Friday, May 26, 2017

EFHW Tri-Band Trapped 17, 20, 30 meter Antenna

I couldn't do any SOTA activations recently due to work commitments, so I spent some spare time building half a dozen unun's on different cores and comparing them to the Par and Packtenna efhw unun's both on the bench and in my backyard. I came up with a design that works well on my bands of interest, 30, 20, and 17 and doesn't require a capacitor. It uses a FT-82-61 core with a 16:2 winding for a 64:1 impedance transformation. My tests confirmed that a 9:1 unun is adequate to bring down the impedance into a range suitable for use with the Elecraft KX2 internal tuner. One advantage of using a 9:1 with the KX2 is the flexibility of using the antenna as a random wire on other bands where a 64:1 impedance transformation would produce a very low impedance/high current situation for the radio which could result in overheating or inability to find a match. However, I wanted something that I could also attach to a radio without a tuner, such as the LNR MTR3, which requires a low SWR.  I use 10 feet of rg174 as a feed/counterpoise, so I didn't want high swr for that reason, as well. My 17 and 20 meter traps are made from the SOTAbeam pico trap parts. Below are the plots of my final design. Plots are with the actual trapped tri-band EFHW. M1, near the top left corner of the chart gives the reading of the frequency under test and the resulting SWR. Green plot is SWR.The SWR's on a recent Dick's Peak activation matched these plots closely. The SARK 110 was indispensable during this project.

        Below: 30m, 20m, 17m field derived plots

Below - Also tested, Clockwise from left: T60-2 (red), FT-114-43 split winding, , NXO-100, FT-114-43 traditional winding.

Final Unun  - FT-82-61 (16:2)
end fed half wave transformer unun

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