HamSandwich

The Elmer you never had.

Study › Technician › T3A

T3A

RADIO WAVE PROPAGATION

Radio wave characteristics: how a radio signal travels, fading, multipath, polarization, wavelength vs absorption; Antenna orientation

Drill results are kept in this browser. Log in to keep them on your account and get a study plan.

T3A011 of 12

Why do VHF signal strengths sometimes vary greatly when the antenna is moved only a few feet?

Why At VHF a signal usually arrives at your antenna by several paths at once: the direct path plus reflections off buildings, hills, towers and vehicles. Those copies arrive with different phase, so they add up in some spots and cancel in others, creating a standing pattern of strong and weak spots. Since a wavelength at 146 MHz is only about 2 meters, moving the antenna a few feet can shift you from a null to a peak. That is why mobile stations hear rapid flutter, or picket fencing, while driving.
Watch out Ionospheric propagation is rarely a factor at VHF for local contacts, and a few feet of path length would be utterly insignificant to skip anyway; Doppler shift changes frequency, not strength, and requires real motion between stations.
Short waves, short nulls: at 2 meters a peak and a null can be a few feet apart. Move the antenna, change the multipath mix.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A022 of 12

How does vegetation affect UHF and microwave signals?

Why At UHF and microwave frequencies the wavelength is only a few centimeters to a few tens of centimeters, comparable to the size of leaves and branches, and the water inside that foliage absorbs RF energy. The result is added path loss, so weak signals through trees or brush get buried in the noise. The higher the frequency, the worse the attenuation, which is why 2.4 GHz links suffer more foliage loss than 2 meters.
Watch out Knife-edge diffraction is a real propagation effect, but it comes from signals bending over a sharp obstruction such as a ridge or building edge, not from vegetation, and nothing passive can amplify a signal.
Wet leaves eat short waves: the higher the frequency, the more trees attenuate.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A033 of 12

What antenna polarization is normally used for long-distance CW and SSB contacts on the VHF and UHF bands?

Why By long-standing convention, VHF/UHF weak-signal work using CW and SSB is done with horizontally polarized antennas, so beams, Yagis and loops for those modes are mounted with the elements parallel to the ground. Horizontal polarization also tends to pick up less man-made noise and suffers less ground-reflection loss over long paths. Matching polarization matters because a cross-polarized station can be 20 dB or more weaker, so everyone sticks to the same standard.
Watch out Vertical is the normal choice for FM repeater and mobile operating on VHF/UHF, not for long-haul SSB/CW; circular polarization is mainly used for satellite work, where the spacecraft's orientation keeps changing.
Horizontal for weak-signal SSB/CW, vertical for FM repeaters, circular for satellites.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A044 of 12

What is the effect of antenna cross-polarization over a line-of-sight VHF or UHF path?

Why Antennas transmit and receive best when their electric field orientation matches. If one station uses a horizontal antenna and the other vertical, the polarizations are 90 degrees apart and only a small fraction of the energy couples into the receiving antenna, typically costing 20 dB or more on a direct line-of-sight path. That shows up simply as a much weaker received signal, not as distortion.
Watch out Echoes and inverted sidebands come from other mechanisms: multipath reflections cause flutter or ghosting, and sideband inversion is a mixing or modulation issue, neither of which is caused by antenna orientation. Saying nothing happens is only close to true for long skywave paths where the ionosphere scrambles polarization anyway.
Mismatched polarization = big signal loss. That's why FM repeaters are vertical and SSB/CW weak-signal work is horizontal.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A055 of 12

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?

Why VHF and UHF signals reflect well off large flat surfaces like metal buildings, water towers and hillsides. If the direct line of sight is blocked, you can rotate a directional antenna away from the repeater and aim at a reflecting object, bouncing the signal around the obstruction. This is a common technique in cities, where you often hear yourself best pointing at a building rather than at the repeater itself.
Watch out Trying the long path applies to HF signals traveling the other way around the world via the ionosphere, not to local VHF/UHF repeater work. Changing polarization on VHF FM usually loses signal, since repeaters are almost always vertically polarized, and raising SWR only wastes power.
Blocked path? Point the beam at a big building and bounce the signal in.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A066 of 12

What is the meaning of the term "picket fencing"?

