E4 - Amateur Radio Technology And Measurements

A spectrum analyzer differs from a conventional oscilloscope in that a spectrum analyzer displays signals in the frequency domain; an oscilloscope displays signals in the time domain. A typical spectrum analyzer displays frequency on the horizontal axis and amplitude on the vertical axis.

A spectrum analyzer is a test instrument that can be used to display spurious signals from a radio transmitter. It can also be used to display intermodulation distortion products in an SSB transmission. An important precaution to follow when connecting a spectrum analyzer to a transmitter output is to attenuate the transmitter output going to the spectrum analyzer.

All of these choices are correct. A spectrum analyzer could be used to determined

  • the degree of isolation between the input and output ports of a 2 meter duplexer
  • whether a crystal is operating on its fundamental or overtone frequency
  • the spectral output of a transmitter

A method to measure intermodulation distortion in an SSB transmitter is to modulate the transmitter with two non-harmonically related audio frequencies and observe the RF output with a spectrum analyzer.

An advantage of using an antenna analyzer vs. a SWR bridge to measure antenna SWR is that antenna analyzers typically do not need an external RF source. It would be best for measuring the SWR of a beam antenna.

When adjusting PSK31 transmitting levels the following most important setting is ALC level.

A useful test for a functioning NPN transistor in an active circuit, where the transistor should be biased "on", is to measure base-to-emitter voltage with a voltmeter; it should be approximately 0.6 to 0.7 volts.

A logic probe can be used to indicate pulse conditions in a digital logic circuit.


A characteristic of a good harmonic frequency marker is frequency stability. An important factor that affects the accuracy of a frequency counter is its time base accuracy. If a frequency counter with the specified accuracy in ppm reads 146,520,000 Hz, the most the actual frequency being measured could differ from the reading is given by the following table,

Accuracy in ppmThe largest difference from the reading
+/- 0.1 ppm14.6520 Hz
+/- 1.0 ppm146.520 Hz
+/- 10  ppm1465.20 Hz

An advantage of using a bridge circuit to measure impedance is that the measurement is based on obtaining a null in voltage, which can be done very precisely.

When a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power, 75 watts is being absorbed by the load.

When using an oscilloscope probe it is a good practice to keep the ground connection of the probe as short as possible.

A characteristic of a good DC voltmeter is high impedance input.

If the current reading on an RF ammeter placed in series with the antenna feedline of a transmitter increases as the transmitter is tuned to resonance it is a good indication that there is more power going into the antenna.

When measuring antenna resonance and feedpoint impedance with a portable SWR analyzer you should connect the antenna feed line directly to the analyzer's connector.

The voltmeter sensitivity expressed in volts per ohm can be used obtain the meter's impedance by taking the full scale reading of the voltmeter multiplied by its ohms per volt rating to get the input impedance of the voltmeter.

To adjust the compensation of an oscilloscope probe a square wave is observed and the probe is adjusted until the horizontal portions of the displayed wave is as nearly flat as possible.

If a dip-meter is too tightly coupled to the tuned circuit being checked a less accurate reading results.

Coil impedance limits the accuracy of a D'Arsonval-type meter.

The bandwidth of the circuit's frequency response can be used as a relative measurement of the Q for a series tuned circuit.


Excessive phase noise in the local oscillator section of a receiver can cause strong signals on nearby frequencies to interfere with reception of weak signals.

The result of the capture effect in an FM receiver is that the strongest signal received is the only demodulated signal. The term for the blocking of one FM phone signal by another, stronger FM phone signal is the capture effect.

The noise floor of a receiver is the equivalent input noise power when the antenna is replaced with a matched dummy load. The value of -174 dBm/Hz with regard to the noise floor of a receiver represents the theoretical noise at the input of a perfect receiver at room temperature. The thermal noise value of a receiver is -174 dBm/Hz. The theoretically best minimum detectable signal for a 400 Hz bandwidth receiver is -148 dBm. Lowering the noise figure of a receiver would increase signal to noise ratio.

The MDS of a receiver represents the Minimum Discernible Signal.

The most likely limiting condition for sensitivity in a modern communications receiver operating at 14 MHz is atmospheric noise.

