This group covers what happens to a signal between your antenna and the receiver: reflections, absorption by weather and foliage, polarization, and the layer of the atmosphere that bounces HF signals back to Earth. Most of it comes down to one idea: radio waves take more than one path, and the paths can help or hurt each other.
Multipath: why signals flutter and fade
A signal rarely travels only in a straight line. Part of it also bounces off buildings, hills, vehicles and the ground. At the receiver these copies arrive with slightly different delays and are added together.
If the copies arrive in step, they reinforce and the signal gets stronger.
If they arrive out of step, they cancel and the signal gets weaker.
On VHF and UHF a wavelength is short (about 2 meters on 2 meters, about 0.7 meter on 70 cm), so moving an antenna just a few feet changes the path lengths enough to swing between a peak and a null. A mobile station moving through these peaks and nulls produces rapid flutter called picket fencing.
Multipath also hurts digital modes. Delayed copies smear one data symbol into the next, so the error rate goes up. There is no rule that ties the data rate to the number of paths; the practical result of multipath is simply more errors.
Reflections can rescue a weak path
When buildings or hills block the direct line to a repeater, a directional antenna can be aimed at a large reflecting surface instead. If the signal bounces to the repeater, you may be able to complete the contact. Switching polarization or deliberately raising your SWR will not help.
Weather and vegetation
Vegetation: Leaves and branches hold water and absorb UHF and microwave signals. The loss gets worse as frequency rises, so weak signals through trees are often lost.
Precipitation: Rain, wet snow and heavy fog absorb and scatter microwave signals and reduce range. At 6 and 10 meters, however, these have little effect because the wavelength is much larger than the droplets.
Polarization
Polarization is the orientation of the electric field of the wave. A vertical antenna sends vertical polarization; a horizontal antenna sends horizontal polarization.
Weak-signal CW and SSB on VHF and UHF use horizontal polarization by convention. FM and repeater work normally use vertical.
If the transmit and receive antennas are polarized at right angles (cross-polarized), the received signal is noticeably weaker, even on a line-of-sight path.
The ionosphere and HF
The ionosphere is a region of ionized gas high above the Earth. It refracts HF waves back toward the ground, which allows long-distance HF contacts. The troposphere and stratosphere do not do this.
Signals returned by the ionosphere often arrive by several paths and combine randomly, producing irregular fading. The refraction also leaves the wave elliptically polarized, so on HF skywave paths either a vertical or a horizontal antenna will work, and the two stations do not need matching polarization.
Key facts to memorize
Multipath = signals arriving by several paths that add or cancel; causes big changes with small antenna moves on VHF and UHF.
Picket fencing = rapid flutter on mobile signals caused by multipath.
Multipath raises the error rate on data transmissions.
Foliage absorbs UHF and microwave signals; rain absorbs microwaves.
Fog and rain have little effect on 10 and 6 meters.
Horizontal polarization is standard for VHF/UHF CW and SSB.
Cross-polarization reduces received signal strength.
Ionospheric signals are elliptically polarized, so either antenna polarization works.
The ionosphere reflects (refracts) HF waves.
Memory hooks
“Picket fence” flutter: picture a car driving past a fence, with the signal blinking on and off between the pickets.
SSB and CW on the weak-signal end of VHF: Horizontal (H for “Hush, weak signals”).
Questions in this group (12): answers shown
T3A01
Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?
A. The signal path encounters different concentrations of water vapor
B. VHF ionospheric propagation is very sensitive to path length
C. Multipath propagation cancels or reinforces signals ✓ Correct answer
D. The Doppler effect causes slight frequency shifts which result in changes in signal strength
Signals can reach you by a direct path and by reflections off buildings, terrain and vehicles. These multipath signals add or cancel depending on tiny differences in path length, and at VHF a wavelength is only a few feet, so moving the antenna slightly changes the result a lot.
T3A02
How does vegetation affect UHF and microwave signals?
A. Causes knife-edge diffraction, distorting voice peaks
B. Absorbs signals, leading to poor reception of weak signals ✓ Correct answer
C. Amplifies signals, improving reception of weak signals
D. Has no effect
Leaves and branches contain water and absorb energy at UHF and microwave frequencies. The effect grows with frequency, so weak signals through foliage can become unusable. Vegetation does not amplify signals.
T3A03
What antenna polarization is normally used for long-distance CW and SSB contacts on the VHF and UHF bands?
