LESSON 31

Propagation, fading, multipath distortion, reflections,
radio horizon, terrain blocking, wavelength vs. penetration,
antenna orientation

When considering VHF and UHF propagation sometimes it helps to remember that radio waves and light waves are both electro magnetic waves. The only difference between radio waves and light is their frequency or wave length (wave length is indirectly proportional to frequency that is the higher the frequency the shorter the wave length.)

VHF, UHF and higher frequencies are normally considered line of sight communication frequencies. This means when the signal reaches the horizon, the point at which the curvature of the earth blocks the signal, it continues off into space. Because of the longer wave length of VHF and UHF signals the horizon, known as radio horizon, for these signals is about one third longer then visual horizon.

Directional antennas (beams) are used to increase the transmitted signal in a given direction while reducing the signal in all other directions. When the beam is being used to receive it increases the received signal level in one direction while reducing the signal level in all other directions. The advantage of this phenomenon is more then just the simple point to point signal increase. VHF and UHF signals reflect more readily from objects like buildings, mountains, or other physical structures then do their HF counter parts. This reflection can cause multipathing which means the signal can be coming in from more then one direction. Because these reflected signals travel different distances they will most likely arrive at different phase angels which will cause the signal to interfere with itself. Beams can be used to increase the strongest signal while reducing all the others thus reducing the results of multipathing.

Sometimes a station which is blocked by an object can be reached by a VHF or UHF signals by reflecting the signals off a structure. This means the strongest signal may be in a direction away from the physical location of the other station.

The blocking of signals and multipathing can cause a fluttering sound to mobiles using VHF and UHF. This is frequently referred to as picket fencing. This same problem is sometimes observed when using a handheld unit by reports of a strong signal being changed to a weak signal though no appreciable change has take place. Because of the lower transmit power of the handheld the receive signal may not change noticeably. The best thing to do in such an event is to move around to look for a place where the signal is again improved.

Repeaters usually use vertically polarized antennas because they are easy to make omni-directional (radiate in all directions equally) then horizontally polarized antenna systems. For the same reason most mobile antenna systems are vertically polarized. When a two stations are trying to communicate with one having a horizontal antenna and the other having a vertical antenna as much as a 10 DB (100 times) loss can be produced. For this reason when using a handheld unit and working a distant repeater the antenna should be held as close to vertical as possible.

UHF signal will normally out perform VHF signals when being operated from within a building because The shorter wavelength of UHF signals allows them to more easily penetrate urban areas and buildings.

HF signals can be heard from stations even from as far as the other side of the world because HF signals can reflect off the ionosphere. VHF and UHF signals are not normally reflected by the ionosphere but sometimes a sporadic reflection from the E layer of the ionosphere will allow long distance VHF communications possible.

Strong VHF or UHF signals which overload the receiver will sometimes cause undesired signals to be heard. This can cause sudden bursts of tones or fragments of different conversations to be heard which interfere with the desired signal.


1. T9B01 (C)
Why are VHF/UHF signals not normally heard over long distances?
A. They are too weak to go very far
B. FCC regulations prohibit them from going more than 50 miles
C. VHF and UHF signals are usually not reflected by the ionosphere
D. They collide with trees and shrubbery and fade out ~~

2. T9B02 (D)
What might be happening when we hear a VHF signal from long distances?
A. Signals are being reflected from outer space
B. Someone is playing a recording to us
C. Signals are being reflected by lightning storms in our area
D. A possible cause is sporadic E reflection from a layer in the ionosphere
~~
3. T9B03 (B)
What is the most likely cause of sudden bursts of tones or fragments of different conversations that interfere with VHF or UHF signals?
A. The batteries in your transceiver are failing
B. Strong signals are overloading the receiver and causing undesired signals to be heard
C. The receiver is picking up low orbit satellites
D. A nearby broadcast station is having transmitter problems
~~
4. T9B04 (A)
What is the radio horizon?
A. The point where radio signals between two points are blocked by the curvature of the Earth
B. The distance from the ground to a horizontally mounted antenna
C. The farthest point you can see when standing at the base of your antenna tower
D. The shortest distance between two points on the Earth's surface
~~
5. T9B05 (D)
What should you do if a station reports that your signals were strong just a moment ago, but now they are weak or distorted?
A. Change the batteries in your radio to a different type
B. Speak more slowly so he can understand your better
C. Ask the other operator to adjust his squelch control
D. Try moving a few feet, random reflections may be causing multipath distortion.
~~
6. T9B06 (B)
Why do UHF signals often work better inside of buildings than VHF signals?
A. VHF signals lose power faster over distance
B. The shorter wavelength of UHF signals allows them to more easily penetrate urban areas and buildings
C. This is incorrect; VHF works better than UHF inside buildings
D. UHF antennas are more efficient than VHF antennas
~~
7. T9B07 (C)
What is a good thing to remember when using your hand-held VHF or UHF radio to reach a distant repeater?
A. Speak as loudly as possible to help your signal go farther
B. Keep your transmissions short to conserve battery power
C. Keep the antenna as close to vertical as you can
D. Turn off the CTCSS tone
~~
8. T9B08 (B)
What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?
A. The modulation sidebands might become inverted
B. Signals could be as much as 100 times weaker
C. Signals have an echo effect on voices
D. Nothing significant will happen
~~
9. T9B09 (B)
What might be a way to reach a distant repeater if buildings or obstructions are blocking the direct line of sight path?
A. Change from vertical to horizontal polarization
B. Try using a directional antenna to find a path that reflects signals to the repeater
C. Ask the repeater owners to repair their receiver
D. Transmit on the repeater output frequency

10. T9B10 (B)
What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting?
A. Flip-flopping
B. Picket fencing
C. Frequency shifting
D. Pulsing
~~
11. T9B11 (C)
Why do VHF and UHF Radio signals usually travel about a third farther than the visual line of sight distance between 2 stations?
A. Radio signals move somewhat faster than the speed of light and travel farther in the same amount of time
B. Radio waves are not blocked by dust particles
C. The Earth seems less curved to radio waves than to light
D. Radio waves are blocked by dust particles


Answers: C D B A D B C B B B C