http://elecraft.85.s1.nabble.com/135ft-flat-top-with-K2-KAT2-BL2-and-450-ohm-feedline-tp394239p394242.html
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
position. Thanks for everyone's feedback.
> 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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>>
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