ANTENNA NOTES FOR A DUMMY
Restricted Space Antennas
If you have to operate out of restricted spaces, you will likely be advised to use an antenna tuner sooner or later. These devices are one of the "must have" items around many ham shacks, however, there is quite a bit of misunderstanding as to what they will do and not do, as well as how to use them effectively. In this section we'll take a look at the subject of antenna tuners.
What Is It? Recall from the discussion of impedance matching and SWR, that it is important to have the transmitter matched to the transmission line to get optimum power from the transmitter. As mentioned, most transmitters work optimally into a 50 ohm load, however, there are times when despite our best efforts, our antenna system does not present a 50 ohm load to the transmitter. So what can we do, besides operate at very low power or get off the air? What we need is a device that will transform the impedance at the transmission line to 50 ohms so our transmitter is happy. Such a device is known as an "antenna tuner."
Note that in my opinion the name "antenna tuner" is a source of much confusion. This device does not "tune" an antenna. You can only tune an antenna by adjusting the antenna itself. It is more properly called an "impedance transformer." All it does is transform the impedance at the transmitter end of the transmission line to a value close to 50 ohms so the transmitter can operate efficiently. If you had a high SWR on the feed line before using a tuner, you still have the same high SWR on the line afterwards. As can be seen from the review of SWR previously presented, the SWR depends on the transmission line and the antenna and not on anything else. So if you have losses in your transmission line, a tuner won't fix those. It will only allow the transmitter to accept the mismatched line and output power. Nothing more.
If this seems confusing, please review the sections on impedance matching and SWR. It is important that this concept be understood!
How Do They Work? We know from basic electronics that we can place resistors in series and parallel to get any particular resistance we want. Of course, resistors dissipate energy in the form of heat, so we wouldn't purposely use resistors in an antenna system. However, from basic electronics we also know that we can use inductors and capacitors to change the impedance of a circuit. Moreover, ideal inductors and capacitors have the very nice property that they do not dissipate energy, but only store energy in electric or magnetic fields. If this doesn't make sense, a review of basic electronics is recommended.
So, by using the energy storage properties of inductors and capacitors, it is possible to transform the impedance of the antenna system to 50 ohms without losing energy in the process. This is how an antenna tuner works.
Most ham tuners use 2 variable capacitors and an inductor arranged in a T configuration, as shown in the following diagram. Due to the configuration, this is commonly called a T-network. Other arrangments are possible, including Pi-networks (which look like the Greek p symbol, and simpler L-networks that use only a single inductor and capacitor arranged in an L configuration. For the remainder of this discussion, we'll consider only the commonly used T-network.
As can be seen in the above diagram, there are no resistive components, so there are no resistive losses. Voltage and current varies in the circuit, but nothing is lost and all power is transferred from the transmitter to the antenna. Unfortunately, in the real world, every component has some resistance and inductors and capacitors are not perfect. The real equivalent diagram, showing the circulating currents in the T-network is shown in the following figure. Note that the resistors, RC1, RC2, and RL are not physical resistors, but represent internal resistance in the wires, capacitors and coil.
Normally we can neglect the resistance in the capacitors and wiring, since it is very small. That means we can take RC1 anf RC2 out of the diagram, since they do not contribute to any significant loss. However, inductors may not be so ideal and the resistance represented by RL is most likely not insignificant. So whatever current flows through the coil also flows throw the associated resistance, RL. This will lead to a power loss equal to the current squared times the resistance or PLoss = IL2 RL. And since whatever power is lost in the resistance RL cannot get to the antenna, it represents a real loss in antenna system efficiency.
So, since we need to transform the impedance so that our transmitter works properly, it is important to understand how to use a tuner.
Tuner Adjustment. Without getting into the details of the T-network, one "feature" of these impedance matching circuits is that there are often multiple values of inductance and capacitance that give the same impedance transformation. For details, check the ARRL Handbook, the ARRL Antenna Book or various articles in both QST and QEX. You can rest assured that antenna system efficiency has been evaluated by hams over the years.
Now, since we have a choice of what values of capacitance and inductance to use, we can devise a strategy that is as efficient as possible. Obviously, from the above discussion, we want to keep the current in the coil as small as possible, so that the losses associated with that current are small. If we can always maintain the minimum necessary current in the inductor, we are guaranteed that our tuner is operating as efficient as possible. And since the losses are proportional to the current squared, if we can maintain half the current IL, we will have only one quarter of the losses.
Consider that the tuner has been adjusted and presents a 50 ohm load to the transmitter. Therefore, no matter what, as long as the transmitter is delivering full power, the current through C1 is the transmitter current. In the case of a 100 watt transmitter, this will be 1.414 amps. (Remember 1.414 amps at 70.7 volts is equivalent to 100 watts across a 50 ohm load.) Thus it can be seen that, if we are able to obtain a match, the capacitor nearest to the transmitter has no effect on the loss. However, by varying the inductance and the capacitor closest to the antenna, the relative amount of current going through the inductor can be controlled. In fact, it can be seen that to get the smallest current through the coil, we would like to have the largest value of XL possible along with the smallest value of XC2.
This, then, indicates
the proper method for adjusting a tuner to minimize losses. The procedure
can be summarized in the following steps:
Set L to the largest inductance (largest possible XL)
Set C1 and C2 to the largest capacitance (smallest possible XC1, XC2)
Adjust C1 for best match. If SWR doesn't drop, leave it at maximum capacitance
Adjust C2 for best match. If SWR drops, alternately adjust C1 and C2
If no acceptable match, reduce L slightly and go to step 2.
By following this procedure it is normally possible to find the minimum loss configuration which matches the transmitter and the transmission line. Note that most tuner manufacturers recommend setting both capacitors to mid-scale and adjusting the inductance, then adjusting the capacitors. While this often works, it does not guarantee minimum losses in the tuner. And, especially if we are in a limited space situation with compromise antennas, we certainly don't want to squander any power or operate at lower efficiency when we don't have to!
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