Introduction |
## ANTENNA NOTES FOR A DUMMY## Restricted Space Antennasby Walt Fair, Jr., W5ALT## Antenna TunersIf 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.
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!
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 π 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, R Normally we can neglect the resistance in the capacitors and wiring, since it is very small. That means we can take R So, since we need to transform the impedance so that our transmitter works properly, it is important to understand how to use a tuner.
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 I 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 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 X
_{L}) - Set C1 and C2 to the largest capacitance (smallest possible X
_{C1}, X_{C2}) - 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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