Answers to RF Cafe Quiz #16: Antennas

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Return to RF Cafe Quiz #16

The subject of Quiz #16 is Antennas. You don't need to be an antenna expert to score well, but if you do or plan to work with antennas and cannot answer a question like, "What does dBi, the most often used unit for antenna gain (or directivity), stand for?," then maybe it is time for some review.

1. What does dBi, the most often used unit for antenna gain (or directivity), stand for?

c) Decibels of gain relative to an isotropic radiator

An isotropic radiator is theoretically a point source (dimensionless), and therefore distributes the input power uniformly across the entire spherical volume surrounding it. Directivity concentrates the input power in a preferred direction, leaving less power to be radiated in the not-preferred directions.

2. For which region of space does antenna gain normally apply?

b) Far field

Almost without exception, specified antenna gain refers to the far field. The common sense proof is that if you had two antennas with a gain of, say, 10 dBi, and placed them face-to-face, you would not realize a gain of 20 dB in signal power while the path loss would be negligible.

One exception would be NFC (near field communications) antennas which are designed to use combinations of inductive and/or magnetic coupling to transfer the signal.

1/2-wave dipole antenna coordinates and radiation pattern - RF Cafe

Elevation Pattern Azimuth Pattern

3. For which type of antenna does the pattern to the right describe?

c) 1/2-wave dipole

See Antenna Patterns page for more patterns.

4. What is the free space impedance that an antenna "sees?"

d) 120p Ω (≈377 Ω)

Z₀ = = 120p Ω (μ₀ = 4p 10^-7 Henries/m, ε₀ = 8.854 x 10^-12 Farads/m)

5. What is the name given to the point where the RF input signal interfaces to the antenna?

b) Feed-point

This is where the signal conductors physically attach to the antenna's radiating element structure.

6. Where is the approximate transition point between near field and far field?

a) λ / (2p)

The explanation is a bit complex, so please see this link on the Conformity site for details.

7. What is an isotropic radiator?

a) An antenna that radiates equally in all directions

See Q1.

8. Which type of antenna would typically have the highest directivity?

d) Parabolic

Depending on the size, degree of curvature, and the edge properties of the parabolic dish, gain (directivity, which is generally interchangeable with gain for high efficiencies) can be very high.

See Antenna Patterns page for gain ranges of various antenna types.

Voltage and current distribution on a half-wave dipole antenna - RF Cafe

9. On a center-fed 1/2-wave dipole, where is the voltage potential the highest?

b) At the tips (see diagram to right).

Intuitively, at the tips of the antenna the current has nowhere to flow, so I = 0 there. The 1/2-wave dipole acts like a capacitor where the voltage lags the current by 90°. As a result, the voltage is at maximum magnitude at the tips.

Yagi-Uda antenna - RF Cafe 10. Yagi antennas are constructed of which three types of elements?

c) Reflector, driven, and director

The Yagi, or Yagi-Uda, is constructed similar to the one shown to the right. One or more reflector elements are behind the driven element, and one or more director elements are in front of the driven element.

Design your own at the DXZone website.

Posted July 12, 2022
(updated from original post on 2/12/2005)