[Antennas] Design your own "J" Pole antenna.

2002年7月12日 10:45:31 -0400

Read a very in-depth article "From a J to a Zepp" by Gary O'Neil N3GO a few
years ago. If you can find a copy of that article, and read it a few times,
you will gain a good understanding of the Zepp antenna. 
One way to view the Zepp (J-Pole) is as a 1/2 wave element which has a
characteristic impedance of around 5000 Ohms. The 1/4 wave matching stub is
shorted at the end. Somewhere between the shorted (0 Ohms) end and the
element (5000 Ohms) end, you find a 50 Ohm point to match your feed line.
Yes, this is a very over simplified analogy, but it may help someone
visualize what happens. (It took me a long time to figure out how I could
connect a transmitter to a dead short (DC) and make an antenna radiate,
especially when I made a 440 MHz J-Pole.)
N3GO also studied the differences in construction material such as copper
pipe, 450 Ohm ladder line, and 300 Ohm Twin-Lead. The 300 Ohm Twin-Lead
seemed to be the best material.
This is HAM radio, Build a few and experiment, Let us know what you find.
300 Ohm twin lead is cheap at Home Depot, so is copper tubing. Don't get me
wrong, the discussion here has been very interesting and your questions are
good, but, you learn allot by cutting wire.
73 de WA2EHZ