When those sidebands are 60 Hz steps from the main signal, then there is a 60 Hz mains and the audio going into an nonlinear part in the transmitter and those signals are mixed together. More often the sidebands are in 120 Hz steps due to full wave rectification in power supply. If in an power amplifier its gain is dependent of voltage, then it acts as an amplitude modulator and the modulation waveform is a kind of triangle 120 Hz that contains multiples of 120 Hz. That happens easier, when most output power is pressed from the amplifier. Keeping amplifier in linear range normally helps. 40 dB down is quite good, but could be better; there are many signal with sidebands a bit more than 30 dB down. 73, Reino oh3ma
-----Original Message----- From: Jim Brown [mailto:k...@audiosystemsgroup.com] Sent: 6. heinäkuuta 2019 9:28 To: wsjt-devel@lists.sourceforge.net Subject: Re: [wsjt-devel] 60 Hz + harmonics sidebands on FT8 signals? There are two common causes of sidebands with a station that is properly adjusted and uses good equipment. 1) The best gear (Elecraft K3, K3S, or a Flex 6000-series), and a first class power amp) have sidebands roughly 40 dB down on both sides of their signal due to IM distortion in the RF stages. These sidebands are generally symmetrical, and the total -40 dB bandwidth is about 680 Hz. 2) Doppler shift produced by reflections from aircraft. These sidebands are generally asymmetrical (that is, on one side or the other of the main signal). Here in the SF Bay Area, which includes Silicon Valley and extends down along the Monterey Bay, we have glide paths for major airports in San Jose and San Francisco. There are roughly a dozen or so locals running 500W with gear like this, and we all regularly experience what you're seeing. 73, Jim K9YC On 7/5/2019 11:02 PM, Paul Kube wrote: > ’ve seen various kinds of problematic FT8 signals that I think I > understand -- “barcode” harmonics etc. – but what is going on here: _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
