[Elecraft] Coax-Fed Antenna for KX1 with ATU

[Charles Greene]

Stuart,
Stuart and All,

LB Cebik has done a lot of work on OCF antennas 
http://www.cebik.com/cgi-bin/cebikswish.pl .  Check it out.  There has been 
a lot of effort over the years to broadband an 80 meter dipole, and all 
sorts of tricks have been tried.  The OCF does it without tricks.  I use a 
balun either at the feed point and use coax cable or place the balun on the 
ground and feed the antenna with 300 ohm ladder line.  The feed point 
impedance is close to 200 ohms, so the 4:1 current balun transforms the 200 
ohm impedance to 50 ohms, but does little to change the inherent unbalance 
of the antenna.  The baluns I use are measured to be 97% efficient working 
into a 200 ohm load.  The wire and coax have no more loss than in a narrow 
band dipole, so you cannot attribute the broadband effect to losses.  One 
way to get around the weight of the balun is to put the balun on a support 
mast which is what I do on my fixed station.  On my 135' portable, the 
balun in its plastic enclosure weight just under one ounce, which is less 
than the ceramic dog bone insulator typically used as a center insulator on 
a dipole.  enough said.

I don't see where Cebik has addressed the broad band nature of the OCF 
antenna, so that would be a good topic for investigation.  It has something 
to do  with the fact that as the frequency is increased above the resonant 
frequency, one side becomes capacitive reactive and the other side 
inductive reactive.  Below the resonant frequency, a similar effect occurs, 
but the sides are reversed.  The capacitive reactance and inductive 
reactance partially cancel each other out over a fairly wide frequency 
range, and thus the antenna feed point remains at near zero reactance and 
close to the resistance at resonance.  As you can see from Cebik's work, 
the resistance varies widely depending upon height and percentage from the 
end of the feed point.

On the KX1 antenna, I am planning to use it on a 56' boat with a high 
mast.  I was thinking about using a real old fashion Windom, with a single 
wire feed line, one end thrown over the horizontal portion of the mast and 
the other end connected to the cabin top.  At worst, it will be a top 
loaded sloping vertical.  I doubt if I will have the KX1 done for a trip 
from Charleston, SC to New England in the near future, so I will use my 
K2/100, as the boat has plenty of power.

cheers,

Chas, W1CG



At 03:43 PM 1/22/2004, Stuart Rohre wrote:
>Charles, I don't doubt the OCF even with feedpoint balun will work.
>My point was why have the extra weight when the balun cannot change the
>inherent balance of the antenna, but only unbalance a balanced dipole to
>unbalance of coax?  Trying to balance something that has unbalanced
>halves is probably not worth the effort.  Just accept some feeder
>radiation, and it may enhance your low angle of take off for OCF users.
>
>There are some who do not have very long feed lines on an OCF and worry
>about it being able to be matched to their rig.  With the proliferation
>of 50 ohm unbalanced output rigs today, the path of least trouble lies
>thru use of center fed antennas.   Hard to go wrong there.  Not much of
>a tuner is needed, if you are making bandwide excursions in frequency.
>
>I just quoted Windom himself that the current distribution only works
>for his feed at a single frequency and its harmonics.
>
>I suspect the perceived broadbandedness of an OCF as compared to a
>dipole may be because of higher losses.  These could be from feedline
>interaction with the OCF, the balun, or a combination.  Remember the
>most broadband antenna of all is a resistor.
>
>I have been looking for a basic antenna project to write a paper about
>for upcoming ham conventions.  I think I might replicate the Ohio State
>"Windom" team's RF current metering set up and measure the current
>distribution on a center fed dipole and on a true OCF and the modern
>OCF.
>That could be a fun little project, say make test antennas on 10m of
>each kind, or rig a pulley arrangement to move the feedpoint along the
>wire.
>Thanks and 73,
>Stuart Rohre
>K5KVH