An undesirable effect of using too wide a filter bandwidth in the IF section of a receiver is that undesired signals may be heard. The desirable amount of selectivity for an amateur receiver for the indicated mode is shown in the following table,

Type of ReceiverDesirable Selectivity  Notes
RTTY HF300 Hz 100 Hz is too narrow for the typical 170Hz shift; 6000 Hz and 2400 Hz are too wide, letting in unwanted signals
Single Sideband Phone2.4 kHz 1 kHz is too narrow for the typical 2.4 kHz phone signal; 4.2 kHz and 4.8 kHz are too wide, letting in unwanted signals
VHF FM15 kHz 1 kHz.2.4 kHz and 4.2 kHz are all too narrow for the typical 15 kHz FM signal

A narrow band roofing filter improves the dynamic range of a receiver by keeping strong signals near the receive frequency out of the IF stages.

The primary source of noise that can be heard from an HF-band receiver with an antenna connected is atmospheric noise.


The blocking dynamic range of a receiver is the difference in dB between the level of an incoming signal which will cause 1 dB of gain compression, and the level of the noise floor. Two types of problems caused by poor dynamic range in a communications receiver are cross modulation of the desired signal and desensitization from strong adjacent signals.

Intermodulation interference between two repeaters occurs when the repeaters are in close proximity and the signals mix in one or both transmitter final amplifiers. An effective way to reduce or eliminate intermodulation interference between two repeater transmitters operating in close proximity to one another is to install a properly terminated circulator at the output of the transmitter.

The unwanted signals produced as the sum and difference of the original frequencies of two transmitters "mixing" together in one or both of their final amplifiers is called intermodulation interference. If a receiver tuned to 146.70 MHz receives an intermodulation-product signal whenever a nearby transmitter transmits on 146.52 MHz, the two most likely frequencies for the other interfering signal are 146.34 MHz and 146.61 MHz.

The most significant effect of an off-frequency signal causing cross-modulation interference to a desired signal is that the off-frequency unwanted signal is heard in addition to the desired signal.

Nonlinear circuits or devices cause intermodulation in an electronic circuit.

The purpose of the preselector in a communications receiver is to improve rejection of unwanted signals.

A third-order intercept level of 40 dBm mean with respect to receiver performance means that a pair of 40 dBm signals will theoretically generate the same output on the third order intermodulation frequency as on the input frequency.

The third-order intermodulation products within a receiver of particular interest compared to other products because the third-order product of two signals which are in the band is itself likely to be within the band.

The term for the reduction in receiver sensitivity caused by a strong signal near the received frequency is called desensitization. Strong adjacent-channel signals can cause receiver desensitization. A way to reduce the likelihood of receiver desensitization is to decrease the RF bandwidth of the receiver.


Ignition noise in a receiver can often be reduced by use of a receiver noise blanker. Signals which appear correlated across a wide bandwidth are the type that a receiver noise blanker might be able to remove from desired signals.

All of these choices are correct. All of the following types of receiver noise can often be reduced with a DSP noise filter,

  • Broadband "white" noise
  • Ignition noise
  • Power line noise

Conducted and radiated noise caused by an automobile alternator be suppressed can be suppressed by connecting the radio's power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads.

Noise from an electric motor can be suppressed by installing a brute-force AC-line filter in series with the motor leads.

A major cause of atmospheric static is thunderstorms.

The type of signal picked up by electrical wiring near a radio transmitter is a common-mode signal at the frequency of the radio transmitter. You can determine if line-noise interference is being generated within your home by turning off the AC power line main circuit breaker and listening on a battery operated radio.

An undesirable effect that can occur when using an IF type noise blanker is that nearby signals may appear to be excessively wide even if they meet emission standards.

All of the following answers are correct. A common characteristic of interference caused by a "touch controlled" electrical device includes all of the following,

  • The interfering signal sounds like AC hum on an AM receiver or a carrier modulated by 60 Hz FM on a SSB or CW receiver
  • The interfering signal may drift slowly across the HF spectrum
  • The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band

If you are hearing combinations of local AM broadcast signals inside one or more of the MF or HF ham bands, the most likely cause is nearby corroded metal joints are mixing and re-radiating the BC signals.

One disadvantage of using some automatic DSP notch-filters when attempting to copy CW signals is that the DSP filter can remove the desired signal at the same time as it removes interfering signals.

All of these answers are correct. A loud "roaring" or "buzzing" AC line type of interference that comes and goes at intervals can be caused by,

  • Arcing contacts in a thermostatically controlled device
  • A defective doorbell or doorbell transformer inside a nearby residence
  • A malfunctioning illuminated advertising display

One type of electrical interference that might be caused by the operation of a nearby personal computer is the appearance of unstable modulated or unmodulated signals at specific frequencies.