Simple 50MHz-J-Pole-Antenna (DK7ZB)
The principle of the Wireman-J-Pole:
The picture shows the currents in the antenna. At the both ends of the Lambda/2-section (L1) you have a high impedance (some KOhms) and a low current.
At the other side at the end of the Lambda/4-line (L2) is a maximum of the current (low impedance of about 20 Ohms). The two ends are connected together through the metal of the coax-socket (see next picture below).
At the points XX you can find a 50-Ohm-point for feeding with the coax-cable.
Because the quarterwave-line is symmetrical, you should feed with a lambda/4-choke for a SWR of 1,0 at 50,150 MHz.
The bandwidth for a SWR <1,5 is greater than 1 MHz. The gain and pattern are the same as with a normal vertical dipole, but feeding is much more easier.
For the elevation plot and gain look at the other vertical with a Collins-filter!
|Practical mounting of the
This J-Pole is mounted on a fishing rod. The radiator L1 is an isolated copper-wire with a length of 281,5 cm (with not-insulated wire about 1-2% longer!). The quarter-wave matching sector L2 is made with 450-Ohm-Wireman-cable (length 133cm).
The matching cable is tapped 65mm from the shortened bottom (coax-socket) and fed with a coax-choke of 1,00 m RG-174, wound on a plastic tube. This choke will handle 100 Wtts output. For RF-power up to 500 Wtts use RG-58 for the choke.