The J-pole antenna has two elements section A is ¾ wavelength and B is ¼ wavelength. The antenna is impedance matched to the coaxial feedline by adjusting the stub C. The coaxial shield is connected to element A and the coax center conductor is connected to element B. Best SWR is achieved by sliding the connection C up and down. The J-pole pattern is circular determined by the phasing of the elements resulting from the spacing D. The best radiation is at low angles for maximum line of sight range. The supporting mast can be any material and any length provided the antenna is at least ¼ wavelengths from everything including the ground to obtain a circular pattern.
Here is an interesting VHF / UHF Antenna project
Building a Dual Band 146 MHz & 444 MHz attic antenna for your HT - Nothing special here or magic, it just works out that a common SO-239 connector has the right spacing for a 70 cm band J-Pole antenna plus the third harmonic relationship supports the 2M band as a 1/4 wavelength monopole antenna.
Building a Dual Band 146 MHz & 444 MHz J-Pole Antenna
This special case takes advantage of the third harmonic relationship of 2M & 70cm Bands
Dual Band 146 MHz & 444 MHz dimensions shown in inches at left include three 20.5 Inch ( 52 cm ) Radials
This indoor dual band 2M and 70cm antenna is easy to build and tune. It operates as a J-Pole at 70 cm and 1/4 monopole at 2M with the help of radials. You will need an SO-239, Chassis mount connector, 1/16 Copper coated steel welding rod and #6 fasteners.
The key step in making this antenna is spacing the elements 0.60 Inches as the SO-239 spacing is closer to 1/2 Inch. Construct an eccentric or off-center connection using a 6-32 screw and solder the side of the screw to the side of the welding rod to form the 1/4 wavelength element. See detail in picture below. This arrangement provides to adjustments; spacing can be varied by rotation to get the 0.60 Inches and fine tuning of the 6.38 Inches with the vertical screw position.
For dual band operation add three 20.5 Inch radials to the unused S0-239 holes.
A common problem with small antenna elements is maintaining the spacing between the elements as the 1/16 Inch Rod is easily bent. Adding a plastic spacer about five inches up the elements with a drop of RTV silicone works great. See detail in picture below. In fact I have found a good use for the indestructible plastic packages used for display packages. Just cut a 1 X 1/2 Inch plastic scrap and drill two holes spaced at 0.60 Inches.