1. Lets Build a Vertical or J-pole
Joe Perry
Wb6dco
Antenna’s by EZNEC
Feb 2010

2. 2 meter Vertical single pole on 146 Mhz

3. 2 meter Vertical pole on 440 Mhz

4. 2m Vertical with non driven 17.7” Stub just sitting next to it.
What we did was move the 2m Vertical 59” antenna over 1.89”. This changed nothing in eznec. Then we add a 17.7” stub at the axis. We still drive the Vertical 59” copper tube. The SWR goes up due to the Coupling of the Vertical to the Stub.

5. Now Drive the Stub only, not changing either vertical tube
The SWR gets worser!! And the Gain is lost

6. Now try a shorting wire from the Stub over to the Vertical tube
Now try a shorting wire from the Stub over to the Vertical tube. Note that the SWR comes down from 8 to 3 now. This is better. But the impedance is way off. The Gain is back up.

7. The J-Pole is almost working.
So, there must be a position on the stub that you can drive the Stub and short to the Vertical tube and get a good swr and impedance match.
The question is how to find it?
By designing a moveable shorting wire you can change its position up and down above the ground bottom position tell you reach Z=50 ohm impedance and best Gain!
To use EZnec to analyze this you need about 60 nodes. So, you cant use the 20 note free trial for final analysis.
The J-pole design programs on the internet show a shorting height of 1.9” above the ground position. I cant test that with EZnec trial as you need 31 segments on the Vertical and 10 segments on the Stub and 1 segment for the shorting wire.