KB6NU's Ham Radio Blog

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Playing with end-fed wire antennas and 9:1 ununs

Posted: 12 Aug 2016 02:00 PM PDT
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For the past couple of weeks, Ive been playing with end-fed wire antennas.
Before I get into the nitty-gritty details, let me first make a distinction
between end-fed half-wave antennas, such as the ones sold by LNR Precision
and end-fed wires that use some kind of tuning to achieve a 50 Ω output
impedance.

End-fed, half-wave antennas (EFHWs) are a half-wavelength long and are
resonant antennas on the band of interest. They use some kind of matching
network to trasnsformÂ*the very high impedance at the end of a half-wave
wire to about 50 Ω. Generally, they are not usable on bands for which they
are not a half wavelength long. You cant, for example, generally use a 40m
EFHW antenna on 20m.

End-fed wire antennas are a different beast. They are not a half-wavelength
long, meaning that, if you choose the length of the radiator wisely, the
impedance at the end of the wire willÂ*not be as high as the impedance of a
half wavelength long wire.

In many cases, the impedance can be transformed with the help of a 9:1
ununÂ*(unbalanced input to unbalanced output). See the figure at right. A
9:1 ununÂ*is a transformer that reduces the impedance at the input by a
factor of 9.Â*So, ifÂ*you connectÂ*a length of wire that presents an impedance
of about 450Â*Ω to the input, youll get an impedance of about 50Â*Ω on the
output.

In fact, the 9:1 ununÂ*that I built is actually an autotransformer. Heres a
video that talks a little bit about autotransformers.

Its relatively easy to build a 9:1 balun. One of the most common designs is
toÂ*wind nine turns of a trifilarÂ*winding around a toroidÂ*core. Trifilar
means that there are three wires wound simultaneously around the core. Im
not sure why there are nine turns, instead of sayÂ*eight or ten, but I
suspect that its a compromise between size and coupling. Nine turns yields
sufficientÂ*couplingÂ*to ensure that the impedance transformation will take
place without taking up too much space.

By the way, the ratio 9:1 isnt really magic. You could choose to build a
transformer with 7:1 or 12:1 ratio, but it just so happens that its much
easier to build a 9:1 transformer than a 12:1 transformer.

I built one on a T80-2 powdered iron core, using some 22-ga. wire that I
scavenged from some four-conductor cable (see photo at left.) I got a
little bit lucky in that the T80 core has a diameter just big enough to
accommodateÂ*nine turns. Using different colored wires (red, black and
white) made it easier to wire it up properly.

I didnt do much engineering when it came to selecting the parts. I just
happened to have a little bag of T80-2 cores that Id purchased cheap at
Dayton a couple of years ago. The short length of four-conductor cable was
something that Id salvaged from some previous project and had just thrown
into my wire box. Â*I havent done the calculations, but as built, Id guess
that its good up to 25 W or so. If youre shooting for an ununÂ*to handle
more power, then go with a T130 core and heavier gauge wire.

Theres also some question about which type of core to use. Some people wind
their ununÂ*on ferrite cores instead of powered iron cores. One manufacturer
even goes so far to say that they use a custom mix instead of one of the
standard ferrite mixes (although I find it hard to believe at the
relatively low quantities that they must be purchasing that theyre getting
a truly custom mix). My friend, Thom, W8TAM, built his 9:1 ununÂ*using
anÂ*FT82-61 core, and it works great. G3TYX has performed a number of
experiments with different core types, and with the antenna he used, found
Type 2 powdered iron cores to be preferable.

So, how long a wire should you use for the antenna? It really depends on
what bands you want to work. Mike, AB3AP, has calculated the lengths that
give good results on various bands. Jack, VE3EED (SK), has also made this
calculation. They differ slightly because VE3EED used the center of the
bands in his calculations, while AB3AP used the center of the CW portion of
the bands.

Last Saturday, I played around with an end-fed with a 36-ft. radiator and
counterpoises of 13-ft. and 25-ft. To be honest, I wasnt really happy with
any of the configurations. The best I was able to do was achieve an SWR of
2:1 on 40m with the 36-ft. radiatorÂ*and the 13-ft. counterpoise. Neither
configuration yielded a satisfactory match on 20m.

Thom, on the other hand, used his 9:1 ununÂ*with a 30-ft. radiatorÂ*and got
fantastic results. He got great signal reports from an NPOTA station, a
special event station in Georgia, and an operator working aeronautical
mobile over Nebraska. So, theres more experimentation in my future.



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