A dummy load is a simple resistive load inside a shielded container that allows a transmitter to be operated without the RF signal being radiated into the atmosphere. You can test the output power of your transmitter to ensure that it is operating within specification. A dummy load is often used while troubleshooting a transmitter problem.
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"Antenna" is the key word for both this question and answer. An antenna analyzer is a very helpful instrument for analyzing the characteristics of an antenna. It can indicate resonant frequency and VSWR by outputting a small adjustable frequency while looking at the amplitude of the forward and reflected!
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Standing Wave Ratio is a ratio, but the answer here that lists "ratio" as an option is not it, so don't be fooled. The most efficient transfer of power occurs when the load and the transmission line have the same impedance; in this case, radios are all designed for 50 ohm, so if your feedline and antenna system are not 50 ohm some of the power will get reflected back to the transmitter. The Standing Wave Ratio is the ratio of how much forward power there (the power out of the transmitter) is to how much power is reflected back (or reflected power), but what it actually measures is how well the antenna and feedline (load) are matched in impedance to the transmitter (transmission line).
Note that although resistance and impedance are both measured in ohms, they are not the same thing! The primary difference for the purpose of this discussion is that resistance is always the same, but impedance changes with frequency; this is why you may have a very close match (and a good SWR) at one frequency but a very bad match at another.
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SWR stands for Standing wave ratio, which indicates how much energy outputted by the transmitter is actually delivered to the load (antenna). A standing wave is created when energy is reflected back from the antenna system and the SWR is the ratio of the maximum to the minimum. The second number is always 1 and a perfect impedance match is when the first number is 1 also. For more information see Standing wave ratio.
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Most radios have protective circuitry that reduces the power output when a high SWR (standing wave ratio) is detected. The power is scaled back when the SWR is approximately 2:1. If this is not done, the radio will over heat and likely damage the output drivers. If you ever sense that your radio is unusually hot after transmitting for a relatively short period of time then it is good to check the cable connections and antenna for problems. Use a VSWR meter if needed. If someone reports that your transmission dropped out after a few moments of transmission, reduce your power setting manually to see if that helps; if it does you likely are having too much reflected energy returned (high SWR) to the transmitter.
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A SWR (Standing Wave Ratio) reading indicates the quality of the impedance match between the radio and the antenna system. When it is not matched, energy is reflected back to the transmitter and lost as heat, sometimes damaging to the transmitter. The goal is to have a 1 to 1 ratio of maximum to minimum standing wave ratio. An SWR of 1.1:1 to 1.5:1 is good.
You can think of this effect much like a glass window. The goal of a perfect window is to have all of the light to go right through the window and not be reflected back. A mirror is a special kind of window that is designed to reflect most of the light back; it does not let the light through so this is a very high reflection ratio. One-way glass is made to be partially reflective so that some light is reflected and some is passed through such that it operates as a mirror in the room with bright lights on and a partial window on the other side where the lights are off or very low. Radios need a good clear window and have very low reflections so that we efficiently transmit our energy into the atmosphere, not back to our electronics.
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All cables, no matter how well designed, have some "loss". This comes from the natural resistance that the cable has to electricity. The amount of resistance that a wire has can be roughly calculated based on the thickness (gauge) of the wire and the length; the longer the length, the more resistance it has. Feed line, like other cables, also has resistance.
This resistance results in power lost when going through the feedline and being converted into heat.
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A directional wattmeter can be connected in-line to measure how much power (in watts) is flowing in a certain direction through a feedline. If the feedline and antenna are perfectly matched, you should have full power (whatever the transmitter is capable of) forward, with little or none reflected.
This is because forward power indicates the power from the radio, and reverse or reflected power is the power that bounces back when it hits the improperly matched antenna.
Iambic pentameter is a commonly used type of metrical line in traditional English poetry and verse drama.
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Moisture in coaxial RF cables will cause impedance changes that cause feedline failure. Water can short out the conductors at the cable ends and connectors. It can corrode the connections which increase the resistance or even open the circuit. Water in the cable can also change the capacitance of the coax which directly changes the impedance. Seal your connectors and cable terminations to prevent moisture contamination and cable failure.
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If you've ever seen something that got left out in the sun and turned hard and brittle, you can understand why that could be harmful for a coax cable. Any coax cable connected to an outside antenna will be exposed to sunlight, ultraviolet light resistance will significantly delay (or even prevent) the cable from becoming brittle and cracked.
If the cable jacket becomes brittle and cracked, water could enter the cable, causing it to fail.
You can also use electrical conduit to protect the cable from damage.
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"Air core" coax can have very low loss even at high frequencies, however unlike other dielectrics used in coax, air is very easily displaced by water, should there be any leaks or problems with the coax jacket.
For more information, see the wikipedia article on Coaxial Cable.
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Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. This makes it ideal for carrying RF signals with relatively low loss and the shield protects the signal against interference from other power sources or metal that the cable may come near to.
Coax has no particular properties that would make it better than anything else rope-like for securing masts, tubing, etc, and it only has two conductors (center and shield) which means it is not sufficient for connecting with a TNC (Terminal Node Connector) which requires more wires than that. You could probably use it for carrying DC power from a battery to a radio, but it wouldn't work any better than other types of cable. In fact, it may not even work as well because the diameter of the center conductor may well be smaller than many common wire types used as power cables.
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The right answer is a resistor that works the same at all frequencies. Some resistors derive their resistance from coiling a length of wire around a form. That coil of wire has inductance, and that can affect how well radio signals travel through the resistor depending on their frequency (a topic covered on the General exam).
So a dummy load usually consists of a composite resistor, one with a resistive material, but no coils (so no inductance), or non-inductive. It usually contains a heat sink, or some way to dissipate the heat, so the resistors don't burn up. Some dummy loads place the resistors in an oil bath and use the energy to heat up the oil. Some use copper plates on both sides of the resistors in order to let the copper plates dissipate the heat.
Yes, most dummy loads are made to match the 50 ohm impedance of an antenna or transmitter, so they can substitute for the antenna and maximize the flow of power from the transmitter into the dummy load. And yes, you can use a dummy load to terminate a transmission line instead of an antenna. But that choice, the "50 ohm reactance one," is just a distractor - remember that they're asking what a dummy load consists of, not what it does. Just forget the rest of the choices.
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