RF Electronics Basics - Answers
RF Cafe Quiz #69

This RF Electronics Basics quiz targets those of you who are newcomers to the world of radio frequency (RF) electronics, but seasoned vets are welcome to give it a go as well. Please report any suspected errors to me via e-mail. Return to Quiz #69.

1. What comprises radio frequency signals?

b)  Electromagnetic waves.

An electromagnetic wave consists of time-varying electric and magnetic waves in phase with and at right angles with respect to each other.

2. How does an antenna achieve gain when it has no active signal amplification?

b)  Gain is achieved by directing (concentrating) a majority of the signal in a preferred direction rather than equally in all direction.

As the radiation pattern plot to the right shows, the power contained in the main lob, compared to all other directions, is greater. That represents a directional gain as compared to if the power was spread out equally in all directions.

3. Why do RF people often speak of power in units of dBm and dBW rather than milliwatts and watts, respectively?

d)  Using dBm or dBW (decibels relative to a milliwatt or watt, respectively) allows multiplication and division of gains and losses to be performed as addition and subtraction, respectively.

Using dBm or dBW (decibels relative to a milliwatt or watt, respectively) allows multiplication and division of gains and losses to be performed as addition and subtraction, respectively.

The multiplication of A and B, A * B, is the equivalent of the addition of the logarithm (log) of A and the log of B, log (A) + log (B).

A * B = log (A) + log (B).

The division of A by B, A / B, is the equivalent of the subtraction of the log of B from the log of A, log (A) - log (B).

A / B = log (A) - log (B).

Therefore, dBm and dBW, both being logarithms of power ratios (relative to a milliwatt or watt, respectively), allows gains and losses expressed in decibels (dB) to be added and subtracted directly to power levels expressed in decibels (dBm, dBW).

Example: An amplification factor of 13 (13x) of a 250 mW signal yields 13 * 250 mW = 3,250 mW. Equivalently, log (13) + log (250) = 1.11394 + 2.3979 = 3.51188. Antilog (3.51188) = 10^3.51188 = 3,250.

Keep in mind that although units of dBm and dBW are numerical ratios as is the dB, dBm and dBW represent actual power levels in milliwatts and watts. There is a tendency for people to confuse and conflate dB with dBm and dBW.

4. Why do coaxial (coax) cables specify a minimum bend radius?

a)  Too tight of a bend alters the internal physical dimension to where the impedance change profoundly affects the internal signal.

The coaxial cable's characteristic impedance is a function of the radii of the outer metal shield and the central conductor, and of the dielectric material that fills the space between them. Ideally, the inner and outer conductors are perfectly round and perfectly concentric (i.e., coaxial). Changing the geometry changes the impedance. A too-sharp bend alters the geometry by smashing dielectric and shifting the center conductor off-center and thereby changes the impedance. Any change in impedance sets up reflected waves inside the cable, which causes variations in the signal amplitude along the length of the cable.

5. Why do discrete components - resistors, capacitors, and inductors - eventually not work as frequencies increase beyond some point?

c)  Parasitic resistance and reactance progressively dominates the component impedance as the frequency increases.

Capacitance exists between parallel conductor surfaces such as between adjacent windings on an inductor or transformer as well as between metal in other nearby components (including a circuit board ground plane and signal traces), etc. Inductance is present in all lengths of metal such as external leads, internal bond wires (in IC's), surface mount pads, etc. Those "parasitic" reactances can be ignored at low frequencies, but as the frequency increases, their effects grow more significant. At a component's self-resonant frequency (SRF), the magnitudes of capacitive and inductive reactance are equal, and above the SRF the components exhibits the opposite form of reactance; i.e., capacitors act like inductors and vice versa.

6. What can cause a poorly shielded AM or FM radio to change its audio level as you vary your distance from it?

a)  Your body creates half of a capacitor (the radio and/or antenna is the other half) that can alter the resonant frequency of the radio's local oscillator(s).

As a result, the intermediate frequencies (IF) shift toward the edges of the IF filters rather than in the center, thereby attenuating the signal more intensely.

7. What does the "S" in S-Parameters stand for?

d)  Scattering.

     The term "scattering" in physics refers to deviation in the intended path of travel due to interruptions. In the case of electrical signals, impedance discontinuities cause reflections. An s-parameter matrix is used to mathematically describe the relative changes in signal strength and phase. Each port (potential entrance or exit of the signal) has a unique set s-parameters that describes it relationship with every other port. For example, in a 3-port device like a circulator, S31 describes the signal exiting Port 1 when it is injected into Port 3. S33 describes the signal exiting Port 3 when it is injected into Port 3; i.e., the portion of the injected signal that is reflected back out the port.

8. What is the phase shift at the shorted end of a coaxial cable?

c)  180°

     A good illustration is a rope attached fast to a pole so it cannot move. Give the far end of the rope a whip and watch the wave travel down the rope toward the pole. When it reaches the pole, the wave is reflected back toward you in the opposite phase (180° shift). This is so because in order for the amplitude of the wave to equal zero (0) at the pole (since that end cannot move), some force must exactly cancel out the amplitude and phase of the wave. The only thing that can do that is a force equal in amplitude and at the opposite phase, hence the 180° phase shift (as with a short circuit). The same phenomenon occurs with an electrical wave.

    Conversely, if the rope is attached to the pole such that the end can freely move up and down the pole, the wave on the rope will cause the end of the rope to move to the position on the pole where the wave would be if the pole was not there. The wave reflects backward beginning at the same amplitude and phase as is arrives. Hence there is 0° of phase shift (as with an open circuit).

9. At what rate does the power of an RF signal attenuate in free space?

c)  6 dB for every doubling of distance.

     An electric field's voltage falls off at a rate of 1/r, where "r" is the distance from the source. At twice the distance, 2r, the field intensity is 1/2r. In terms of decibels, the relative voltage level is 10 * log (1/2) = 10 * (-0.301) = -3.01 dB. Since power is proportional to the square of the voltage, the relative power power level is 10 * log (1/2²) = 10 * log (1/4) =  10 * (-0.602) = -6.02 dB. Note: The negative sign indicates a reduction in gain, hence, attenuation.

10. What is the name of the frequency band occupied by license-free devices such as WiFi routers and Bluetooth headphones?

a)  ISM (Industrial, Scientific, and Medical)

     Many frequency bands are designated by the FCC and other countries' regulatory bodies for unlicensed use. They were originally created for use in the industrial, scientific, and medical communities, but have evolved to include many other domestic and commercial uses.