Test Equipment

Station operation and set up

Modern HF transceivers have features that make operating a breeze, but to use them properly, you have to know when to use them and how to use them. The notch filter is a good example. The notch filter actually introduces a notch into a receiver’s passband to block interfering signals.

QUESTION: What is the purpose of the “notch filter” found on many HF transceivers? (G4A01)
ANSWER: To reduce interference from carriers in the receiver passband

Another feature that helps reduce interference from nearby stations is the IF shift control. It shifts the passband of the IF filter to the right or left of the center frequency.

QUESTION: Which of the following is a use for the IF shift control on a receiver? (G4A11)
ANSWER: To avoid interference from stations very close to the receive frequency

One type of interference is called overload. This occurs when a strong incoming signal is close to the frequency that you’re monitoring. To help prevent this type of interference, many transceivers have an attenuator, which you can switch in to reduce the signal level reaching the RF amplifiers.

QUESTION: What is one reason to use the attenuator function that is present on many HF transceivers? (G4A13)
ANSWER: To reduce signal overload due to strong incoming signals

Noise is quickly becoming a big problem for amateur radio operators, whether it be from power lines, atmospheric noise, or man-made noise from electronic devices in one’s home or neighbors’ homes. To combat this, most modern transceivers have noise blankers and noise reducers. Noise blankers attempt to reduce pulse type noise, such as from a car ignition, by reducing the receiver gain during a noise pulse. Noise reducers are designed to reduce the level from other types of noise. One disadvantage of a noise reducer is that when you increase the noise reduction level, the incoming signal may become distorted.

QUESTION: How does a noise blanker work? (G4A16)
ANSWER: By reducing receiver gain during a noise pulse

QUESTION: What happens as the noise reduction control level in a receiver is increased? (G4A17)
ANSWER: Received signals may become distorted

Modern transceivers also have features that make operating CW more convenient and effective. For example, most transceivers nowadays have built-in electronic keyers. Keyers automatically generate dots and dashes (dits and dahs) of exactly the same length and evenly space them as well.

 

Another feature designed for easier CW operation is the ability to select which sideband that you want to use to receive a CW signal. This feature can reduce or eliminate interference from signals in the receiver passband. By selecting the opposite, or “reverse,” sideband, you shift the receiver’s passband up or down, so that the interfering signal can no longer be heard.

QUESTION: What is the purpose of an electronic keyer? (G4A10)
ANSWER: Automatic generation of strings of dots and dashes for CW operation

QUESTION: What is one advantage of selecting the opposite, or “reverse,” sideband when receiving CW signals on a typical HF transceiver? (G4A02)
ANSWER: It may be possible to reduce or eliminate interference from other signals

Many modern transceivers also have the ability to receive on one frequency and send on another. This is called “split mode.” Some high-end transceivers also have two or more variable frequency oscillators, or VFOs. Both of these features allow you to receive on one frequency and transmit on another.

These features are especially useful when operating DX, that is when contacting foreign stations. You set your receiver frequency to the frequency on which the DX station is calling and your transmitter frequency to a frequency that won’t interfere with the DX station. That allows everyone to hear the DX station and allows the DX station operator to hear and call stations trying to call him or her. If your radio has only a single VFO, it switches rapidly between the transmit and receive frequencies. If your radio has two VFOs, it can monitor both frequencies simultaneously.

QUESTION: What is normally meant by operating a transceiver in “split” mode? (G4A03)
ANSWER: The transceiver is set to different transmit and receive frequencies

QUESTION: Which of the following is a common use for the dual-VFO feature on a transceiver? (G4A12)
ANSWER: To permit monitoring two different frequencies

It has become quite popular to operate “digital modes” by supplying an audio signal, usually from a computer sound card, to a transceiver set to transmit single sideband. This type of operation is called audio frequency shift keying, or AFSK, because the audio tones will shift the transmitted frequency.

In order to send a clean signal, you must set the mic gain and automatic level control (ALC) properly. If not set properly, your signal could become distorted and you might actually transmit spurious signals.

QUESTION: What is likely to happen if a transceiver”s ALC system is not set properly when transmitting AFSK signals with the radio using single sideband mode? (G4A14)
ANSWER: Improper action of ALC distorts the signal and can cause spurious emissions

One problem that may occur when transmitting AFSK signals is that the audio cable or USB cable connecting the computer to the radio may pick up stray RF. Should this happen, your signal might become distorted or the radio might be disconnected from the computer.

QUESTION: Which of the following can be a symptom of transmitted RF being picked up by an audio cable carrying AFSK data signals between a computer and a transceiver? (G4A15)
ANSWER: All of these choices are correct
– The VOX circuit does not un-key the transmitter
– The transmitter signal is distorted
– Frequent connection timeouts

Many amateurs buy linear amplifiers to make their signals stronger. Knowing how to set the controls on a linear amplifier, including the load or coupling control, ALC, and drive power, is important so that you transmit clean signals and avoid damaging the amplifier.

QUESTION: What is the correct adjustment for the load or coupling control of a vacuum tube RF power amplifier? (G4A08)
ANSWER: Maximum power output without exceeding maximum allowable plate current

QUESTION: What reading on the plate current meter of a vacuum tube RF power amplifier indicates correct adjustment of the plate tuning control? (G4A04)
ANSWER: A pronounced dip

QUESTION: What is a reason to use Automatic Level Control (ALC) with an RF power amplifier? (G4A05)
ANSWER: To reduce distortion due to excessive drive

QUESTION: What condition can lead to permanent damage to a solid-state RF power amplifier? (G4A07)
ANSWER: Excessive drive power

When using a linear amplifier, a time delay is sometimes included in a transmitter keying circuit. The reason for this is to give the system time to completelyl change over from receive to transmit operation.

