[Elecraft] Coax-Fed Antenna for KX1 with ATU

Wed Jan 21 19:48:00 2004

Stuart and All,

Speaking from experience (I first put up an OCF antenna in 1949, and have 
had one ever since), it works well with a 4:1 balun at the feed 
point.  It  is not balanced, but works well anyway.  I took down a G5RV to 
put this one up, as it works well without a tuner on even harmonics and 
better on 80 than the G5RV.  The 135' antenna covers the entire 80 40 20 
and 10 meter band with less than a 1.5:1 SWR, at an average height of 
32'.  I'm using a radio works 2KW 4:1 balun at the feed point.  I have 
another 135' one with a 4:1 current balun on a single F114-43 core for 
portable operations, good for 100 watts.  It has a 3:1 SWR on 80 at 20' 
and >1.5:1n the other bands.  I also have one for 40-20-10 and 30-15 for 
portable operations.  These latter two are fed with 300 ohm ladder line, 
with the balun on the ground.  They don't work as well as the ones with the 
balun in the air.  I'm not sure why.

With an antenna tuner, you can tune my 135' OCF on all bands except 160, 
but it has a higher SWR on them.

The OCF works as well as a dipole, but is much broader.  There's a reason 
for this, but I can't quote you the theory; I would have to get my friend 
KC1SD who also uses one to explain it again, as he is a much better 
engineer than I and understands these things.

The feedline is RG8X on my current one.  It can be a random length, and 
changing the length does not change the SWR.  If you get up here sometime, 
stop in, and I will be glad to demonstrate it to you.  K3IU has seen it and 
has used the portable 135' one and can attest to its performance.  In fact 
he has the G5RV I took down to install the OCF, as I wanted to give it a 
good home.

Cheers

CHAS, W1CG

At 05:51 PM 1/21/2004, Stuart Rohre wrote:
>I would expect any OCF antenna, of which the Carolina Windom is an example;
>to have more problems tuning than the G5RV.   I have in hand the original
>1929 QST piece on the Windom antenna, which was not developed by Windom, (he
>was only the scribe he says); but was team work from Ohio State Univ.  It
>maybe should be the OSU antenna.
>
>It is emphasized that it only works on its fundamental frequency and its
>harmonics; ie discrete frequencies, one per band; and was not regarded as an
>antenna you could tune widely over a band.  That is because, once off center
>fed, you violate any "balance" point you might have found, when you deviate
>from the frequency the length of the antenna was naturally resonant at.
>Indeed, they put up a wire, then tuned around to find the one frequency
>where its single wire feeder did not have standing waves, allegedly.  That
>became the frequency of use for that antnena, or on its harmonic bands.  No
>scanning around the bands. The antenna and its feeder were one system.
>
>It is a matter for debate if you can achieve no feedline radiation from an
>OCF.  It is inherently unbalanced.  You would expect unequal currents to
>flow in the two legs, one longer than the other and hence more impedance.
>"perhaps" you could attain balance in a given installation using cable choke
>beads on the feeder, and prevent feedline radiation of an objectionable
>amount.  No balun is going to force equal currents into inherently unequal
>arms of an antenna.  They are for the opposite case, going from a state of
>balance, (equal current legs, and thus equal sized legs) to an unbalance
>state as coax is.
>
>Maybe avoid the OCF for multiband tuned antennas, or accept that the feeder
>will be radiating.  In that case, abandon the balun at a feedpoint, and
>simply add line length until you do not have a "hot" (RF) in the shack
>syndrome.  On the other hand, Jack Belrose notes his simple L Network (2
>components) tuner, handles the same range of matches as commercial Tee net
>tuners.  Most ATUs use an L net, and then the issue becomes one of tuner
>losses if a compact tuner with low Q components.
>73
>Stuart
>K5KVH
>
>
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