The J-pole antenna earned its name as one of the most popular VHF antenna because of its simplicity and its considerable gain when compared to any dipole or ground plane. Aside from its low angle radiation, the J-pole can be DC grounded which provides excellent protection from lightning overload that pose potential damage to the receiver.
The J-pole antenna evolved in may ways but its basic parts remained the same. The bottom portion is the quarterwave matching section wherein balance feeding can be connected. Connected above the quarterwave matching section is its radiator. The radiator can be cut 1/2 wavelength but if you want greater gain it can be cut to 5/8 wavelength without degrading the SWR at the feed point.
In my project, the bottom portion which is the quarterwave matching section and the lower 5/8 wave radiator is made of 1/2" OD anodized aluminum for added rigidity to sustain strong wind loads from storm. The upper 5/8 wave radiator is made of 3/8" OD anodized aluminum. The two radiator is connected via 3/8 wave phasing stub and is made of # 10 AWG solid electrical wire localy available at hardwares. The transmission line is coupled by 1:1 balun at the feed point of the antenna wherein minimum SWR is found. Once fully constructed, the j-pole exibits a theoritical gain of approximately 6 dB in reference to a standard dipole. --- 73 de hevir
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