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K0CQ > HELP 10.04.07 16:34l 90 Lines 4113 Bytes #999 (0) @ WW
BID : 34459_W0AK
Read: GUEST
Subj: Re: Info, 1/2 Wave End Fed
Path: DB0FHN<DB0RGB<DB0PM<OE5XBL<OE2XUM<OE5XBR<OE1XAB<HG8LXL<7M3TJZ<ON0AR<
DB0RES<ON4HU<ON0BEL<N4ZKF<KQ0I<W0AK
Sent: 070408/2226Z @:W0AK.#CIA.IA.USA.NOAM #:34459 [Des Moines] $:34459_W0AK
From: K0CQ@W0AK.#CIA.IA.USA.NOAM
To : HELP@WW
> Depends what your definition of "feed line" is. The J-pole's
> quarter-wave matching stub is really a matching device with one
>end shorted and with a tappable feed point. Is this the normal
>configuration for a feed line? Its dimensions, except for its
>length, are uncritical, unlike a true transmission line where
>the spacing, conductor diameters (and dielectric) determine the
>line's characteristic impedance.
My definition of a feed line is two wires running along side,
whether shorted and resonant (as the Zepp feed is) or relatively
impedance matched, its a feed line.
>>Since the feed line section
>>is balanced it doesn't radiate much (almost not at all). Its a
>>shorted quarter wave resonator fed near the shorted end. It
>>connects to the end of a half wave dipole where the impedance is
>>very high. Since that connected load is high that's not much
>>different from the open on the other side so its pretty close to
>>balanced.
>Yes, although I think you're confusing the issue by continuing to
>refer to the quarter-wave stub as a feed line.
Its made of transmission line wire, it looks like a transmission
line, it IS a transmission line. Sure it has to be quarter wave
resonant in this application but parts of it can be wrapped up in
a tuner, or it could be any odd multiple of a quarter wave long,
still its not part of the radiator.
>>The J (pole) was used in the 1930s on VHF, its not new,
>I find no reference to the J-pole, or its method of feed under some
>other name, in either my 1936 or 1963 ARRL Handbooks. The earliest
>reference I have is a detailed description of it in my ARRL Antenna
>Book, 1974 Edition, pp 226-227 and 246-247. If you have an earlier
>source, I'd be pleased if you would quote the reference.
I checked my library, without digging to the bottom of the piles
(which would take years). I found references to the J and the end
fed Zepp in my 1944 ARRL Handbook (21st edition) where they had
the same complaint about the high angle radiation due to feed
line radiation that I've expressed for years. I found reference
to the Zepp in the 1941 8th edition of the Radio Handbook (by
Editors and Engineers). I know I've seen the J in earlier
publications, like late 30s used alone and as none rotating feed
for a vertically polarized VHF beam. Those aren't in my library
or at least not on top the piles.
>>Its not
>>all that good because it doesn't decouple the feed line from the
>>antenna so it lets quite a bit of RF run down the coax and
>>radiate like a really long wire looking radiation at the horizon
>>when used at VHF.
>The matching stub is balanced (as you yourself pointed out in an
>earlier paragraph, above) and should really be fed with a balanced
>feeder such as 300-ohm ribbon or 600-ohm spaced feeder, or via a
>balun to 50-ohm coax. However many including myself, feed direct
>with 50-ohm coax on 2m with very good results.
And you probably have more radiation from the feedline reducing
the purity of polarization and directivity which is not all that
unhandy in a ham antenna.
>>You asked, "Can it work?" Yes it can, it has for at least 3/4
>>century.
>I've never seen any reference to a horizontally oriented J-pole.
Its been always used bent in the form of the end fed Zepp. Since
the feedline doesn't radiate (much) its position with respect to
the 1/2 wave radiator isn't critical and the traditional right
angle of the end fed Zepp is handier than the straight
configuration of the J.
I suspect the J fell out of favor when the ground plane came
along and gave far superior VHF results because the radials are
quite effective at limiting currents on the outside of the
coaxial feed line>. The reinvention of the J in 1974 was not a
forward step for amateur radio. Yes its better than a rubber
duck, but the only poorer antenna than a rubber duck is a
shielded dummy load.
>73 de Neil ZL1ANM
73, Jerry, K0CQ @ W0AK.#CIA.IA.USA
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