[Antennas] RE: J POLE QUESTION

[Cletus W Whitaker]

de WB2CPN South Central Pennsylvania   2002.07.11

Sorry, but the J-Pole as we know it is a 1/2 wave end-fed vertical
antenna.  It is a 1/2 wave from the top of the matching section on up.
It's an excellent way to mount a 1/2 wave vertical antenna without 
having to insulate the bottom end from ground, and not worry about the
high voltages found at the ends of 1/2 wave antennas.  It is really a 
vertical Zepplin antenna if you think about it, and why Zepplin antennas
are called that.  The quarter wave matching section at the bottom is not
supposed to radiate if its designed right. The max lobe is at a lower
angle than a 1/4 wave with radials.  It is not as good as a 5/8 wave
because the 5/8 wave flattens the pattern somewhat.  The coaxial vertical
1/2 wave, where the top 1/4 wave sticks up and the bottom 1/4 is a sleeve
sticking down, never works perfect because of the difference in velocity 
between the outside of the sleeve, (234), and the inside of the sleeve
which is a 1/4 wave coax whose velocity we do not know.  The length of
the sleeve can be tweaked to get the best SWR you're ever going to get.
Coaxial antennas aren't J-Poles. Maybe I-Poles.        73  Clete