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Subelement E4
AMATEUR PRACTICES
Section E4B
Measurement technique and limitations: instrument accuracy and performance limitations; probes; techniques to minimize errors; measurement of "Q"; instrument calibration; S parameters; vector network analyzers
Which of the following factors most affects the accuracy of a frequency counter?
  • Input attenuator accuracy
  • Correct Answer
    Time base accuracy
  • Decade divider accuracy
  • Temperature coefficient of the logic

A frequency counter has an internal frequency reference it uses to compare against the signal being measured. "Time base" is another term for "frequency reference" (frequency being the reciprocal of time, therefore a time reference is also a frequency reference). The more accurate the time base, the more accurate the frequency counter.

The other 3 factors mentioned are insignificant -- an attenuator changes the amplitude of the signal but not the frequency; and the logic components (including a decade divider) have no cumulative effect on accuracy since they are referenced to the time base.

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What is an advantage of using a bridge circuit to measure impedance?
  • It provides an excellent match under all conditions
  • It is relatively immune to drift in the signal generator source
  • Correct Answer
    It is very precise in obtaining a signal null
  • It can display results directly in Smith chart format

A bridge circuit uses an adjustable known reference impedance connected to the unknown impedance. The reference impedance is adjusted until a signal null is achieved. At that point, the reference impedance is equal in value to the unknown impedance. The reference impedance can then be measured.

Memory aid: In folktales, a troll (sounds like null) lives under a bridge.

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If a frequency counter with a specified accuracy of +/- 1.0 ppm reads 146,520,000 Hz, what is the most the actual frequency being measured could differ from the reading?
  • 165.2 Hz
  • 14.652 kHz
  • Correct Answer
    146.52 Hz
  • 1.4652 MHz

There could be as much as 1 Hz error for every million Hz in frequency.

So to calculate the maximum possible error - or the max difference between read frequency and the actual frequency.

Divide the frequency (in Hz) by 1,000,000 and multiply by the “parts per million” (also in Hz) to get the answer.

146,520,000 / 1,000,000 x 1.0 gives us 146.52 Hz

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If a frequency counter with a specified accuracy of +/- 0.1 ppm reads 146,520,000 Hz, what is the most the actual frequency being measured could differ from the reading?
  • Correct Answer
    14.652 Hz
  • 0.1 MHz
  • 1.4652 Hz
  • 1.4652 kHz

\[1 \text{ million} = 10^{6}\] \[0.1 \text{ ppm} = \frac{0.1}{10^6}=\frac{10^{-1}}{10^{6}}=10^{-7}\]

Move decimal point seven places to the left, or: \[\pm 0.0000001 \times 146,520,000 \text{ Hz} = \pm 14.652 \text{ Hz}\]


Better done: divide the frequency by 1,000,000 and multiply by the “parts per” to get the answer in Hz.

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If a frequency counter with a specified accuracy of +/- 10 ppm reads 146,520,000 Hz, what is the most the actual frequency being measured could differ from the reading?
  • 146.52 Hz
  • 10 Hz
  • 146.52 kHz
  • Correct Answer
    1465.20 Hz

There could be as much as 10 Hz error for every million Hz in frequency.

So to calculate the maximum possible error - or the maximum difference between read frequency and the actual frequency, divide the frequency (in Hz) by 1,000,000 and multiply by the “parts per million” (also in Hz) to get the answer.

Because the ppm is 10 in this problem, you can also simply move decimal point five places to the left. +/- .00001 \(\times\) 146,520,000 = 1465.2 Hz.


\[1 \text{ million} = 10^{6}\] \[10 \text{ ppm} = \frac{10}{10^6}=10^{-5}\]

Move decimal point five places to the left, or: \[\pm 0.00001 \times 146,520,000 \text{ Hz} = \pm 1465.20 \text{ Hz}\]

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How much power is being absorbed by the load when a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power?
  • 100 watts
  • 125 watts
  • 25 watts
  • Correct Answer
    75 watts

Where \(P\) is power:

\begin{align} \text{(load absorption)} &= P_\text{forward} - P_\text{reflected}\\ &=100-25\\ &=75\:\text{Watts} \end{align}

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What do the subscripts of S parameters represent?
  • Correct Answer
    The port or ports at which measurements are made
  • The relative time between measurements
  • Relative quality of the data
  • Frequency order of the measurements

S parameters are a way of measuring the frequency response of devices and can refer to as many ports as are on the device.

