Building J-Pole AntennasThe J-pole antenna has two elements section A is ¾ wavelengths and B is ¼ wavelengths. 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 making the J-Pole suitable for DX. 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.
F Mhz >>> A= 8460/F, B= 2808/F, C= 276/F, D= 264/F (answer in Inches)
146 Mhz >>> A= 57.9 Inches, B= 19.2 Inches, C= 1.9 Inches, D= 1.8 Inches
440 Mhz >>> A= 19.22 Inches, B= 6.38 Inches, C= 0.63 Inches, D= 0.60 Inches
Building a Dual Band 2M / 70 cm J-Pole AntennaThis antenna easy to build indoor or portable on 70cm or dual band 2M & 70cm. This combination antenna operates as a J-Pole at 70 cm and 1/4 wavelength at 2M. You will need an SO-239, Chassis mount connector, 1/16 Copper coated 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.
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.
For dual band operation add three 20 Inch radials to the unused S0-239 holes.70 cm Measurements