J-Poles aren’t so simple afterall

I finally got my copper pipe J-Pole up on a modest mount and cabled into my ham shack. It works, but I stumbled into many interesting things that most folks seem to ignore. In addition to the many J-Pole tutorials on the Internet you should also consider:

  • Any metal below the bottom of the J will conduct RF currents and mess with the intended pattern of the antenna; No, the parallel conductors of the J-Pole, even if perfectly dimensioned, do not prevent RF current flowing to any other conductor connected including the mounting arm.
  • This coax feed line conducts RF current and becomes part of the radiating portions of the antenna also complicating the antenna pattern.
  • The J-Pole is not a smooth omni directional antenna, but it it close. This is true no matter how well you build and isolate it with chokes, but the twin-lead types keep these effects to a minimum.

This leads to these additional points when considering a J-Pole:

  • Do not put any metal portion of the antenna below the J even if you think you need it to mount the antenna.
  • You must choke the RF currents passing back along the coax.
  • You must choke the RF current passing back along the bottom metal mast if you just have to have a piece of copper pipe attached to the bottom of the J.

Better is the Enemy of Good Enough

Of course do this only if you are trying to maximize the performance of this antenna. You might not care for perfect performance. A good engineering rule is “Better is the Enemy of Good Enough.” That’s fine, but still please consider adding a Type 43 ferrite core to the coax to choke off the RF currents on the outside of the coax; You don’t want this coming back into your shack. Yes you can spool up some coax in a air coil; Ensure the choke impedance is at least 10x the natural impedance of the area where you are choking. However, ferrite materials are a much simpler and better looking alternative and are easy to buy, purchase and heat shrink in place.

Leave a Reply

You must be logged in to post a comment.