Simple test antennas
There often arises a need for a simple antenna to be used for various test purposes. This column will describe some simple antennas that can be easily constructed using readily available materials.
Half-wave dipole The half-wave dipole is an excellent 0dB gain reference antenna. The simple dipole shown in Figure 1 below can be constructed by using «-inch copper pipe fastened to a small board. Two copper pipes are used, each cut to l/4 at the operating frequency.
In Figure 1 note that a balun is used to convert the balanced antenna to an unbalanced transmission line (coax). Balun is an acronym for balanced to unbalanced. The balun shown in Figure 1 is made of a Pawsey stub. The Pawsey stub is made of a quarterwave section of transmission line identical to that used to connect to the antenna feedpoint. The Pawsey stub permits the use of a coax (unbalanced) line with a balanced antenna while maintaining a 1:1 impedance ratio. Thus, we get 75V balanced to 75V unbalanced.
Coaxial dipole The coaxial dipole shown in Figure 2 bottom right is made by folding a l/4 section of braid back over the outer jacket of the coax cable. This antenna is more easily constructed at UHF and higher frequencies because the length of braid that must be folded back over the cable is shorter than at the lower frequencies.
Impedance matching If the equipment to which you are connecting the antenna has a 75V input impedance you can connect the antenna directly to the instrument (receiver or spectrum analyzer, for example). If the instrument has a 50V input impedance (which is the case for most or our work), then the 75V antenna impedance must be matched to the 50V instrument impedance. There are several ways by which the 75V antenna can be matched to the 50V impedance of the test equipment or receiver.
Resistive pad A simple “L” network resistive matching pad can be used to make the transition from 75V to 50V. The resistive network is shown in Figure 3 on page 53. This is a minimum-loss “L” pad. The loss of the matching pad is 5.7dB. It is easy to build these pads for use at lower frequencies, but at the VHF highband and higher, it is difficult to build a high-quality pad. At these higher frequencies it is best to buy a commercially manufactured pad. Reasonably priced pads are available that will operate well into the gigahertz range.
Transmission line stubs One of the easiest ways of matching impedance is through the use of a tuning stub located at a point on the transmission line. Shorted or open stubs may be used. The trick is to determine the length of the stub and where on the transmission line it should be placed. This is where the Smith Chart (1) shines! Using a computerized version of the Smith chart (ARRL MicroSmith) the exact location and length of the tuning stub is easily determined.
The results are posted in the sidebar on page 53. Results are shown for both an open and a shorted stub for use at 155MHz. The distance (D) is the distance from the load (antenna feedpoint) in inches. The length (L) is the length of the stub in inches. The formulas in the sidebar on page 54 allow you to calculate the length (L) and distance (D) for any frequency. You must enter the velocity factor (V) of the transmission line. The calculations are only valid for matching a 75V (75 6 j0) load (antenna) to a 50V receiver, transmitter or other 50V device.
The data in the sidebar at the right show that a 4.7″ open stub is placed at a distance of 27.35″ from the antenna feedpoint to achieve a match between the 75V antenna and the 50V device to which it is connected. The center frequency is 155MHz. The matching stub provides a VSWR of 1.1:1 or less over the frequency range of 148MHz-162MHz and a VSWR of 1.2:1 or less over the frequency range of 140MHz-168MHz.
To illustrate the use of the formulas in the sidebar on page 54, suppose that a 75V antenna such as the dipole must be matched to a 50V device at a frequency of 159MHz. The transmission line is 50V low-density foam coax with a velocity factor of 0.79. If the tuning stub is to be the open type then formula (1) is used. Substituting, we have:
D= [ (1811)(0.79)(0.359) ] = 21.067 inches 159
Next, the length (L) of the open stub is determined by using formula (2). Substituting, we have:
Lo = [ (1811)(0.79)(0.062) ] = 3.64 inches 159
Thus, an open transmission line stub 3.64 inches in length is placed at a distance of 21.067 inches from the load (antenna feedpoint) in order to match the 75V antenna to a 50V device. It is important to note that the transmission line is 50V, as is the stub. Formula (1) for determining distance (D) applies to the use of shorted stubs or open stubs. However, the length of the shorted stub is determined from formula (3) in the sidebar. In these formulas the shorted stub will be l/4 longer than the open stub. The open stub is easier to fine tune by simply snipping off small sections of the stub while observing the effect on VSWR.
For the coaxial dipole shown in Figure 2, if the distance (D) falls within the folded back section of braid, then simply add l/2 to the distance and place the stub there. You can always add l/2 section of cable without any adverse effects. The overall arrangement for stub-tuning the halfwave dipole of Figure 1 is shown in Figure 4 at the left.
As you can see, the transmission line stub matching scheme is practical and easy to implement once you know how long the stub should be and where it should be placed on the transmission line. Thanks to late Phillip H. Smith, inventor of the Smith chart, we can determine length and location of the tuning stub quite easily.
L-C Pi Network: The L-C pi network is so named because of the pi shape of the design. (See Figure 5 above.) The pi network is a valuable matching circuit. With tunable capacitors on each end the circuit can be fine tuned for excellent matching results over a broad frequency range. In Figure 3 on page 53, if C1 is 13.54pF; L1, 60.37nH; and C2, 11.59pF, then the resulting impedance match is shown by the return loss graph shown in Figure 6 at the right.
The higher the return loss, the better the match and, hence, the lower the VSWR. A return loss of 230dB is equal to a VSWR of about 1.065:1. The dip in the curve at 155.3MHz corresponds to a VSWR of about 1.001:1. The pi matching network would produce an excellent match at any frequency shown on the graph.
An extra benefit of the pi matching network is the reduction of harmonics. Commercially manufactured pi matching networks are available from several of the RF equipment manufacturers under trade names such as “Line Matcher,” and “Z Matcher.”
RF transformer Another method of matching impedance is through the use of an RF transformer. Such transformers are typically of the toroid type. You can wind your own or buy commercially produced transformers for higher quality and better performance. These impedance transformers are usually broadband and have fairly low insertion loss.
Conclusion As you can see, it is fairly easy to construct simple antennas for test purposes or other special needs. With a little care in dimensions and impedance, matching such antennas can give surprisingly good results. Go ahead…experiment! Until next time…stay tuned!