[Elecraft] Coax-Fed Antenna for KX1 with ATU

George, W5YR [email protected]
Wed Jan 21 19:33:01 2004


In his book "HF Antennas for all Locations" Moxon speaks of the OCF antenna
on page 42 with some perceptive observations.

73, George W5YR
[email protected]


----- Original Message ----- 
From: "Stuart Rohre" <[email protected]>
To: "Phil Wheeler" <[email protected]>; "George, W5YR" <[email protected]>
Cc: "Ron D'Eau Claire" <[email protected]>; "'Elecraft Reflector'"
<[email protected]>
Sent: Wednesday, January 21, 2004 4:51 PM
Subject: Re: [Elecraft] Coax-Fed Antenna for KX1 with ATU


> 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
>
>