U0430. Which of the following methods can reduce the RF current
that may be induced in the audio cable of a microphone or
headphones?
下面哪种方法可以减小话筒或耳机用音频电缆可能感生的射频
电流？
Explanation · 解析
RF current induced on an audio lead is best suppressed by a
ferrite choke (铁氧体磁环) slipped over the cable — it adds
common-mode impedance at RF without affecting the audio. Option A is correct. Putting filters in the core wire (B/D) would
distort the audio; a mic amp (C) does nothing for RFI. 天线的作用 Antenna Functions
U0431. The function of a transmitting antenna is:
发射天线的作用是：
Explanation · 解析
An antenna (天线) is a transducer: it converts the guided RF current from the transmitter into radiating electromagnetic
waves (radio waves). Option A is correct. Gain (B) is a
directional focusing effect, not power amplification; an antenna does not generate power. C is a receiver/detector
function; D is wrong.
U0432. The function of a receiving antenna is:
接收天线的作用是：
Explanation · 解析
A receiving antenna is the reciprocal of a transmitting one: it intercepts passing radio waves and converts them back into a
tiny RF voltage delivered to the receiver input. Option A is
correct. Gain (B) focuses, it does not "amplify" free-space waves; audio (C) is produced later by the receiver, not the
antenna; D is wrong.
U0433. Regarding antenna gain, which of the following statements is
correct? (Choose all that apply.)
关于天线的增益，以下说法正确的是：
Explanation · 解析
Antenna gain (增益) is a directional measure: the radiation intensity (power density) in the main lobe relative to a
reference antenna (isotropic for dBi, dipole for dBd), i.e. how much the antenna concentrates power in a given direction. A
and B both express this. C is radiation efficiency (辐射效率), not
gain; D is dissipation/loss ratio. Hence A, B correct.
U0434. Antenna gain expressed in dBi means:
以dBi为单位的天线增益是指：
Explanation · 解析
dBi = gain relative to an isotropic (理想点源) radiator — a
theoretical point source radiating equally in all directions. So dBi compares your antenna's main-lobe power density to that
of an isotropic source, in dB. Option A is correct. B is the
definition of dBd (relative to a half-wave dipole); C and D are other references.
U0435. What is an "ideal point-source antenna"? And what is its
significance for amateur radio?
什么是“理想点源天线”？对业余无线电又有什么意义？
Explanation · 解析
The ideal isotropic radiator (理想点源天线) is a theoretical
lossless point source that radiates equally in every direction —
it cannot be built, but it is the reference for dBi gain. Option A describes it correctly. B/C/D invent false physical antennas; it is
purely a mathematical reference.
U0436. Antenna gain expressed in dBd means:
以dBd为单位的天线增益是指：
Explanation · 解析
dBd = gain relative to a half-wave dipole (半波长偶极天线),
which itself has 2.15 dBi. So dBd compares your antenna's main-lobe density to that of a dipole, in dB. Option A is correct.
B is dBi; C and D are other references.
U0437. The half-wave dipole is a practical antenna capable of
radiating radio waves. To obtain the lowest input impedance,
it is usually fed at the center point. This forms a resonant
dipole antenna with two 1/4-wavelength regions of different
polarity, also called a half-wave dipole. This antenna's gain is
specified as 0 dBd. Then, compared with an ideal point-source
antenna, its gain is:
A half-wave dipole has a gain of 0 dBd by definition; since 0
dBd = 2.15 dBi (the dipole is 2.15 dB stronger than an isotropic point source), its gain versus the ideal isotropic antenna is 2.15
dBi. Option A is correct. D (1.64) is the aperture efficiency factor, not the gain in dBi.
U0438. The antenna gain indicator given in a certain commercial
antenna manual is expressed in dB. Its meaning is:
某商品天线说明书给出的天线增益指标以dB为单位。其意义
为：
Explanation · 解析
A bare "dB" with no stated reference (neither dBi nor dBd) is
meaningless for comparing antennas, because gain must be relative to a defined reference antenna. Such a spec lacks
reference value. Option A is correct. B is the dBi definition, C a dB-vs-vertical, D a front-to-back ratio — but the question says
only "dB" with no basis, so none of those apply.
U0439. The reason we call a vertical grounded antenna an
"omnidirectional antenna" is:
我们之所以称垂直接地天线为“全向天线”，是因为：
Explanation · 解析
A vertical grounded antenna radiates equally in all compass bearings around it — its horizontal (azimuth) pattern is a circle,
i.e. no horizontal directivity — which is why it is called
omnidirectional (全向). Option A is correct. It does have vertical (elevation) directivity/nulls (B false); its patterns are not
perfect circles in both planes (C false); and it is not truly isotropic (D false — only an ideal point source is isotropic).