By Jonathan Imberi
If the transmitter output impedance is different from the impedance of the feed line connected to it, the transmitter probably will not operate very well. Most transmitters have an output circuit designed for a 50 ohm load, but not all antennas and feed lines will present a 50 ohm load. In this case, an impedance-matching device is needed to provide the proper impedance correction. In some cases this can be as simple as an inductor and a capacitor.
A transmatch is one type of matching device which contains variable matching components (inductors and capacitors) and a band switch. These are commonly used because they work over a wide frequency range and offer the flexibility of matching a wide range of impedances. Using a transmatch it is possible to operate with a single antenna on several bands. Each band will have its own transmatch settings, one for 80 meters, one for 40 meters, one for 15 meters, and one for 10 meters, just as an example.
The transmatch is connected between the antenna and the SWR meter. An SWR meter is used to measure the impedance mismatch on the feed line between the two pieces of equipment it is connected to. You want to adjust the controls on your transmatch for a minimum SWR, and anything below 2:1 will be fine.
A dipole, or a center-fed wire with open ends, is a balanced antenna. In this type of antenna the current flowing into one half of the antenna is equal to the current in the other half, but are opposite in phase. Phase refers to the relative positions of two points on a wave, or on two different waves at a particular point in time. Since these two currents are opposite in phase, one is in the positive half of the cycle and the other is in the negative half cycle. On a balanced antenna, neither side connects to ground. Instead, the antenna is balanced with respect to ground.
We upset the system balance when we feed a dipole at the center with coax. This happens because one side of the antenna connects to the center or inner conductor of the coax and the other connects to the coax shielding or braid. Normally the outside shield of a coax cable is connected to ground at the transceiver, making it an unbalanced line. This unbalanced condition could allow some antenna current to flow down the shielding of the coax braid from the antenna. This can cause several antenna problems and should definitely be avoided.
Amateurs use a balun when connecting coax (unbalanced) feed lines to balanced antennas. "Balun" is a simple contraction for "BALanced to UNbalanced". A balun is installed at the antenna feed point between the coax cable and the antenna.
There are many types of baluns which are available commercially, but it is just as easy to make your own. One common type of balun is a balun transformer, which is simply wire wound on a toroidal core. Aside from just providing balance, these baluns can also transform an impedance, such as from 50 to 75 ohms. Another common type is the bead balun. On this type several ferrite beads go over the outside of the coax, one after the other. These beads are used to choke off any RF current that might otherwise flow on the outside of the shield.
You can also make another type of choke balun from just the coax line itself. At the feed point coil up ten turns of coax into a roll about six inches in diameter. Take or secure these rolls together, and the resulting inductance of the coil turns will choke off RF currents on the shielding.
When an antenna system does not match the characteristic impedance of the transmitter, some of the transmitter's power is reflected from the antenna. The power traveling from the transmitter to the antenna is known as forward power. When that power reaches the antenna of an unmatched system some of the power is reflected back down the feed line to the transmitter. Some of the power is also radiated from the antenna, which of course is what you want to happen. The power returning to the transmitter from the antenna is known as reflected power.
When the forward power and reflected power pass each other on the feed line it causes voltage standing waves to appear on the line. When this occurs the current and RF voltage are not uniform along the line. The standing-wave ratio (SWR) is the ratio of the maximum voltage on the line to the minimum voltage. These two points will always be 1/4 wavelength apart. A SWR meter measures the relative impedance match between the feed line and its antenna. This is done by measuring the voltage of the RF signal on the line. When a good impedance match exists between the transmitter and the antenna system you will have a low SWR value. If a perfect match exists, the SWR is 1:1. In short, your SWR meter gives a relative measure of how well your transmitter matches the antenna system impedance.
Most transmitters are designed to match 50 ohm coax lines and antennas. Most commercially made antennas are also designed to have nearly the same characteristic impedance when they are properly adjusted. If your SWR reading is higher than 2:1 it means that your antenna is not adjusted properly for the frequency on which you are operating. It is a good idea to adjust the antenna for a minimum SWR (rarely 1:1) some where in the middle of the band in which you are operating. When changing to other frequencies the SWR may be higher, but if the antenna is assembled properly, an SWR of 2:1 or less is probably okay, and an SWR measurement of 1.5:1 indicates a good impedance match.
Generally the only way to obtain an SWR meter reading of 1:1 is to use a matching device. The SWR on the transmission line between the tuner and the antenna will be different, however. This is because the matching device adjusts the impedance match between the transceiver and the matching device.