[Elecraft] 135ft flat-top with K2, KAT2, BL2, and 450 ohm feedline

[Zac Brown]

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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>>
>>
>>
>