Wave propagation 1

Propagation of Waves

Ham Radio ClassWeek 5

Wave propagation 2

Propagation of waves

Radio waves spread out from the antenna in straight lines (like light)

Wave propagation 3

• The farther it travels the weaker it becomes, eventually becoming too weak to be received.

The distance over which a transmission can be received is called range.

Possible routes of wave propagation

Wave propagation 5

• The distance over which a transmission can be received is called range.The curvature of the earth sets a range limit for The distance over which a transmission can be received is called range.The curvature of the earth sets a range limit for many signals; this is called a radio horizon. (T3C10) many signals; this is called a radio horizon. (T3C10)

The curvature of the earth sets a range limit for many signals; this is called a radio horizon.

Wave propagation 6

Communications at VHF and UHF are generally “line of sight” communications.

That is they travel directly from the transmitting station to the receiving station.

Normally used for local communication.

Direct (not via a repeater) UHF and VHF signals are rarely heard from stations outside your local coverage area because these highfrequency signals arenot reflected by the ionosphere.

Wave propagation 8

Longer Radio waves can travel along the ground

Possible routes of wave propagation

Wave propagation 10

Radio waves can be reflected or refracted by sudden changes in the media through which they travel.

Knife-edge propagation is the term used to describe when signals are partially refracted around solid objects exhibiting sharp edges.

Obstructions can create radio shadows

Wave propagation 12

Refraction also makes the earth seem less curved to VHF and UHF signals making them able to travel further that the line of sight.

Wave propagation 13

Shorter waves lengths can more easily penetrate buildings, making communication from inside a structure more effective when using UHF signals.

Wave propagation 14

Radio signals may take different paths from the transmitter to the receiver. Thisis called multipath.

Wave propagation 15

Signals may interfere with each other if they are out of phase.

Wave propagation 16

Moving your antenna just a few feet may avoid the random reflections causing the multipath distortions.

Problems/solutions with multipath propagation

The irregular fading of signals from distant stations is caused by the random combining of signals arriving via different paths.

Wave propagation 17

Signals from mobile stations moving through an area with multipath havecharacteristic rapid variations in strength known as picket-fencing.

Wave propagation 18

What should you do if another operator reports that your station’s signals were strong just a moment ago, but now they are weak or distorted?

Move a few feet, as random reflectionsmay be causing multipath distortions.

Wave propagation 19

Why are UHF signals often more effective from inside buildings than VHF signals?

The shorter wave length allows them to more easily penetrate the structure of the building.

Wave propagation 20

What term is commonly used to describe the rapid fluttering sound heard from mobile stations that are moving while transmitting?

Picket fencing

Wave propagation 21

What is the cause of irregular fading of signals from distant stations during times of generally good reception?

A.absorption of signals by the D layer of the ionosphere.

B.absorption of signals by the E layer of the ionosphere.

C.random combining of signals arriving via different path lengths

D.distortion in the local receiver

Wave propagation 22

What may occur if VHF or UHF data signals propagate over multiple paths?

A. Transmission rates can be increased by a factor equal to the number of paths

B. Transmission rates may decrease by a factor equal to the number of paths

C. No significant changes will occur if the signals are transmitting using FM

D. Error rates are likely to increase

Wave propagation 23

What does the term knife-edge propagation mean?

A. Signals are reflected back toward the originating station at acute angles

B. Signals are sliced into several discrete beams and arrive via different paths

C. Signals are partially refracted around solid objects exhibiting sharp edges

D. Signals are propagated close to the band edge exhibiting a sharp cutoff

Wave propagation 24

What is the radio horizon?A. The distance at which radio signals between two points are effectively blocked by the curvature of the EarthB. The distance from the ground to a horizontally mounted antennaC. The farthest point you can see when standing at the base of your antenna towerD. The shortest distance between two points on the Earth’s surface

Wave propagation 25

Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations?A. Radio signals move somewhat faster than the speed of lightB. Radio waves are not blocked by dust particlesC. The Earth seems less curved to radio waves than to lightD. Radio waves are blocked by dust particles

Wave propagation 26

Wave propagation can be assisted by atmospheric phenomena

Wave propagation 27

Layers caused by temperature inversions form ducts that can carry signals much farther than typical line of sight communications

Wave propagation 28

Layers with different properties refract waves with slight variations.

This “tropospheric scattering often allows contacts over 300 miles.

Wave propagation 29

The ionosphere is an area from 30 to 260 miles above the earth that has some unique electrical properties.

Wave propagation 30

The ionosphere can completely reflect HF and VHF waves back toward the earth. (sky wave or skip)

Multiple reflections between the ionosphere/earth allows radio waves to be received around the world!

The earth’s surface can also reflect radios waves. (hop)

Wave propagation 31

The ability of the ionosphere to refract waves depends on the frequency of the waves.

Wave propagation 32

High frequency waves usually pass right through, this is why VHF and UHF signals are rarely heard beyond the radio horizon without being relayed by a repeater

Wave propagation 33

As the sun illuminates the ionosphere the maximum usable frequency increases.This is why upper HF bands are more likely to by usable during the day.

Wave propagation 34

During solar cycles patches of the ionosphere become sufficiently charged to reflect VHF and UHF signals back to Earth.

This is called sporadic E propagation.

Wave propagation 35

Sporadic E Propagation is most common during early summer and mid winter months on the 10, 6 and 2 meter bands.

Wave propagation 36

Radio signals reflected off the aurora borealis exhibit rapid fluctuations and are often distorted.

Wave propagation 37

It is possible to bounce radio signals of meteor trails. The best band for “meteor scatter” is the 6 meter band.

Wave propagation 38

What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?A. Tropospheric scatterB. D layer refractionC. F2 layer refractionD. Faraday rotation

Wave propagation 39

What band is best suited to communicating via meteor scatter?A. 10 metersB. 6 metersC. 2 metersD. 70 cm

Wave propagation 40

What causes “tropospheric ducting”?

A. Discharges of lightning during electrical storms

B. Sunspots and solar flares

C. Updrafts from hurricanes and tornadoes

D. Temperature inversions in the atmosphere

Wave propagation 41

What is generally the best time for long-distance 10 meter band propagation?A. During daylight hoursB. During nighttime hoursC. When there are coronal mass ejectionsD. Whenever the solar flux is low

Wave propagation 42

Which part of the atmosphere enables the propagation of radio signals around the world?A. The stratosphereB. The troposphereC. The ionosphereD. The magnetosphere

Wave propagation 43

Why are “direct” (not via a repeater) UHF signals rarely heard from stations outside your local coverage area?A. They are too weak to go very farB. FCC regulations prohibit them from going more than 50 milesC. UHF signals are usually not reflected by the ionosphereD. They collide with trees and shrubbery and fade out

Wave propagation 44

Which of the following might be happening when VHF signals are being received from long distances?A. Signals are being reflected from outer spaceB. Signals are arriving by sub-surface ductingC. Signals are being reflected by lightning storms in your areaD. Signals are being refracted from a sporadic E layer

Wave propagation 45

What is a characteristic of VHF signals received via auroral reflection?A. Signals from distances of 10,000 or more miles are commonB. The signals exhibit rapid fluctuations of strength and often sound distortedC. These types of signals occur only during winter nighttime hoursD. These types of signals are generally strongest when your antenna is aimed to the south (for stations in the Northern Hemisphere

Wave propagation 46

Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands?A. BackscatterB. Sporadic EC. D layer absorptionD. Gray-line propagation