w7why1 at gmail.com
Sat Jul 15 12:20:58 EDT 2017
I built 3 UNUN's for some EFHW antennas I plan to use. On 2 of them, I
used 1 FT-240 mix 43 core (will be running these as portable antennas on
LP) and one one of them I used 2 of the same cores.
With around 135 feet of wire, the 80 meter SWR is right where I want it -
around 3550. But - when I go to 40, the SWR is lowest around 6950. If I
increase the length of the wire to bring the frequency up on 40, it will
resonate around 3700, on 80 which is not what I want.
The UNUN's I build have 2 turns of twisted wire wrapped around the core,
and then 5 more to make 7 turns. Then it crosses over to the other side
and has 7 more turns. This seems to be what most of the ones I found on
the 'net have, Across the coax connector I have 150pF capacitance.
Why would the 2nd harmonic be so far off on 40? Is there a better turns
ration solution for the cores?? Tanks and 73
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