[Antennas] 17 Meter J-pole

[Terry Conboy]

After some more EZNEC twiddling, I made a model for a 17m J-Pole made from 
#14 wire.  NEC2 is a little touchy about closely spaced parallel wires, 
such as those in the 1/4 wavelength matching section of a J-pole, but the 
answers seem reasonable.

The 1.5:1 bandwidth is just under 300 kHz, with 1.01:1 match to 50 ohms at 
18.118 MHz and under 1.2:1 at the band edges.  The gain in the main lobe is 
1 dBi at a 13 degree takeoff angle.

Here are the dimensions I used:

  Main radiator -
   Bottom: 20 feet
   Top: 46.29 feet

  Matching section -
   Bottom: 5.7 feet
   Taps: 6.9 feet (feedpoint)
   Top: 20 feet
   Spacing: 6 inches

This is probably a good starting point for bare #14 wire.  Using insulated 
wire may require shortening the dimensions by a couple of 
percent.  Changing the height will also involve some tweaking.

It will help to use a choke balun at the feedpoint to avoid detuning and 
feedline radiation.

It's interesting that the matching arrangement in a J-pole narrows the 
bandwidth by more than a factor of 2 compared to a directly center-fed 
vertical dipole.  However, it does eliminate the concern about routing the 
feedline away from the center of a dipole.

Probably the simplest alternative would be a ground plane antenna with 3 
drooping (-40 degree) radials to give a 50 ohm feed.  You just need a 
little room to spread out the radials.  Very close to the same gain, 
pattern, and bandwidth as a vertical dipole, assuming the same tip height.

EZNEC model files are available for all three antennas.

73, Terry