Physics & RF
B14 EM Fields & Wave Behavior — Study Guide
This guide covers EM Fields & Wave Behavior as part of Physics & RF. Start with the vocabulary, then try to answer each question before revealing the answer. Connect these facts to a real radio link or electrical circuit and practice the subject in the arcade when you are ready.
Acronyms first
| Term | Meaning |
|---|---|
| EM | Electromagnetic |
| E | Electric field |
| H | Magnetic field |
| C | Speed of light |
Key facts
Unit of the electric (E) field:
Volts per meter (V/m)
Unit of the magnetic (H) field:
Amperes per meter (A/m)
An EM wave is composed of…
an electric field and a magnetic field
The E-field, H-field and direction of travel are…
mutually perpendicular
As the wave propagates forward, the E-field moves ____ and the H-field moves ____.
up/down; side-to-side
Which field exists even when charges are STATIONARY?
Electric (E) field
Which field is created by MOVING charges (current)?
Magnetic (H) field
Unlike sound, EM waves need no medium and can travel through space.
True
Which field determines polarization and drives current in antennas?
E-field
Which field do loop antennas sense for direction finding?
H-field
Operator analogy: E-field is the ____ effect; H-field is the ____ effect.
voltage; current
Per Maxwell, a changing E-field creates…
an H-field
Per Maxwell, a changing H-field creates…
an E-field
Scenario: a 100 MHz signal — how many times per second do both fields oscillate?
100 million
Approximate impedance of free space:
~377 Ω
Standard impedance of antenna systems and coax:
50 Ω
The antenna acts as an impedance transformer between…
the 50 Ω wire and 377 Ω free space
Why 50 Ω for coax?
A compromise between lowest attenuation and highest power handling
EM waves are born from…
accelerating charges, like electrons vibrating in an antenna
When an electron accelerates, its changing E-field induces an H-field. This is…
Faraday's law
The changing H-field then induces a new E-field, sustaining the wave. The slides attribute this to…
Ampère's law
A typical transmit/receive antenna made of two metal rods end-to-end:
Dipole
Scenario: a dipole driven at 100 MHz radiates a wave with wavelength…
3 m
Common wire dipole configurations:
Flattop, Inverted V, Sloper
The near field (within ~1 wavelength) is…
reactive — used for short-range sensing
The far field (beyond ~1 wavelength) is…
where waves propagate — critical for long-range jamming or radar
Scenario: a 1 GHz jammer targets enemy comms 10 km away. Which region?
Far field
Polarization describes the orientation of the…
E-field
Which polarization is typical for standard comms?
Linear (up-down or left-right)
Which polarization resists interference — used for anti-jamming?
Circular (rotating)
Scenario: a stealth aircraft evades radar partly by exploiting…
polarization mismatch
Who unified electricity and magnetism in the 1860s, predicting EM waves?
James Clerk Maxwell
'Electric charges make electric fields' is…
Gauss's law for electricity
'No magnetic charges (monopoles) exist; field lines form closed loops' is…
Gauss's law for magnetism
'Changing magnetic fields make electric fields' (how generators work) is…
Faraday's law
'Currents AND changing electric fields make magnetic fields' is…
Ampère–Maxwell law
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