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1:16 transformer & feedline in one (for end-fed 800ohm)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AB1WX, Nov 14, 2021.

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  1. AB1WX

    AB1WX Ham Member QRZ Page

    I wanted to make a quick end-fed for 40m to see how a particular space/arrangement would perform. I found a few pieces of wires that added up to about 18m, so that part was not a problem. But I don't have a 1:9 or higher ratio transformer handy, so I had to make something.

    I had a lot of old CAT5 ethernet cables. I cut one about 8m segment and parallel-connected the four pairs at the TRX end, and series-connected at the antenna end to make a Guanella 1:16 transformer. (I could use 3 pairs to make a 1:9 but why not 1:16?) This was also a convenient length for the feedline. (If too long, I would wind the ethernet cable like you would a coax to form a common mode choke.) This arrangement requires a decent counterpoise at the antenna end of the feedline since the feedline itself does not function as a counterpoise.

    The antenna worked as expected. Performance was only ok because it was only 3m above the seawater surface (the antenna was almost parallel to the seawater surface). I heard stations from within 1000 km but not much farther. That's expected from low-height horizontal antennas, so there is no disappointment.

    I didn't measure the S parameters or anything, but I think this approach is not too suitable for wideband designs (unless you wind the twisted pairs on a ferrite core so that a shorter wire length is used with enough coupling at the lowest operating freq.).

    The CAT5 cable handled 100W without any issue.

    Using a twisted pair wire as a transmission line transformer is a common technique in RF engineering but I don't see that used in amateur radio antennas, so I thought to post it here.

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