[hfsig] Sidebands on Raspberry Pi WSPR carrier - how much does a homebrew 1:1 balun help?

Ron Ott ronott at sbcglobal.net
Wed Sep 7 22:14:36 EDT 2016

Construction:  I received the parts to make a 1:1 balun this afternoon and wound 5 turns each of #30 insulated wire for the primary and secondary on an Amidon FT 37-75 toroid core.  The primary and secondary windings are bifilar wound (interleaved) around the toroid with leads coming from opposite sides of the "O".  The balun is installed in a small plastic box with BNC connectors on each end, no potting or glue was used.
Insertion loss: I measured 1dB loss at 14MHz when the balun was inserted in a coaxial line from my HP signal generator and terminated in 50ohms; with some experimentation, this loss could probably be reduced as I just guessed at the number of turns based on the AMRAD design for a LF balun.  As a result, my ERP will decrease from 18dBm to 17dBm.
Effect on sidebands due to AC modulation:  Listening to myself on the air on 20m while transmitting with the Raspberry Pi, I still get occasional double spots due to a sideband 120Hz from the carrier frequency. However, the difference betwen the carrier and sideband signal strengths can be over 50 dB with the balun, according to WSJT-X, making the RPi WSPR transmitter apparently within FCC specs for spurious transmissions.
Looks like this is a worthwhile addition to the RPi TAPR kit setup and helps reduce the sidebands 10s of dB, at least at my station.
73, Ron

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