Winter 2006 Issue

ANTENNAS

The 1/4-Wave Whip

By Kent Britain, WA5VJB

The classic 1/2-wave dipole antenna is shown in figure 1. The 1/4-wave ground plane uses the ground reflection to act as the other half of the dipole. Okay so far, but in photo A we have a small VHF transmitter from RF Monolithicsģ. That half-inch-square board is not going to act like a VHF ground plane; itís far too small. Therefore, a 1/4-wave antenna is simply not going to work well, as itís only half of the antenna.

When you feed a dipole antenna right in the middle, the feed impedance is about 72 ohms (figure 2). There are few fudge factors thrown in for the diameter of the wire, height above ground, whether or not there is insulation on the wire, etc., but 72 ohms is okay for this demonstration.
As the feed point is moved toward one end, the impedance increases, until near the end the feed impedance is close to 1000 ohms. A half-wave antenna is a great antenna. Itís just hard to impedance match from the end.

Many of the simple single-chip transmitters or SAW (surface acoustic wave) oscillators are not really designed to drive a perfect 50-ohm load. Most are happy driving higher impedances. How high? Well, we really donít know in most cases. Also, the transmitter usually has switches, batteries, and other pieces hanging off the PC board. I have worked with over 50 different kinds of low-power transmitters, and Iíve never been able to calculate the best length for the antenna beforehand.

I start with a wire about 1/2 wave long and start snipping while measuring the transmitterís field strength (figure 3). With a few snips, the point where the transmitter impedance is happiest and the antenna is happiest is experimentally found. This is typically about .4 wavelength long. This is quite a bit longer than the typical 1/4-wave antenna, and I have seen as much as a 6-dB improvement.

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