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

[Zac Brown]

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
>> _______________________________________________
>> Elecraft mailing list
>> Post to: Elecraft at mailman.qth.net
>> You must be a subscriber to post to the list.
>> Subscriber Info (Addr. Change, sub, unsub etc.):
>> http://mailman.qth.net/mailman/listinfo/elecraft
>>
>> Help: http://mailman.qth.net/subscribers.htm
>> Elecraft web page: http://www.elecraft.com
> 
> 
>