Simon Küppers designed an antenna by using a “standard” dual-band Dipole design (using sleeving bars). The antenna acts as a dipole at 2m, and as a dual-ended half wave at 70cm giving about 3dB of gain. Dipole wires are just 2mm diameter spring steel held in place using 3D prints.
I feed the dipole from a single-ended coax using a marchand balun. This is a type of balun usually used on PCBs (or MMICs) consisting of coupled lines. By choosing the correct even/odd impedances (essentially trace widths) and trace lengths, you can achieve a very broadband balun. In fact, in theory this balun has to transmission poles that are spaced at fcenter/2 and 3*fcenter/2 which fits perfectly into the 70cm/2m band if the designed center frequency is around 290 MHz. You can find some info on Microwaves101 or in Electronics page 142.
Read more about the design in the GitHub post here.
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