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

Sun Oct 1 16:30:16 EDT 2006

    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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>>>
>>>
>>