For example, an antenna has 1 port, a filter may have 2 ports, a power divider may have 3 or more ports.

The subscripts tell us from which ports the measurements were made and are in the order of "To -> From" or S<out><in>

So S11 would be a measurement to Port 1, from Port 1. (Perhaps an antenna measurement, or other reflection measurement).

S21 would be a measurement made at port 2 with the signal being delivered from port 1. (Such as measuring a filter to see what frequencies are blocked or passed through).

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Which of the following is a characteristic of a good DC voltmeter?
  • High reluctance input
  • Low reluctance input
  • Correct Answer
    High impedance input
  • Low impedance input

A voltmeter should be an infinite impedance attachment to the circuit of interest so that it has no effect in the circuit. In practice, it becomes part of the circuit and affects the signal being measured. Keeping the impedance as high as possible minimizes this effect.

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What is indicated if the current reading on an RF ammeter placed in series with the antenna feed line of a transmitter increases as the transmitter is tuned to resonance?
  • There is possibly a short to ground in the feed line
  • The transmitter is not properly neutralized
  • There is an impedance mismatch between the antenna and feed line
  • Correct Answer
    There is more power going into the antenna

The magnitude of a complex impedance is always higher than its resistive component (see Pythagorean Theorem). This means that a resistive load with a reactive component will always draw less current than the same load with no reactive component.

When an antenna is tuned to resonance its inductive and capacitive reactances add to zero, leaving only the resistive component of the antenna's impedance. As the antenna's reactance is reduced, more current flows into the antenna. Current is maximum when the antenna's reactance is zero.

Increased current means more power is being delivered to the antenna.

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Which of the following describes a method to measure intermodulation distortion in an SSB transmitter?
  • Modulate the transmitter with two non-harmonically related radio frequencies and observe the RF output with a spectrum analyzer
  • Correct Answer
    Modulate the transmitter with two non-harmonically related audio frequencies and observe the RF output with a spectrum analyzer
  • Modulate the transmitter with two harmonically related audio frequencies and observe the RF output with a peak reading wattmeter
  • Modulate the transmitter with two harmonically related audio frequencies and observe the RF output with a logic analyzer

A simple elimination leads to the correct answer. We use audio to modulate the carrier in SSB mode so the answer saying use radio frequencies is clearly wrong.

The question is asking about measuring the resulting inter-modulated signal. A logic analyzer is clearly not applicable and a peak reading wattmeter is not used to measure distortion.

This leads us to the correct answer:

Modulate the transmitter with two non-harmonically related audio frequencies and observe the RF output with a spectrum analyzer

Still, read the answers slowly as it's very easy to mistakenly choose the answer that says radio and not audio (ericthughes)

Hint: only the correct answer contains both "audio" and "spectrum analyzer" (KG5KOU)

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How should an antenna analyzer be connected when measuring antenna resonance and feed point impedance?
  • Loosely couple the analyzer near the antenna base
  • Connect the analyzer via a high-impedance transformer to the antenna
  • Loosely couple the antenna and a dummy load to the analyzer
  • Correct Answer
    Connect the antenna feed line directly to the analyzer's connector

A portable antenna analyzer is a device that is used to analyze the characteristics of an antenna and often the feed line. You can see some pictures of analyzers on the Wikipedia page, but generally it has at minimum a connector to attach an antenna / feed line to, a readout and dial for selecting the frequency, and a readout for the SWR of the antenna/feed line system at that frequency.

There is no need for a dummy load with an antenna analyzer, and the analyzer is designed to connect to an antenna so it generally connects the same way a transceiver would -- by connecting the feedline directly to the analyzer.

-kd7bbc

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What is the significance of voltmeter sensitivity expressed in ohms per volt?
  • Correct Answer
    The full scale reading of the voltmeter multiplied by its ohms per/volt rating will indicate the input impedance of the voltmeter
  • When used as a galvanometer, the reading in volts multiplied by the ohms/volt rating will determine the power drawn by the device under test
  • When used as an ohmmeter, the reading in ohms divided by the ohms/volt rating will determine the voltage applied to the circuit
  • When used as an ammeter, the full scale reading in amps divided by ohms/volt rating will determine the size of shunt needed

\begin{align} \frac{\text{Ohms}}{\text{volt}} \times \text{full scale volts} &= \text{full scale impedance} \\ &=\text{input impedance} \end{align} (drichmond60)

Hint: all the "When used as..." answers are incorrect. (KG5KOU)

Hint: The word "voltmeter" appears in the question and in the correct answer.

