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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