THE 3/4 METER BAND IS BECOMING almost as popular as the 2 meter band among radio amateurs. A lot of 2 meter band projects have been published in books and magazines for the amateur radio enthusiast, but there is a definite shortage of projects that address the 3/4 meter band.
This article describes the J-440, a very simple J-pole gain antenna for the 440 to 450 MHz band that you can hang in just almost any convenient location for operation. The J-440 is a Hertz antenna that does not require grounding, and it can be made in a few minutes with simple tools from materials costing only a few pennies.
The J-pole antenna
What is a J-pole antenna? It is a popular form of Hertz antenna that can be hung from a window frame, a ceiling, or even a tree trunk if you want to operate in the field. The J-pole provides higher gain than the 1/4 wave whip antenna that is widely used on portable transceivers that operate in the 440 MHz amateur region. The 1/4 wave whip, a Marconi antenna, is really one-half of a half-wavelength antenna fed at its center, its low-impedance point. Although quite short in this band. it requires a ground plane or radials to provide the necessary reflected image to make it work.
By contrast, the J-pole antenna described here is an end-fed half-wave antenna that does not require a reflected image. At the high frequencies of the 3/4 meter hand. the antenna is only 17 1/2 inches long. which makes it easy to move around. While the J-440 antenna is longer than the 1/4 wave whip, it is also more efficient because of its higher radiation resistance.
The J-440's angle of radiation is lower than that of a quarter- wave whip antenna. Theoretically there should be about a 2 decibel improvement in gain. However, you should be able to obtain better results because vertical whips on hand held transceivers are far from 100% efficient.
The J-440 antenna includes a half-wave antenna, and a quarter-wave section of transmission line coupled in series giving it the properties that make it useful as a tuned circuit. Neglecting losses, the impedance's at each end of a half wave transmission line are very high, as shown in Fig. 1-a. Therefore. a quarterwave matching section is needed to transform that high impedance to a low impedance, preferably about 50 ohms.
A 1/4 wave shorted transmission line, shown in Fig. 1-b, looks like an open circuit at its open end at the length corresponding to the desired frequency. At a point between the shorted and open ends of the transmission line, a low impedance point that provides an adequate match to 50-ohms can be found. Figure 2 illustrates what happens when the voltage and current waveforms shown in Figs. 1-a and 1-b are connected in series.
Figure 3 is a scaled drawing of the J-440 antenna made from readily available, low-cost. unshielded television lead-in cable. It consists of two parallel stranded No. 22 to 20 AWG copper conductors. Although three different styles are available with typical impedance's of 300 ohms, the lowest priced cable was selected. That cable has parallel stranded conductors covered and joined together with polyethylene insulation that forms both the jacket and spacer.
The velocity factor in transmission lines determines the difference between the physical length of the antenna and its electrical length. The velocity of wave travel along the antenna is less than it would be in free space. This has the effect of making the physical half wave length too long. To compensate for this, the physical length of the antenna must be made shorter than the corresponding wavelength in free space. This is calculated by making use of known velocity factors.
The velocity factor for uninsulated wire is 95%. while the velocity factor for 300-ohm TV lead-in cable is 85%. The upper 1/2-wave section has only a single active insulated wire so a velocity factor of 90% was used, but the 85% factor was used in calculating the length of the 1/4 wave stub. The antenna length was determined as follows:
1. Wavelength = 300/f (MHz) meters = 300/446 MHz = 0.067 meter 0.067 meter x 39.37 inches/ meter = 26.48 inches
2. 1/2 wave section length = wavelength/2 x velocity factor = 26.5/2 inches x 0.90 = 11.9 inches
3. 1/4 wave matching section = wavelength/4 x velocity factor = 26.5/4 x 0.85 = 5.6 inches
4. Overall J-440 antenna length =11.9 + 5.6 = 17.5 inches
The calculated 17.5-inch over- all length provides the best standing-wave ratio (SWR). the ratio of the maximum voltage or current to minimum voltage or current distribution. The author has built several of these antennas and has obtained favorable results with each. Therefore. you can cut the cables to the dimensions shown in Fig. 3. and be certain that your antenna wtll work well in the 3/4 meter band.
Before cutting the cable to length. strip approximately an Inch of insulation from both lead-in cable conductors on one end. twist the bare wires together to form a short, solder the joint, and trim off the excess wire. The optimum 50-ohm tap point was found to be 5/8 inch up from the short. Carefully trim back the insulation around both conductors at that point.
Strip the end of the coaxial cable to your transceiver about 1/2 inch, and make one turn of the center conductor of the coax around one lead of the TV cable. Then connect the coax shield to the other twin lead conductor with a short length of wire as shown in Fig. 3. Solder both connections and trim off any excess wire.
Notch out a 1/4 inch section of the lead-in cable 5 5/8 inch up from the shorted end, as shown. This 1/4-wave matching section also doubles as a 1:1 balanced- to-unbalanced transformer (or balun). Punch a hole with a diameter of about 0.050-inch in the polyethylene webbing between the conductors about 1/2 inch from the open end of the cable. This permits the antenna to be hung from a convenient hook or tied up with a nylon cord for operating either indoors or out-of-doors.
The standing-wave ratio was measured on several antennas made to the dimensions shown In Fig. 3. The measured SWRs at three different frequencies were found to be: 1.5 @ 442 MHz. 1.25 @ 446MHz, and 1.40 @ 449MHz
These measurements verified that the lowest standing-wave ratios were obtained at 446 MHz, which is the middle of the 3/4-meter band.
Phil Salas "Electronics Now " February 1993
Note: I have built several of these antennas and they have all performed as described in the article. However, I would like to point out that placing them in fiberglass tubes or pvc piping changes the velocity factor and affects the performance considerably. Depending on the material used you will need to keep this into account when modifying the original design. 73 Lance WA3YXK