QUESTION: Why is a time delay sometimes included in a transmitter keying circuit? (G4A09)
ANSWER: To allow time for transmit-receive changeover operations to complete properly before RF output is allowed

Antenna tuners, also known as antenna couplers, are also common accessories in an amateur radio station. They ensure a good match between the transmitter and the antenna system.

QUESTION: What type of device is often used to match transmitter output impedance to an impedance not equal to 50 ohms? (G4A06)
ANSWER: Antenna coupler or antenna tuner

Test and monitoring equipment, two-tone test

When you set up your amateur radio station, sometimes called your “shack,” you’ll not only want to acquire radios, but also some test equipment. The most basic piece of test equipment is the voltmeter. Voltmeters may be either analog or digital, but most amateurs now choose digital meters because they are cheaper and more accurate than analog meters. One of the reasons that digital voltmeters are more accurate is that they have a higher input impedance than analog voltmeters. The higher input impedance reduces the load on the circuit being tested, and thereby improves the accuracy.

QUESTION: What is an advantage of a digital voltmeter as compared to an analog voltmeter? (G4B06)
ANSWER: Better precision for most uses
QUESTION: Why is high input impedance desirable for a voltmeter? (G4B05)
ANSWER: It decreases the loading on circuits being measured

The use of an analog meter might, however, be preferred over a digital meter in some applications. The reason for this is that with an analog meter you can more easily see how a circuit’s output changes as you tune it.

QUESTION: What is an instance in which the use of an instrument with analog readout may be preferred over an instrument with a digital readout? (G4B14)
ANSWER: When adjusting tuned circuits

An oscilloscope is another handy piece of test equipment to have in your shack. With an oscilloscope, you can see how the amplitude of complex waveforms change over time. To do this, they have both vertical and horizontal amlifiers, that let you adjust the scale of the amplitude and the time period that you see on the screen.

QUESTION: What item of test equipment contains horizontal and vertical channel amplifiers? (G4B01)
ANSWER: An oscilloscope

QUESTION: Which of the following is an advantage of an oscilloscope versus a digital voltmeter? (G4B02)
ANSWER: Complex waveforms can be measured

One application for an oscilloscope is to check the keying waveform of a CW transmitter. With an oscilloscope, you can see how fast the RF signal reaches full amplitude when the transmitter is keyed, and how quickly the signals falls off when the key is released. Because oscilloscopes are not designed to handle high power, you must attenuate the transmitter output before connecting the oscilloscope.

QUESTION: Which of the following is the best instrument to use when checking the keying waveform of a CW transmitter? (G4B03)
ANSWER: An oscilloscope

QUESTION: What signal source is connected to the vertical input of an oscilloscope when checking the RF envelope pattern of a transmitted signal? (G4B04)
ANSWER: The attenuated RF output of the transmitter

Antenna analyzers are instruments that can measure a number of different parameters associated with antennas, such as SWR and coaxial cable impedance. To make measurements with an antenna analyzer, you disconnect the antenna system from the transmitter and connect it to the analyzer. One problem that you may encounter is that nearby transmitters can affect measurements that you make with an antenna analyzer. The reason for this is that the antenna being analyzed will pick up RF energy from the nearby transmitters, and this energy will be read as excessive reflected power.

QUESTION: Which of the following must be connected to an antenna analyzer when it is being used for SWR measurements? (G4B11)
ANSWER: Antenna and feed line

QUESTION: What is a use for an antenna analyzer other than measuring the SWR of an antenna system? (G4B13)
ANSWER: Determining the impedance of coaxial cable

QUESTION: What problem can occur when making measurements on an antenna system with an antenna analyzer? (G4B12)
ANSWER: Strong signals from nearby transmitters can affect the accuracy of measurements

Another instrument that you can use to make SWR measurements is a directional wattmeter. To measure the SWR with a direction wattmeter, you first measure the forward power, then the reverse power, and finally calculate the SWR with the equation: SWR = (1 + √(Pr/Pf))/(1- √(Pr/Pf)).

QUESTION: Which of the following can be determined with a directional wattmeter? (G4B10)
ANSWER: Standing wave ratio

The field strength meter is another instrument useful for making antenna measurements. You can use a field strength meter to make relative field strength readings, and by making a series of these readings in a circle around an antenna determine the antenna’s radiation pattern.

QUESTION: Which of the following instruments may be used to monitor relative RF output when making antenna and transmitter adjustments? (G4B08)
ANSWER: A field-strength meter

QUESTION: Which of the following can be determined with a field strength meter? (G4B09)
ANSWER: The radiation pattern of an antenna

To test the linearity of an SSB transmitter, you run a test called the two-tone test. You modulate the transmitter with two non-harmonically related tones, usually 700 Hz and 1900 Hz. If the transmitter is perfectly linear, those two tones will appear on the output. If not, non-linear products of those two frequencies will appear on the transmitter output, and you’ll be able to detect those products using an oscilloscope or spectrum analyzer.

QUESTION: What type of transmitter performance does a two-tone test analyze? (G4B15)
ANSWER: Linearity

QUESTION: What signals are used to conduct a two-tone test? (G4B07)
ANSWER: Two non-harmonically related audio signals