IN3LBQ on the Hühnerspiel (Amthorspitze) - photo by IN3OTD

Homemade 6 dB hybrid coupler

A long time ago, well before the end of the last century, I was curios to know the 3rd order intercept point (IP3) of my HF receiver but I was missing quite some of the equipment needed. The two signals generator needed for the test tones were built with two XTALs for the 20 band about 20 kHz apart, found at a ham radio fair. Then I was missing a signal combiner, so I just built a basic one, following the classical circuit for a so called "6 dB hybrid coupler".

I had some 50.5 Ω, 1%, resistors around (non-standard value for the E96 series, don't remember where they came from) and for the transformer I tried several small ferrite cores scrounged from old TV and radio sets, the main source of electronics components at that time; in the end I settled on a big ferrite bead which seemed to provide less losses than the others on the HF band.
And for many years I have used this combiner (well, not much, honestly), without really characterizing its performances first:

Homemade 6 dB hybrid coupler

Then recently I had to opportunity to measure it on a modern multiport network analyzer, so I quickly got some nice graph showing all its S-parameters:

Homemade 6 dB hybrid coupler measured S parameters More in detail, the following graph shows the ports couplings: Hybrid coupler measured coupling Hybrid coupler measured return loss