2M / 70 cm J-Pole
Building J-Pole Antennas
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 wavelength at 2M with the help of radials. The 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 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.
F in Mhz equals A= 8460/F, B= 2808/F, C= 276/F, D= 264/F (units in Inches)
146 Mhz equals A= 57.9 Inches, B= 19.2 Inches, C= 1.9 Inches, D= 1.8 Inches
440 Mhz equals 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 Antenna
This 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.