[Amps] T networks and harmonics

[Jim Kearman]

From: "Gary Schafer" <garyschafer at comcast.net>

> I guess this was in response to your response to someone that said that "a T
> network tuner after the amp would not help reduce harmonics as it was
> basically a high pass filter". You said that "it would help some".

Of course, this is beyond the scope of my original post about the need for pi-L output tanks to meet current FCC requirements, but thanks for the backup. The Regs require spurious emissions be below a certain level _at the output of the transmitter_, which would imply the output of the amplifier used in conjunction with a transmitter or transceiver. Not the output of the antenna tuner unless it is integrated into the amp/transmitter. 

Also, while a T-network _may_ attenuate harmonics sufficiently so that the total system meets the spirit of the Regs, it's hard to know the effect of any combination of antenna and tuner settings. If the second harmonic at the amp output is -35 dB and the tuner adds another 8 dB, you have the -43 dB required by the Regs, though not in the right place. Yes, I doubt FCC is going to come knocking, but we want to build according to the best Amateur standards.

So I think we can draw two conclusions: A pi network is inadequate, and a t-network antenna tuner is neither a legal nor a necessarily adequate solution.

In case the Commission is reading this, I assure them my pi-net amplifier was "installed" prior to 1978! 

73,

Jim, KR1S
http://kr1s.kearman.com/