a value that would closely resonate the inductor in use to the second harmonic.? This worked well in every case and had no effect on the output at the fundamental.

In this case, if L7 is indeed 1.1 uH then the 100 pF cap resonates it to some 15 MHz which reduced the second harmonic in my Bitx40 to -56 DBC on the low end of the band and some 60 dB below the fundamental on the high end.? A 110 pF cap would resonate it to about 14.4 MHz but the 100 pF was close enough.

In terms of voltage rating we usually see 100V units in output filters for rigs of 5 t0 10 Watts or so, 200 Volt units in rigs of 25 Watts or so, and 500 Volt or higher caps in rigs of 100W and up.? In this case a 100V unit is fine if the PA is run off 12 to 13.8 Volts.

Whether or not you have the means to test your unit, if you parallel L7 (or L4) with a 100 pF NP0 (C0G) 100V cap you can rest assured your second harmonic will no longer be a problem.

It is simply good amateur practice to ensure that our transmitters meet current specs.

73,

Wayne

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