Antennas and SWR
The antenna is connected to a coaxial cable.
An antenna tuner is not used.

There are many types of antennas. Each antenna has a specific feed-point impedance where the coaxial cable is connected to the antenna. This feed-point impedance is dependent on the type of antenna, the materials used in making the antenna, its distance vertically from ground, and its distance from nearby objects such as trees, buildings, etc. Other factors, such as rain or snow, also affect the feed-point impedance.

When the impedance of an antenna is stated it is assumed that the antenna is some optimum distance above ground and away from all external objects and that the weather is good.

If the feed-point impedance is close to 50 ohms it makes sense to connect it to the radio with a coaxial cable that has an impedance of 50 ohms, resulting in a very low SWR. Some antennas at optimum height, away from external objects and in good weather have a feed-point impedance closer to 75 ohms (such as an 80 meter dipole) and are best fed to the radio with a coaxial cable of 75 ohms to get a low SWR.

The SWR (standing wave ratio) is a measure of the effectiveness of an antenna to radiate all of the energy from the radio with no energy returning to the radio. An antenna which reflects no energy back to the radio has an SWR of 1.0.

Given that an antenna’s feed-point impedance is not the same as that of the coaxial cable, and IF it were possible to measure the current at the feed-point (the maximum current) and also IF it were possible to measure the current at various points along the coaxial cable until one found the lowest current value, then the SWR could be determined experimentally by dividing the maximum current by the minimum current. This is simple in theory, but NOT easily done in practice!

However, there are simpler ways to obtain the SWR. A rather expensive instrument such as an antenna analyzer can be used to directly read out the SWR, and is the simplest method, without having to connect a radio to the antenna to determine the SWR.

Some SWR meters are equipped with two needle pointers in the meter and the SWR can be read directly if the meter is placed in line between the radio and the coaxial cable.

If such instruments are not available it is still possible to determine the SWR with the use of a simpler SWR meter having only one needle pointer placed in line between the radio and the coaxial cable. SWR meters of this type have a sensitivity control and a switch for selecting forward and reverse currents. The radio must be operated at a very low power level and the sensitivity control set to allow only a small amount of signal current to pass. With the switch set for forward current, and the radio in operation with a steady tone, the sensitivity control is changed until the meter reading is maximum on the dial. The switch is then set for reverse current and the meter reading is noted. The radio can then be shut down and the SWR can then be calculated. Of course, if one performs these measurements and finds that the reverse current is VERY low, then he can rest assurred that the SWR is indeed low enough and is not to be concerned about. On the other hand, if the reverse current reading is not near the zero value it may be desired to determine the SWR by mathematical means using the following formula:

SWR = (1 + r) / (1 – r) , where r is the reverse current value divided by the forward current value.

When obtaining an instrument for measuring SWR it is important to know whether the instrument is designed to operate with accuracy at the frequency at which it is intended to be used.

VHF and UHF radios have no external means of adjustment to account for a high SWR, and if the SWR is much over 2.0 they are designed to reduce the power level so the output transistors are not damaged (a good thing). Hence, it becomes important to know that the SWR of the antenna system is as low as possible. However, an SWR of 2.0 does not significally reduce the power to where it would make a difference to the human ear at the receiving end of the transmission. It is important to know that most radios will not accept an SWR greater than 3.0.

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