J-Pole antennas

J-POLE ANTENNAS

by Mike Walkington, VK1KCK

 

The J-Pole antenna is an omnidirectional antenna that can be used for base, mobile and field day stations. It does not need a ground plane, radials or a complicated matching system. The J-Pole can be cheaply, simply and quickly constructed using a variety of techniques, some of which are discussed in this article.

THEORY

The J-Pole antenna consists of a half-wavelength radiator fed by a quarter-wave matching stub. Effectively, the antenna is an end fed dipole. The antenna has an omnidirectional pattern with a low take-off angle. The quarter-wave stub is a transformer that provides a means of transforming the high impedance of the antenna to that of the transmission line.

The ARRL Antenna Book describes two common configurations of the J-Pole antenna: an open-stub version and a shorted-stub version. Figure 1 shows the two configurations.

 

According to the ARRL Antenna Book, the open-stub version, Figure 1A can be connected directly to low-impedance, 50 ohm, coax lines with good results; however, the lack of a movable balun allowing impedance adjustment may make this version of the antenna difficult to adjust for minimum SWR. Alternatively, the shorted-stub version, Figure 1B, which is usually fed with 200-600 ohm open-wire line, or 50 ohm coaxial cable using a 4:1 balun, allows easy adjustment for minimum SWR. The ARRL Antenna Book does not recommend feeding the shorted-stub version directly with 50 ohm coaxial, citing less than optimum results, a lack of reproducibility and heavy coupling with nearby objects. Despite this, many J-Pole antenna designs are based on feeding the shorted-stub version with 50 ohm coax; and this type of design will be discussed here.

 

Dr John S. (Jack) Belrose, VE2CV has been interested in J-Pole antennas for many years, and has written extensively about them. He particularly noted that the diameter of the radiating element affects the bandwidth and the physical length required for a given operating frequency.

 

As the diameter of the radiating element is increased, the useable bandwidth increases and the physical length of the radiating element required for a given operating frequency decreases with respect to the free space half-wavelength. Thus, by using a larger diameter radiating element a larger bandwidth can be realised with a physically smaller antenna. In reality this is a velocity factor effect. This effect is described at length in a theoretical sense in the ARRL Antenna Book. VE2CV has empirically determined the relationship between the antenna diameter and the physical length, and the dimensions of the J-Pole antenna can be calculated using his measurements.

 

Dimensions

Figure 2 shows a diagram of a shorted-stub J-Pole with the significant design dimensions.

 

hR = (149.9 ¸ fMHz) ´ k in metres

Where k is related to the antenna diameter in degrees and can be found from Figure 3. k is the fractional decrease of the antenna compared to the electrical length.

hS = (74.6 ¸ fMHz) ´ v in metres

Where v is the velocity factor of the stub, which is an air-insulated transmission line. For this situation the velocity factor is 0.975.

s » 6400 ¸ fMHz in mm

The feed points need to be found by trial and error. If co-axial cable is used the centre conductor is attached to the radiating (l /2) element.

 

Using the equations, the dimensions can be calculated. Alternatively, readers may like to surf to the web page of Buck, K4ABT via http://www.users.bigpond.com/rripp/jpole.html where an automated design and description of how to build a J-Pole Antenna may be found.

 

CONSTRUCTION TECHNIQUES

Materials

Almost any conceivable material may be used to construct a J-Pole. The main considerations being where the antenna will be located, the effects of weather, construction of the short-circuit between the two elements and the method of attaching the transmission line to the antenna.

Some possible materials are aluminium tubing, 300 W twin lead, wire, copper tape and copper tubing. 300 W twin lead J-Pole antennas are light and very portable. K1YPP has provided a design for a weatherproof 300 W twin lead J-Pole by installing it in a PVC tube.

 

Shorting Element

The shorting element between the two elements may be constructed in a variety of ways depending on the type of material used. The simplest is possibly to weld a plate between the two elements. But this does not provide for any adjustment. A bracket may be made up as suggested by Drew Diamond, VK3XU, which allows the elements to be moved up and down to simplify tuning.

 

Attaching the Feedline

The feedline can be attached by welding it to the elements or by making or using commercially available clamps. Some designers suggest using self-taping screws, but I do not recommend this as it can weaken the elements and provide a hole for water to seep into the antenna. Lugs can be soldered to the end of the transmission line to simplify attachment. The locations of the transmission line feedpoints usually needs to be determined by trial and error and adjusted from design guidelines to give the lowest SWR.

 

Sealant

If the antenna is too be installed outside for prolonged periods of time then all joints and the co-axial cable connections should be sealed using a neutral cure (one that doesn�t smell like vinegar) sealant to prevent water ingress and corrosion.

 

Balun or Choke

Co-axial cable is an unbalanced feedline, and it will radiate from the outer shield and affect the radiation characteristics of the antenna. Therefore, if co-axial cable is used as the feedline then a choke or balun is recommended.

A choke wound from four to six turns of coaxial cable with a diameter of 125 mm can be used. Alternatively, depending on the type of co-axial cable used, a ferrite bead balun or other current type balun could be used. One suggestion is to use a 50-ferrite-bead (FB-73-2401) sleeve-over-coax balun (a W2DU type balun) taped to the base of the antenna.

 

Mounting the Antenna

The antenna may be fastened to any supporting structure including grounded metal. Ideally, the antenna should be mounted at least a quarter-wavelength above any metal structures.

 

Indicative Designs

Attached are two designs for J-Pole antennas. One is a 300 W twin lead design by VK4ZZ and the other is a dual band version covering 2 m and designed by VK5TZX.