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

[Stephen W. Kercel]

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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