After spending some time modeling my antenna in EZNEC and then running
some calculations in TWL, I added 30 ft to the feedline. That brings
the total feedline length up to 90 ft. So far, the KAT2 seems to be
able to tune all bands very quickly with the BL2 set to the 1:1
position. Thanks for everyone's feedback.
73,
Zac
KD5IEF
Zac Brown wrote:
Steve,
The feedline is actually 65 ft long. It happened to be the perfect
length from the antenna to my radio, but I could add some more line in
the middle if needed. I'd just have to make the feedline take a less
direct path through the attic. What would you suggest as a good
feedline length? 100 ft?
I read Cebik's paper at http://www.cebik.com/gup/gup5.html and was not
sure if the 65 ft would be a problem on 80m. In one of the first tables
in that article, he calculates the feedpoint impedance at 80m for my
setup for a 57.7 ft feedline and then for a 72.1 ft feedline.
80 57.7 17.6 2450 + 770
100 72.1 22.0 680 - 1107
I figured that I'd be somewhere between those two, maybe around 1600
ohms, but I didn't do any calculations. I guess I was a bit anxious to
get the antenna built and installed.
Thanks,
Zac
KD5IEF
K2# 4907
Stephen W. Kercel wrote:
Zac:
I'm guessing that the configuration that you propose could run you
into all sorts of trouble on 80 M. The problem is that the 135 ft flat
top would have low impedance, about 50 Ohms. The transmission line is
disturbingly close to a quarter wavelength. A 450 Ohm line with an
exact electrical quarter wavelength and a 50 Ohm load would have 4050
Ohms impedance at the transmitter end, and the KAT-2 would see it as
an SWR of about 80:1. Even if your line is not an exact quarter
wavelength, if it is within a few feet either way, the impedance that
your KAT2 will see is very high.
You need to do a bit of studying with EZNEC and a Smith Chart. You
need to find out the feedpoint impedances that the flat top will give
you on all the bands on which you want to operate, and then find a
length of 450 Ohm line that presents acceptable impedances to your
KAT2/BL2 setup. This same set of calculations would tell you whether
you are better off with a 1:1 or 4:1 balun (or it might show you that
you need one balun for some bands, and the other balun for other
bands). You may discover that no single length of feedline works for
all bands 80-20 M. In that case you need to get a very big DPDT knife
switch (Military surplus outlets carry them) so that you can switch an
alternate section in or out to change the length of your line for
different bands.
73,
Steve Kercel
AA4AK
At 12:18 PM 10/1/2006, Zac Brown wrote:
Fellow Elecrafters,
I am putting up a 135 ft flat-top at my QTH. I'll be feeding it with
about 60 ft. of 450 ohm ladder line, connecting the feedline to the
BL2, and tuning it with the KAT2.
I was wondering if anyone on the list has tried this arrangement and
could comment on whether the KAT2 will have any problems tuning this
setup from 80 - 20m. I'm also curious about which bands work better
with the balun set to 1:1 vs. 4:1 with this setup.
Thanks,
Zac Brown
KD5IEF
K2# 4907
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