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5/8 wave end fed j-poles.
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Reply
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by XY3LON4 on April 9, 2007
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while looking at plans for the copper cactus that I built (it works super by the way). I found photos of a 5/8 wave end-fed j-pole what looks like it was made out of mobile cb antennas. It looks like it was made with Francis whips. Would anyone have plans or drawings (no tech talk or formulas please, I am not at that level yet)? The antenna that I am talking about is two Francis whips mounted on a 3-way antenna mount and the 3-way antenna mount is mounted on a length of square tubing and the square tubing is mounted to a fiberglass pole.
http://www.packetradio.com/images/JPOL4E1.jpg
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RE: 5/8 wave end fed j-poles.
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Reply
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by WB6BYU on April 10, 2007
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A 5/8 wave J-pole DOES NOT WORK. Well, it may radiate somewhat, but
certainly not in the intended mode. The dimensions should be close
to those that you probably used - the length of the long pipe past the
end of the shorter one should be half a wavelength, that is 36 to 38 inches
for 2m. I have seen versions with 5/8 wave whips (about 48") but
the builder clearly didn't understand how they worked.
However, the antenna you linked to can be built using the standard
dimensions and it should work fairly well. The long whip attached to
the coax fitting should be 3/4 wavelength, or about 54 to 57 inches.
(I'd start with 57".) The shorter whip is a quarter wavelength, about
19 inches. Note that these are NOT wire-wound whips, but straight
antennas, such as the old metal 108" whips cut down to length, or a
hollow fiberglass whip with a wire running up the center. The
spacing between the two antennas will affect the match, and will
depend on the diameters of the whips themselves. You'll have to try to
estimate the spacing from the photo, then try it and see how it works.
You may have to cut a slot in the mounting plate so you can vary the
spacing between the two whips to get a good match.
(I suspect that there is a miscommunication regarding a "5/8 wave
antenna". For 2m that is a straight whip about 48" long, which may
use a loading coil at the base to match it to 50 ohm coax. The
equivalent CB antenna is a 20' ground plane, generally with 3 or 4
radials under it. Some advertisers may refer to their wire-wound shortened
CB mobile antennas as "5/8 wave whips" if they have wound 20' of wire
onto a fiberglass rod to achieve resonance on 27 MHz, but that is
strictly marketing hype and the resulting antenna is simply a short
helical-wound whip antenna with nothing in common with what hams
universally understand as a "5/8 wave antenna".)
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