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Subelement T7
Station equipment: common transmitter and receiver problems; antenna measurements; troubleshooting; basic repair and testing
Section T7C
Antenna measurements and troubleshooting: measuring SWR; dummy loads; coaxial cables; causes of feed line failures
What is the primary purpose of a dummy load?
  • Correct Answer
    To prevent transmitting signals over the air when making tests
  • To prevent over-modulation of a transmitter
  • To improve the efficiency of an antenna
  • To improve the signal-to-noise ratio of a receiver

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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Tags: troubleshooting antenna radio components arrl chapter 5 arrl module 11

Which of the following instruments can be used to determine if an antenna is resonant at the desired operating frequency?
  • A VTVM
  • Correct Answer
    An antenna analyzer
  • A Q meter
  • A frequency counter

"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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Tags: antenna swr instruments arrl chapter 4 arrl module 9

What, in general terms, is standing wave ratio (SWR)?
  • Correct Answer
    A measure of how well a load is matched to a transmission line
  • The ratio of high to low impedance in a feed line
  • The transmitter efficiency ratio
  • An indication of the quality of your station's ground connection

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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Tags: antenna swr definitions arrl chapter 4 arrl module 8

What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
  • 2 to 1
  • 1 to 3
  • Correct Answer
    1 to 1
  • 10 to 1

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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Tags: antenna swr arrl chapter 4 arrl module 8

Why do most solid-state amateur radio transmitters reduce output power as SWR increases?
  • Correct Answer
    To protect the output amplifier transistors
  • To comply with FCC rules on spectral purity
  • Because power supplies cannot supply enough current at high SWR
  • To improve the impedance match to the feed line

The SWR, or Standing Wave Ratio, is a measure of how well the impedance of the transmitter (or receiver) matches the impedance of the antenna system. When the impedance (effective resistance at a given frequency) of each are the same it results in the most efficient transfer of power from the transmitter to the antenna.

(this is a little counter-intuitive; you might expect that the lower the impedance the more power emitted, but maximum power transfer occurs when the impedances are the same)

When your antenna system produces a higher SWR some of the power is reflected back to the transmitter; at higher power levels this can damage the transmitter. With older radios this was a common problem that needed to be protected against, but most solid-state (read: modern) transmitters automatically protect against that by reducing the power in cases of high SWR so that the reflected power will not cause damage to the transistors in the output amplifier.

There aren't generally any major problems caused by bad SWR in your emissions, the power required is the same, and lowering power does not affect the impedance match of the feedline... it's strictly to protect the transmitter from damage.

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Tags: arrl chapter 4 arrl module 8

What does an SWR reading of 4:1 indicate?
  • Loss of -4 dB
  • Good impedance match
  • Gain of +4 dB
  • Correct Answer
    Impedance mismatch

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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What happens to power lost in a feed line?
  • It increases the SWR
  • It comes back into your transmitter and could cause damage
  • Correct Answer
    It is converted into heat
  • It can cause distortion of your signal

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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Tags: antenna troubleshooting feedline arrl chapter 4 arrl module 8

What instrument other than an SWR meter could you use to determine if a feed line and antenna are properly matched?
  • Voltmeter
  • Ohmmeter
  • Iambic pentameter
  • Correct Answer
    Directional wattmeter

SWR is short for Standing Wave Ratio.

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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Tags: swr instruments arrl chapter 4 arrl module 9

Which of the following is the most common cause for failure of coaxial cables?
  • Correct Answer
    Moisture contamination
  • Gamma rays
  • The velocity factor exceeds 1.0
  • Overloading

Moisture in coaxial RF cables will cause impedance changes that cause feed line 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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Tags: troubleshooting feedline arrl chapter 4 arrl module 9

Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
  • Ultraviolet resistant jackets prevent harmonic radiation
  • Ultraviolet light can increase losses in the cable's jacket
  • Ultraviolet and RF signals can mix, causing interference
  • Correct Answer
    Ultraviolet light can damage the jacket and allow water to enter the cable

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 an electrical conduit to protect the cable from damage.

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Tags: antenna arrl chapter 4 arrl module 9

What is a disadvantage of air core coaxial cable when compared to foam or solid dielectric types?
  • It has more loss per foot
  • It cannot be used for VHF or UHF antennas
  • Correct Answer
    It requires special techniques to prevent water absorption
  • It cannot be used at below freezing temperatures

"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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What does a dummy load consist of?
  • A high-gain amplifier and a TR switch
  • Correct Answer
    A non-inductive resistor and a heat sink
  • A low-voltage power supply and a DC relay
  • A 50 ohm reactance used to terminate a transmission line

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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