J Pole Antenna
- the J pole antenna is a vertical RF antenna that does not require radials
The J antenna or J pole antenna has found favour in many applications. The J antenna has a number of advantages over the standard vertical RF antennas such as the quarter wavelength vertical antenna and the five eights wavelength antenna.
Unlike the other vertical RF antennas just mentioned, the J pole antenna does not require radials for its operation. In applications where radials may appear unsightly or where they may not be suitable for other reasons, the J pole antenna provides a useful alternative.
Additionally its length means that the J pole antenna also provides some gain over a normal quarter wavelength vertical. These two attributes make the j pole antenna the ideal type for many applications. As a result the J Pole antenna is finding many applications, many of which are at VHF and above. Here it forms a compact self contained RF antenna that can fit in many locations and can give a high level of performance without a large visual impact.
Although the fact that the J antenna does not have any radials may make it appear that it will not work, it is a well established RF antenna design. It is a form of RF antenna design known as a Zepp or Zeppelin antenna that found favour in the 1930s as an HF antenna. This antenna gained its name from the fact that it was used on the Zeppelin airships. The Rf antenna design consists of a half wave radiating element which is end fed using a quarter wave stub of open wire or 300 Ohm balanced feeder used to match the impedance to the normal 50 Ohm coaxial feeder.
The diagram shows the development of the J pole antenna and its operation. This shows the RF antenna radiating element which is a half wavelength. Being end fed this presents a high impedance to the feeder and this is matched using a half wave matching stub. In the first form of the antenna, the radiating element is fed from the source, with the other leg of the stub providing a passive balance. It can also be seen that it is possible to feed the antenna using the other arm of the stub.
The final implementation of the J pole antenna uses the stub to provide a good match to 50 Ohm cable. The feed point is moved up or down the stub to provide the best match, and adjustment can be made once the antenna is in position if required. In this way any spurious changes resulting from the position, etc can be removed.
The J pole antenna is quite easy to construct and gives good results. The main disadvantage is that it can be a little more difficult to adjust than some other forms. The reason for this is that impedance matching has to be accomplished by altering the trimming length of the stub.
The length of the half wave radiating stub for the j pole antenna can be determined using the same formula as used in calculating the length of a half wave dipole. The physical length of the balanced feeder will depend on the velocity factor of the feeder in use. For open wire feeder the velocity factor is nearly unity and the length will be very close to that of the free space quarter wavelength. If 300 twin feeder is used then the length required will be shorter because its velocity factor is about 0.85.
The J pole antenna is a very useful form of RF antenna design. It can be used in many applications and can be particualrly useful where an RF antenna desihgn is needed without radials. Also providing some gain, it provides an efficient radiator for the available space. Against this it can be detuned by nearby objects.
By Ian Poole
Popular radio antenna types . . . . .
|• Dipole||• Discone||• Ferrite rod||• 5/8λ vertical|
|• Horn antenna||• J antenna||• Log periodic||• Loop antenna|
|• Parabolic reflector||• λ/4 vertical||• Yagi|