Wires & Pliers

2 Meter portable Zigzag antenna

Jay Jaffery, WV8R

A few years ago I needed a portable 2 meter antenna that I could fit in a briefcase and take along on trips, one that could be hung on a door or in a window. In order to be worthwhile, it had to be capable of doing a better job than a rubber duck or a telescoping antenna on my HT. At the time, there weren't any commercial ones available, so I decided to design one.

Beginning with a vertical halfwave dipole mounted on CPVC pipe, I tried bending the dipole sections into various vshapes. The object was to bring the impedance down to 50 Ohms. The shape that resulted in an omnidirectional antenna with the correct impedence and reasonable gain turned out to be a zigzag. The shape also had the advantage of reducing the length of the antenna by a significant amount. This may be seen in Figure 1. Figure 2 shows how the antenna can be folded for carrying. It does indeed fit in a briefcase.

Fig. 1(left) Zigzag antenna in operating position.

Fig. 2 (right) Zigzag folded for carrying.

The construction of the antenna is fairly simple. A 15 inch piece of CPVC pipe (5/8 inch o.d.) is used to mount the dipole sections and to contain the coax feed line. The pipe could be made shorter if your briefcase is smaller than mine. However, the coax should be 15 feet long.

Details of the bolt and wingnut arrangement that holds the dipole sections are shown in Figure 3. Note the large holes drilled into the back of the pipe to facilitate mounting and connecting the bolts and coax. Some white plastic tape can be used to cover these holes after the connections are made. Rubber tips or something similar should be used on the ends of the dipole for eye safety.

Pipe caps serve to waterproof the connections. Also, the top cap can be used to mount a hook for hanging the antenna, and the bottom cap locks the coax in the side of the pipe.

The dipole sections can be made of thick copper wire, copper rods, or brass rods. The material should be sturdy enough to keep its shape when the antenna is folded and stuffed in a briefcase because the shape determines the impedance. The wire or rods should be at least oneeighth of an inch diameter. At one end of each section there should be an eye formed that will fit on the mounting bolts. Measuring from the end of the eye to the tip, the upper section should be 19.75 inches long and the lower section 19.25 inches long as shown in Figure 3.

Tuning the antenna is done in the following way. Bend the upper section to a 120 degree angle and the lower one to a 90 degree angle. Place them as shown in Figure 1. Make sure the bottom of the lower section is bent so that it is 2 inches away from the support pipe. With an SWR meter in the line, transmit briefly at low power on a clear frequency. If the SWR is low, you are done tuning. If not, decrease the angle of the upper section slightly and check again. If the SWR improves, bend a little more until it is good. But if decreasing the angle makes the SWR higher, try increasing the angle instead. Use the same procedures with the lower section. Make sure the ends of the sections are in line with the support pipe. By the way, a good antenna analyzer makes adjustment a lot easier.

Coax length, type, and quality can have a big influence on the impedence adjustment. For example, one particular zigzag I built had a 12 foot coax (RG58) and required 105 degrees on the top section and 70 degrees on the bottom section in order to have a 1 to 1 SWR. At any rate, experimenting with the shaping can be an interesting project, and once it is done for, say, 146.00 MHz, it will be good for the whole band and will only require readjustment if it is bent out of shape by accident.

Fig. 3 Details of dipole connector

By substituting proportionally shorter lengths for the dipole sections, the antenna can be operated on other VHF and UHF bands. However, these higher frequencies are more difficult to tune, and the nature and length of the coax is more of a problem.

Incidentally, never touch the antenna during transmission. Like any antenna it is dangerous when it's radiating RF, especially if the power level is high.

Finally, once the antenna is tuned, it is reliable and very durable. I have used the same zigzag for a number of years and have never been gentle with it. Some of my friends have them and report the same experience. It's a handy gadget.

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