"You can have the best transmitter in the world, but it is not worth a hoot without a good antenna!"
So said an old ham friend of mine. And he is absolutely right.
You can spend thousands of dollars for top of the line equipment. But if the signal from that radio is going into a lousy antenna, no one is going to hear you.
In amateur radio, there are several different types of antennas. Many hams like long wires, while others prefer beams or cubical quads. Others have small yards or neighborhood restrictions, so they compromise and use a vertical or some sort of stealth antenna.
Receiving radio signals is fairly easy. A shortwave listener can easily run a long piece of copper wire from the back of the receiver to a nearby tree, and pick up stations all over the world.
Transmitting amateur radio, (and commercial radio and TV for that matter), requires an antenna that is resonant. And it also needs to be long enough to handle the wavelength.
A resonant antenna is one where the capacitive reactance and inductive reactance cancel each other out. All that is left is pure resistance, and the antenna will allow the maximum transmitter power.
If an antenna is not resonant, it reflects some of the power back down the feed line. This is power that is lost in the form of heat. Too much reflected power, and a transmitter will either burn up or shut down. Most modern ham transceivers are designed to shut down before the reflected power, or Standing Wave Ratio (SWR) gets too high.
Commercial grade antennas for AM, FM and TV are designed to transmit a specific frequency, like 700 WLW. Engineers use different formulas to determine the length of the antenna.
The lower the frequency, the larger the antenna. 700 WLW, for example, requires a larger antenna than 1360 WSAI.
The higher the frequency, the shorter the antenna. Notice the antennas on a Cincinnati Police car are only about four inches long. That is because they work on 800 mHz, an ultra-high frequency.
Because amateurs use a number of different bands, they usually have multiple antennas. These antennas are usually designed to be resonant in the ham bands. The ham is able to adjust resonance by using an Antenna Tuner. This device adjusts capacitance and inductance to make the antenna resonant at different frequencies.
Sometimes a ham can adjust and tweak, and still not get perfect resonance. So she operates with a certain amount of SWR on the line.
Hams usually have a power meter that shows output and SWR. A ham operating within a ratio of 1.3 reflected power to one or less will usually still get a decent signal over the air. But her power will be reduced. Anything much more than 2 to 1 is not advisable.
One way to cut the SWR is to cut the power output of the transmitter.
Hams that do not use a tuner will often tune their antennas for a specific band of frequencies. This can be done by either cutting or adding length.
Standing Wave Ratio can be a real problem for FM and TV broadcasters too. Did you know that in freezing rain, those antennas have built-in heaters which are usually automatic? Those heaters prevent ice from forming and blocking the signal.
AM towers will usually transmit through ice as long as the ground and the upper part of the tower are not iced together.
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