Who needs low pass filters (LPF)? Today’s state-of –the-art transceivers and amplifiers can indeed pass the FCC requirements for spurious emissions. Nonetheless, they can still generate enough spurious RF energy to interfere with the TV and broadcasting receptions. Your aim is not only to pass the FCC’s test but your neighbor’s, isn’t it? When you run high power, an external LPF is advisable. Suppose you are running 100 W barefoot on HF with 43 dB attenuation for spurious emission as per FCC requirement, your spurious emission power is 0.005W or 5mW. You want to run a kilowatt power amplifier. You get 1000W on the fundamental frequency along with spurious emission of 0.05W or 50mW. It is sort of QRP on harmonic frequencies! Simply put, when you run power amplifier, your spurious emission is also boosted by the same formula. When you run legal limit 1500 W, your spurious emission power is 0.075W or 75mW. Solution: Improve your spurious emission by adding an external LPF. Use the 120 dB attenuation external LPF. Your spurious emission power is down to 1.5 × 10-10 W (0.00000000015) or 150 pW (picowatt) at 1500W operation level. Even if an internal LPF in the transmitter is failing, you will still get a PPM level cleaner signal! Satoru Matsushima JA1EUI, a founder of Sagami Engineering Mfg has designed uncompromised high quality LPF’s, CMF’s and coaxial antenna switches for the amateur radio community. Satoru is an ex-engineer at Anritsu in Japan. He was involved with radios and measuring instruments designs there. Anritsu’s flagship measuring instruments are famous and that what he uses for his own design works and quality assurances.
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