The J pole antenna has become a popular form of vertical antenna, giving a number of advantages over a normal quarter wavelength or 5 eighths wavelength vertical which are widely used for everything from point to point mobile radio communications to amateur radio and much more.
The J antenna or J pole antenna takes its name from the letter J as it has some resemblances to this letter in the forma of the wires or conductors which can be seen to almsot follow the shape of the capital letter J.
One of the main advantages of the J antenna is that it does not require the use of radials or a ground plane. This can sometimes be difficult if the antenna is to be roof mounted, mounted on a pole for a mast or other similar situations.
The fact that no radials or ground plane is needed can reduce wind resistance, make the overall construction simpler and provide a more convenient solution for many radio communications situations.
J pole antenna basics
The J pole antenna is effectively a half wavelength antenna that is fed at one end - this is the pint at which the voltage is at a maximum and the current is at a minimum.
Accordingly the impedance at this point is very high and would not normally be suited as a feed point without reducing the impedance to a more convenient level.
This impedance transformation is undertaken using a quarter wavelength of balanced feeder. Although this makes the antenna longer, it does enable the antenna to be fed with a more suitable form of feeder such as coaxial cable.
This form of antenna was used many years ago by the Zeppelin airships. They used a length of wire equal to a half wavelength or multiple of half wavelengths of the operating frequency and then sued a quarter wavelength of open wire feeder to provide the matching. The whole antenna could be hung out of the gondola of the airship and the solution worked well.
The Zeppelin antenna was also used for many ground based stations as well as it enabled an end fed half wavelength antenna to be used.
J antenna construction
A J pole antenna can be made in very many ways. There is no set right way to manufacture them and no set components to use. Instead there are a few points to not when putting one together.
it is also likely that a little adjustment and experimentation will be needed as even small changes can make variations to the lengths and settings.
For a J pole antenna a length of 450Ω or 300&Omega line is normally used and the coaxial feeder is attached
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