Rasmus SA5RJS on Nostr: 6) RF choke. This is made with two turns of 1mm enameled copper wire around a ...

6) RF choke.

This is made with two turns of 1mm enameled copper wire around a binocular core. It was difficult to bend and thread the thick wire through the core and I accidentally scraped away some enamel in the process. The turns were stiff and didn't touch, so I didn't worry too much about it. I fixed everything in place with hot glue later.

The inductance of the choke turned out to be 4 μH. The reactance becomes roughly 180 Ω for 7 MHz and 350 Ω for 14 MHz. The 50 Ω output impedance of the output transformer secondary looks like 0.6 Ω from the primary (assuming a 1:3 turns ratio, and thus a 1:9 impedance ratio). So in the current divider formed by the choke and the output transformer, most of the current goes to the output transformer (more than 99% I think?) because 0.6 is much smaller than 180.

Actually, the amplifier is a push-pull design with the output transformer doubling as a combiner. So my back of the envelope calculation is probably off by a factor of two in some direction. If you know how to reason about the output impedance of a push-pull amp like this, please feel free to explain it to me!

#AmateurRadio #HamRadio #electronics