The antenna J , J-pole , J-Pole or just J , is an omnidirectional antenna half wave fed at one end that can be used both in mobile or portable base. No need ground plane or radial. It is cheap, simple and fast to build, as it can use a wide variety of materials and techniques.
Summary
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- 1 Features
- 2 Advantages of this design
- 3 Construction of the Antenna
- 1 Short circuit element
- 2 Placement of the power line
- 3 Sealing
- 4 Balun or Choke
- 5 Mounting the antenna
- 6 Antenna Adjustment
- 4 Sources
characteristics
The J-Pole antenna consists of a half wavelength powered radiator and a quarter wave connected to the mesh of the coaxial cable . Basically the antenna is a dipole powered at one end and shorted. It has an omni-directional pattern with a low radiation angle.
The quarter wave element acts as an impedance transformer to that of the transmission line. The ARRL antenna book describes two configurations of the J-Pole antenna: a free version stub and a short-stub version. The image shows the two settings. One directly connected to a low impedance line (50 Ohms ) and the other powered by an open line from 200 to 600 Ohms. This last configuration can replace the open line with a 4: 1 ratio balun and a 50 Ohm coaxial cable. This work recommends direct feeding with 50 ohm coaxial cable, for giving better results and for its coupling with nearby objects.
Dr. John S. (Jack) Belrose, VE2CV, interested in J-Pole antennas for many years, noted that the diameter of the radiating element affects the bandwidth and physical length required for a given operating frequency. As the diameter of the radiating element increases, the usable bandwidth increases and the physical length of the radiating element required for a given operating frequency decreases with respect to the half-wavelength free space.
Thus, using a radiant element with a larger diameter, it is possible to have a greater bandwidth than with a physically smaller antenna. This is actually an effect of the velocity factor. This effect is described in detail in a theoretical sense in the ARRL Antenna Book. VE2CV empirically determined the relationship between antenna diameter and physical length, and the dimensions of the J-Pole antenna can be calculated using its measurements.
Advantages of this design
The letters A, B, C and D correspond to the lowest band (VHF) and the letters A´, B´, C´ and D´ to the highest band (UHF).
Like the Slim jim , the J-Pole has the following characteristics, both in the frequency range of 140-150 MHz and 430-450 MHz; and with respect to other vertical antennas:
- it is easy to erect.
- does not need radials.
- it has a low radiation angle.
- works with greater bandwidth than other verticals.
- it is less sensitive to “noise”.
- it is ideal for local networks or distant repeaters.
- It has more gain than other antennas with ground planes.
- stands the test of time well.
- very strong against tropical cyclones and other natural disasters.
Antenna Construction
Almost any imaginable material can be used to build a J-Pole antenna, but keep in mind where the antenna is going to be installed, the effects of the weather, how we are going to short-circuit the two elements and the method of fixing the transmission line to the antenna.
Some of the possible materials are aluminum tubes, two-wire line, wire, copper tape, copper tubes, etc. Those built with a parallel line are light and very easy to transport; being able to roll up. The fixing of the two-wire J-Pole antenna can be seen with the help of a PVC tube .
If a copper tube is used, it is recommended to weld plugs on the ends of each tube to prevent the entry of water and moisture.
In the case of the double-band antenna, each element must be powered with a coaxial cable. In our case we would have one for VHF and one for UHF . The UHF would go down attached to the longest element to the base of the antenna, where it would join the VHF.
Short circuit element
It can be built in a variety of ways, depending on the type of material used. The simplest is possibly to solder a plate between the two elements. But this does not provide for any adjustment. A bracket can be formed as suggested by Drew Diamond, VK3XU, which allows elements to move up and down to simplify setup, using clamps.
Laying the power line
The power line can be connected by welding to the elements or by means of clamps available in hardware stores. Some designers suggest the use of screws, but this system is not recommended because it can weaken the elements and leave water and moisture in the antenna. The location of the feeding point of transmission lines, in general, has to be determined by trial and error, to obtain the minimum ROE , moving the feeding point up or down – both the live and the mesh.
It is advisable to power each J independently and connect the feeds with a duplexer at the base of the antenna, from which the power cable goes down to the transmitter.
Sealed
In the event that the antenna is to be left installed for extended periods of time, all joints and coaxial cable connections should be sealed with a neutral sealant that will not attack the tubes and prevent water ingress and corrosion.
Balun or Choke
The coaxial cable is unbalanced feed line, therefore, if a coaxial cable is used as the feed line after a balun is recommended that four to six turns of coaxial cable with a diameter of 125 mm . One suggestion is to use ferrite beads (FB-73-2401) (W2DU balun type) sticking to the base of the antenna. M0UKD recommends 9 turns of coaxial cable over 40mm shape.
Mounting the antenna
The antenna can be attached to any supporting structure, including grounded metal ones. Ideally, the antenna should be mounted at least a quarter wavelength above any metal structure, it should be placed vertically and avoid inclinations of it so that its radiation pattern is stable and does not suffer deformations.
Antenna Adjustment
To adjust the antenna to a minimum of ROE, the contacts of the live (in the longest element) and the mesh (in the shortest element) must be moved up or down until obtaining the minimum possible ROE.
In addition, a fine adjustment can be made, if we have used copper tubes for construction, by placing a screw or piece of threaded rod in the center of the plug of the shorter element, in order to shorten or lengthen it.
Once the adjustments are made, the connections must be sealed properly.