[Elecraft] Coax-Fed Antenna for KX1 with ATU

[Charles Greene]

Stuart,

  I thought long and hard about putting the balun at the top and feeding 
with coax, or just bringing the 300 ohm ladder line into the shack.   The 
coax won out, but it was more for the mechanical arrangement than anything 
else.  As I mentioned before, I have two portable OCF antennas fed with 
ladder line with the balun on the ground one for 40 and one for 30, and one 
portable antenna with the balun in the antenna for 80.  The balun on the 
ground is somewhat better than the little guy I use at the top on the 135' 
antenna that weighs less than one oz.  Ken K3IU and John K1JD and I used 
all three the last International Light House, at Prudence Is Light House in 
Narragansett Bay in RI, using John's K2/100.  The 135' was the favorite, 
although it sloped to about 15' at the long end.  The 135' one worked 
better on 20 than the 40 meter one, but it could have been factors other 
than the type of feed line used.  We made lots of contacts on it and lots 
of DX too.  I also used the 135' at the Harvest Faire special event in 
October.  I used my K2/100 on 40 with no antenna tuner, as the SWR was less 
than 1.2:1 over the 40 meter phone band, where it worked well.  The Antenna 
was only 25' high, and the SWR was 3:1 on 80, principally due to the low 
height.

You are going to have unbalanced current wether you use twin lead or coax, 
and if you bring the feed line off perpendicular to the antenna, you 
minimize it.

I had trouble modelling the OCF using EZNEC 3.  I'm not sure why.

At 09:43 PM 1/22/2004, you wrote:
>Very good point Charles,
>Harmonic pattern is certainly going to be multi lobed and pretty hard to
>notice unbalance current.
>
>Why L. B. Cebik finds that unbalance current in an End Fed Zepp was only 10
>per cent, which was surprising and perhaps function of feeder length too.
>No, I am not against unbalance current, unless it causes Rf in the shack.
>Just pointing out conventional transmission line balun is not going to
>remove it from the elements.
>
>A good point was made that the impedance transformation of the "balun" was
>what it was doing that was helpful, and the size did not put too much weight
>on center.   I wonder if open wire would be just as forgiving of what it was
>loaded into.  Probably based, on the Cebik Zepp example.
>
>Antennas are fun,  probably any one with match issues could correct the
>mismatch to his tuner by adding some feedline to move things around ie the
>hot spot, to get the impedance within range of the tuner.  Just that doing
>that with coax carries a bit more loss penalty than open feeders.  Since you
>may have unbalance anyway, I would use lower cost feeder---open wire line
>and live with any imbalance current.
>-Stuart
>K5KVH