I'm sorry but I need to add this before Zac Brown does something
real stupid. When using 450 ohm feed line you must use this EXACT
length. The exact length is that which goes from the antenna to the
balun in your shack.
Any talk about matching the 450 ohms is not correct. The feed line
runs with a very high SWR and since it is low loss there is just a tiny
loss. It will correct to a 1:1 SWR between your tuner and the radio.
And about the balun. Seems if you have a 4:1 or 1:1 to choose from I
would choose the 1:1 because it does a better job of keeping the current
balanced in the balanced feed line.
This is pretty complex stuff but you Zac do not need to do the math.
Just put it up and use it. I have been using it for 8 years now. Had to
get feed line. A hail storm just beat up my feed line :-)
I have done the math. It takes several Smith Charts to plot the
current through the system.
73 Karl K5DI
Zac Brown wrote:
So what's the best way to add some more line to the middle of my
feedline? Is there a technique for joining two pieces of 450 ohm
ladder line that doesn't create an imbalance in the line? Can I just
leave one "leg" straight and then wrap the corresponding "leg" (on the
other piece) other around it, then solder them together? Or should I
just leave both legs straight and then solder them together? Or how
about using those twist locks that you use for wiring up lights?
This is probably a silly question, but I took great care to route the
line away from anything that would imbalance it, and would hate to
mess that up now. This is my first experience with balanced line :)
Thanks,
Zac
KD5IEF
K2# 4907
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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