Radio wave theory

1. RADIO WAVE THEORY
AND
FUNDAMENTALS OF
COMMUNICATIONS
GySgt N.J. Guidry USMC
Ver 31 May 01

2. Radio waves
Radio signals are electromagnetic waves. Two important characteristics of
radio waves are:
-Frequency: The number of cycles over a given period of time, usually a
second. Expressed in Hertz (Hz)
-Wavelength: The distance traveled by a radio wave in the time it takes to
complete one cycle.
These two characteristics have direct and inverse relationship
i.e. as the wavelength increases, the frequency decreases and vice
versa

4. Radio wave behavior
Propagation- The movement of radio waves through a medium
Absorption- Dissipation of a wave’s energy as it travels through a medium
Reflection- When a wave strikes a denser medium and is reflected off the
medium at the same angle at which it struck
Diffraction- The bending of a radio wave around an obstacle in its path to
reform the wave some distance behind the obstacle
Refraction- Occurs when a radio wave strikes a region of different density
and is bent off its original course

5. Ground Wave Propagation
- Direct Wave- Radio wave goes directly from
transmitting antenna to receiving antenna.
- Reflected Wave- Radio wave leaves transmitting
antenna and bounces off the Earth’s surface to the
receiving antenna.
- Surface Wave- Radio waves travels in direct
contact with the ground. The surface wave suffers
much attenuation.
The three components of the ground wave are:

8. Sky wave propagation
- Radio waves are transmitted to the upper
levels of the atmosphere where they are
refracted or reflected back down to the
receiving station.

9. IONOSPHERE
An electrically charged region of the
Earth's atmosphere that lies between the
altitudes of 50 to 500 km above the
surface of the earth.
Consists of four layers

10. "D" - LAYER
- Present only during periods when the sun
is above the horizon.
- Not sufficiently ionized to refract a radio wave.
- Absorbs energy from a radio wave as it passes
through.

11. "E" - LAYER
- The lowest layer that is sufficiently ionized
to refract radio waves, but generally only during
daylight hours.
- The behavior of the E-layer can be predicted
to some extent based on time of day, season,
and solar activity.

12. "F" - LAYER
- Composed of two layers (F1 and F2)
during daylight hours but merge into one
during night hours.
- Behavior is irregular and is difficult to predict
on a day to day basis.

14. HF SKY WAVE CIRCUITS
- Skip Distance-Distance between where
the signal leaves the transmitter and where
it returns to Earth
- Skip Zone- Distance between the end of
the ground wave and the point where the
skywave first returns to Earth

15. RT
Ground wave
Skip distance
Skip Zone
Ionosphere
Antenna

16. NVIS - Near Vertical Incidence Skywave
- Skywave propagation that uses high
take-off-angle antennas to radiate the signal
almost straight up. The signal is then
refracted back to earth.

17. SOLAR EFFECTS
- Regular variations: Predictable changes that
allow general planning when designing and
operating an HF skywave circuit.
- Irregular variations: Caused by the
unpredictable, short term events occurring on
the sun.

18. DAILY VARIATIONS
- Caused by the rotation of the earth.
- The portion of the earth exposed to the sun
will receive increased ionization.
- Higher frequencies can be used.
- Lower frequencies are increasingly
absorbed.
- During night hours ionization fades.
- Lower frequencies are required.
- Less absorption by the D-Layer.

20. 27 DAY CYCLE
- Caused by the radiation of the sun on it's axis.
- Exposes different levels of solar activity as
it rotates.

21. 11 YEAR CYCLE
- A general cycle of increasing and decreasing
solar activity.
- During the11 year cycle, the solar activity will
increase to a high and then decrease to a low.

22. SUN SPOTS
-Dark spots on the sun caused by magnetic storms
on the surface of the sun.
-An increase in sun spots is associated with an
increase in ionization. The higher the sun spot
number, the higher the usable frequencies.

23. SPORADIC "E"
- Areas of high electron density that sporadically
occurs in the lower levels of the "E"-Layer.
- Causes unusually high frequencies to be reflected
or refracted.
- Normally dissipates within a few hours.
- Triggered by bombardment of solar particles.

24. SOLAR FLARES
- Large sudden releases of energy from the sun
which can last from a few minutes to hours.
- Immediate effect is a large increase of solar noise.
- Triggers the start of a Sudden Ionospheric
Disturbance which can last minutes to several hours.
- Normally a daytime phenomenon.

25. SID - SUDDEN IONOSPHERIC DISTURBANCE
- Associated with Solar Flares, and occur without warning.
- SIDs are a large increase in ionization in all layers.
- Results in higher frequencies being able to be refracted,
but also increases the absorption of the "D"-Layer.
i.e., normal HF frequencies are absorbed by D-layer

26. Maximum Useable Frequency
(MUF)
-The MUF is the highest frequency at which
communications can take place via the
ionosphere over a given path
-Transmitter power is NOT a factor

27. Frequency of Optimum Transmission
(FOT)
The frequency at which the best skywave
propagation occurs over a given path
-Is usually 85% of MUF
-Transmitter power is NOT a factor.

28. Lowest frequency that will allow communications via
the ionosphere over a given path
Frequencies below the LUF will attenuate completely
or be absorbed
Transmitter power is a factor- Increasing the power
can decrease the LUF
Lowest Useable Frequency
(LUF)

29. FREQUENCY BAND
CHARACTERISTICS
The frequency spectrum currently usable for
communications ranges from 3 KHz to 300 GHz and is
divided into different bands.
The Marine Corps uses 4 of these bands:
HF, VHF, UHF, and SHF. With the addition of new
equipment, the USMC is expanding into the EHF.
First Force Reconnaissance Company uses HF,
VHF, and UHF.

30. HF High Frequency Communications
(2- 30 MHz)
Advantages:
- Low cost and equipment already in inventory.
- Viable alternative to satellite communications and not
as vulnerable to EMP as other systems.
- Can be highly portable and is capable of front line
operations as well as vehicular/mobile employment.

31. HF - High Frequency Communications
Disadvantages:
-High level of interference exists in the HF spectrum
-Propagation conditions are not consistent
-Narrow bandwidth
-HF frequencies have a larger wave length which
may require an extremely large antenna

32. VHF - Very High Frequency Communications
- VHF communications provide the primary means for
ground-to-ground, LOS, short range communications
- Advantages:
-VHF provides good ground wave signal normally free
from fading and minimum environmental noise
- Disadvantages:
- Limited to ground wave communications and LOS range
- Frequency availability is limited

33. UHF - Ultra High Frequency Communications
Advantages:
-UHF circuits are free from fading and
environmental noise
Disadvantages:
-No useable surface wave
-Frequency availability is limited
-Strictly LOS
Uses: Air-to-ground communications,
ground-to-air communications, and satellite
communications

34. QUESTIONS ?