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Which S parameter is equivalent to forward gain?
  • S11
  • S12
  • Correct Answer
    S21
  • S22

The 2-port S (scattering) voltage parameters for a linear electrical network are defined as

S11 = input reflection coefficient
S12 = reverse gain
S21 = forward gain
S22 = output reflection coefficient

Therefore, S21 is the forward gain

See https://en.wikipedia.org/wiki/Scattering_parameters for a summary of the technical explanation


Poor man's hint: In the United States, you look forward to turning 21 years old.

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What happens if a dip meter is too tightly coupled to a tuned circuit being checked?
  • Harmonics are generated
  • Correct Answer
    A less accurate reading results
  • Cross modulation occurs
  • Intermodulation distortion occurs

Remember that a dip meter is an instrument used to check the circuit without direct connection to the circuit under test. This way it does not cause harmonics, cross modulation or intermodulation distortion to occur. What results is the readings are less accurate.

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Which of the following can be used as a relative measurement of the Q for a series-tuned circuit?
  • The inductance to capacitance ratio
  • The frequency shift
  • Correct Answer
    The bandwidth of the circuit's frequency response
  • The resonant frequency of the circuit

Quick silly attempt at remembering the right answer:

The right answer has "frequency response" in it. So, Every Question (Q) deserves a response

(you're welcome)


Definition Of \(Q\) Factor: In the context of resonators, \(Q\) is defined in terms of the ratio of the energy stored in the resonator to the energy supplied by a generator, per cycle, to keep signal amplitude constant, at a frequency \(f_r\) (the resonant frequency), where the stored energy is constant with time:

\[\begin{align} Q &= 2π \times \left( \frac{\text{Energy}_{\text{stored}}}{\text{Energy}_{\text{dissipated per cycle}}} \right) \\ &= 2π\times f_r \times \left( \frac{\text{Energy}_{\text{stored}}}{\text{Power}_{\text{loss}}} \right) \end{align}\]

http://en.wikipedia.org/wiki/Q_factor#Explanation

There are a few ways to define \(Q\). With regard to this question, the bandwidth is the width of the range of frequencies for which the energy is at least half its peak value. The higher the \(Q\), the narrower the bandwidth. That is,

\[\text{bandwidth} = \frac{f_r}{Q}\]

-wileyj2956

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Which S parameter represents return loss or SWR?
  • Correct Answer
    S11
  • S12
  • S21
  • S22

The 2-port \(S\) (scattering) voltage parameters for a linear electrical network are defined as

\[\begin{align} S_{11} &= \text{input reflection coefficient} \\ S_{12} &= \text{reverse gain} \\ S_{21} &= \text{forward gain} \\ S_{22} &= \text{output reflection coefficient} \\ \end{align}\]

SWR and signal return loss are both calculated by using the input reflection coefficient. Therefore, \(S_{11}\) represents return loss or SWR.

See https://en.wikipedia.org/wiki/Scattering_parameters for a summary of the technical explanation

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What three test loads are used to calibrate a standard RF vector network analyzer?
  • 50 ohms, 75 ohms, and 90 ohms
  • Correct Answer
    Short circuit, open circuit, and 50 ohms
  • Short circuit, open circuit, and resonant circuit
  • 50 ohms through 1/8 wavelength, 1/4 wavelength, and 1/2 wavelength of coaxial cable

\(50\Omega\) ohms is the most common impedance used in RF power systems, and amateur transmitters also use this standard. Therefore it is a useful point to calibrate to. After that, short-circuit and open-circuit are two "boundary cases" that ensure the analyzer behaves correctly at the edges of its range.

Calibrating to \(75\Omega\) or \(90\Omega\) ohms might be somewhat helpful, but after covering the full range with the 3 correct answers there is limited value in any further calibration. The other answers are not simple loads or make no sense at all.

Funny reminder: The movie "short circuit" and the robot Johnny 5. Only 1 answer has "short circuit" and 5 (\(50\Omega\)) in it.

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