A. Right-hand circular
B. Left-hand circular
C. Horizontal ✓ Correct answer
D. Vertical
Horizontal polarization is the convention for weak-signal work such as CW and SSB on VHF and UHF. Using the same polarization as everyone else avoids losing signal strength to a polarization mismatch. Vertical is the norm for FM and repeaters.
T3A04
What is the effect of antenna cross-polarization over a line-of-sight VHF or UHF path?
A. Modulation sidebands might become inverted
B. Received signal strength is reduced ✓ Correct answer
C. Signals have an echo effect
D. Nothing significant will happen
If the transmitting and receiving antennas are polarized at right angles to each other (for example one vertical and one horizontal), the received signal is noticeably weaker. Over a line-of-sight path the loss can be large, though it does not invert sidebands or cause echoes.
T3A05
When using a directional antenna, how might your station be able to communicate with a distant repeater if buildings or obstructions are blocking the direct line of sight path?
A. Change from vertical to horizontal polarization
B. Try to find a path that reflects signals to the repeater ✓ Correct answer
C. Try the long path
D. Increase the antenna SWR
Signals can bounce off buildings, hills or other large objects. Aiming your directional antenna at a reflecting surface may create a usable path to the repeater when the direct path is blocked. Changing polarization or raising SWR does not get around an obstruction.
T3A06
What is the meaning of the term “picket fencing”?
A. Alternating transmissions during a net operation
B. Rapid flutter on mobile signals due to multipath propagation ✓ Correct answer
C. A type of ground system used with vertical antennas
D. Interference from cable TV in the form of carriers at fixed intervals across the band
Picket fencing is the rapid, repetitive flutter heard on mobile stations. It happens because a moving vehicle passes through peaks and nulls created by multipath reflections, like a stick dragged along a picket fence.
T3A07
What weather condition might decrease range at microwave frequencies?
A. High winds
B. Low barometric pressure
C. Precipitation ✓ Correct answer
D. Colder temperatures
Rain, wet snow and heavy fog absorb and scatter microwave energy, shortening the usable range. The effect gets worse as frequency rises. Wind, low pressure and cold are not significant absorbers.
T3A08
What is a likely cause of irregular fading of signals propagated by the ionosphere?
A. Frequency shift due to Faraday rotation
B. Interference from thunderstorms
C. Intermodulation distortion
D. Random combining of signals arriving via different paths ✓ Correct answer
Ionospheric signals often arrive over several paths of slightly different length. These components combine randomly, sometimes reinforcing and sometimes cancelling, which produces irregular fading. This is a path-combining effect, not a distortion product from the equipment.
T3A09
Which of the following results from the fact that signals propagated by the ionosphere are elliptically polarized?
A. Digital modes are unusable
B. Either vertically or horizontally polarized antennas may be used for transmission or reception ✓ Correct answer
C. FM voice is unusable
D. Both the transmitting and receiving antennas must be of the same polarization
Ionospheric refraction twists the wave so it becomes elliptically polarized, with no fixed orientation on arrival. As a result, either a horizontally or a vertically polarized antenna can work for HF skywave contacts, and polarization matching matters little.
T3A10
What effect does multi-path propagation have on data transmissions?
A. Transmission rates must be increased by a factor equal to the number of separate paths observed
B. Transmission rates must be decreased by a factor equal to the number of separate paths observed
C. No significant changes will occur if the signals are transmitted using FM
D. Error rates are likely to increase ✓ Correct answer
Multipath delivers delayed copies of each data symbol, which interfere with the following symbols and corrupt bits. The usual result is a higher error rate. Changing the data rate by the number of paths is not how it works.
T3A11
Which region of the atmosphere can reflect HF radio waves?
A. The stratosphere
B. The troposphere
C. The ionosphere ✓ Correct answer
D. The electrosphere
The ionosphere is a region of electrically charged (ionized) gas high above the Earth. It refracts HF waves back toward the ground, which makes long-distance HF skywave contacts possible. The troposphere and stratosphere do not reflect HF.
T3A12
What effect does fog or rain have on 10-meter and 6-meter band signals?
A. Absorption
B. Little effect ✓ Correct answer
C. Deflection
D. Increased range
At 6 and 10 meters (about 50 and 28 MHz) the wavelengths are long compared to raindrops and fog droplets, so these weather conditions have little effect. Absorption by rain becomes a problem only at much higher frequencies, such as microwaves.