On 7/15/2017 9:20 AM, Tom Osborne wrote:
Hi All
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
Tom W7WHY
First of all, a 1 wavelength wire (the wire itself, not including any
matching network) always resonates a few percent higher in frequency
than twice the resonant frequency where it is 1/2 wavelength. "Why"
is this? Because the "end effect" has less influence on the 1
wavelength band due to there being more wire. Not only can you prove
this on EZNEC, but I used to have a 135 foot end fed wire and it
did exactly that.
When you measure your wire through your UNUN, and you see the opposite
situation, it is because your matching network is pulling the resonance
from the upper end of 40 to below the bottom of the 40 meter band. If
I understand the construction of your UNUN, it has a rather substantial
amount of leakage inductance that will put the frequency lower to a
greater extent on 40 than 80. The turns ratio (again, if I understand
what you are describing) is 14:2 or 7:1, which results in an impedance
step up of 49:1. That would have been about right for my old wire,
which ran around 3000 ohms on 80 meters. It would not be right for that
wire on 40 meters. On 40 meters, a turns ratio of 4:1 or 5:1 would be
more appropriate.
A quick fix would be to add a series LC tank resonant around 5 MHz in
series with the antenna. This will add capacitance on 80 and add
inductance on 40. With the right choice of L and C, you can
tune each band to the frequency you want it.
Rick N6RK
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