Amazon.com: Balun One Nine v1 - Tiny Low-Cost 1:9 HF Antenna Balun and Unun with Antenna Input Protection for Ham It Up, RTL-SDR, HackRF and Other HF-Capable Radios. Great for DIY Dipole
SPICE model of Zcm of a Guanella 9:1 current balun – owenduffy.net