Why As a mobile station moves, signals arriving over multiple paths alternately reinforce and cancel, so the received signal strength rises and falls many times per second. At VHF and UHF the wavelength is short, so the vehicle passes through those peaks and nulls very quickly, producing a rapid flutter or buzz on the signal. The name comes from the sound, like the rhythmic flicker you get looking through a picket fence as you drive past it.
Watch out The cable TV answer describes leakage producing evenly spaced carriers, which is a different problem entirely, and the ground system choice is describing radials under a vertical.
Picket fence = moving car + multipath = fast flutter. Think of driving past fence slats.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A077 of 12

What weather condition might decrease range at microwave frequencies?

Why At microwave frequencies the wavelength shrinks toward the size of raindrops, so water droplets absorb and scatter the signal instead of letting it pass. This 'rain fade' grows worse as frequency climbs, becoming very noticeable above roughly 10 GHz, which is why satellite TV dishes drop out in a heavy downpour. Snow, fog and hail cause similar loss, so any form of precipitation in the path can cut your microwave range.
Watch out Wind and barometric pressure by themselves do not absorb RF; pressure and temperature matter only indirectly, by shaping temperature inversions and ducts that can actually extend VHF/UHF range rather than shorten it.
Microwaves + water drops = rain fade. Think of satellite TV going out in a thunderstorm.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A088 of 12

What is a likely cause of irregular fading of signals propagated by the ionosphere?

Why A signal refracted by the ionosphere can reach you over more than one path: different hop counts, different layers, or slightly different launch angles. Those copies arrive with slightly different travel times, so their waves sometimes add in phase and sometimes cancel, and the result is signal strength that rises and falls irregularly. This is multipath fading, and on HF it produces the familiar slow flutter and occasional deep nulls.
Watch out Faraday rotation is real, but it twists the polarization of the wave rather than shifting frequency, and it is more of a satellite-path concern; intermodulation is a receiver or transmitter mixing problem, not a propagation effect.
Fading = the same signal arriving twice by different roads, sometimes helping, sometimes cancelling.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A099 of 12

Which of the following results from the fact that signals propagated by the ionosphere are elliptically polarized?

Why When a signal passes through the ionosphere, the magnetized plasma rotates and reshapes the wave's polarization (Faraday rotation), so what arrives is elliptically polarized and its orientation drifts constantly. Because the arriving wave has components in both planes, a vertical or a horizontal antenna will pick up usable signal. That is why HF skywave operators mix dipoles, verticals and beams freely and still work each other.
Watch out The idea that both stations must match polarization is true for line-of-sight VHF/UHF work, where cross-polarization can cost 20 dB or more, but it does not apply to ionospheric paths. Polarization has nothing to do with whether a mode like FM or a digital mode works.
Skywave scrambles polarization: on HF, vertical or horizontal both work. Matching polarization matters only line-of-sight.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A1010 of 12

What effect does multi-path propagation have on data transmissions?

Why Multipath means copies of the same signal arrive by different routes, so they show up at slightly different times and phases. The delayed copies smear into the following data symbols and can partially cancel the direct signal, so the receiver misreads bits and the error rate climbs. This is why packet and digital modes often need retries or error correction in areas with lots of reflectors like buildings and terrain.
Watch out The idea that FM is immune is wrong: FM's capture effect helps with weak interfering stations, but delayed reflections of your own signal still distort the data. And multipath does not force any particular change in baud rate by the number of paths; higher rates are actually more vulnerable because symbols are shorter.
More paths, more smeared bits: multipath means more errors, not a new speed setting.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A1111 of 12

Which region of the atmosphere can reflect HF radio waves?

Why HF skywave propagation depends on the ionosphere, the upper region of the atmosphere (roughly 60 to 400 km up) where solar ultraviolet and X-ray radiation strips electrons from gas atoms, creating free ions and electrons. These charged layers (D, E, F1, F2) bend HF signals back toward Earth, allowing worldwide contacts far beyond line of sight. The amount of refraction depends on the ionization level, which varies with the sun, time of day, season and solar cycle.
Watch out The troposphere is the weather layer nearest the ground, and it can duct VHF and UHF signals over the horizon, but it does not return HF waves. The stratosphere is not ionized enough to refract radio waves, and 'electrosphere' is not a real atmospheric layer.
Ionized means bent back: 'ion' in ionosphere is the clue for HF skip.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T3A1212 of 12

What effect does fog or rain have on 10-meter and 6-meter band signals?

Why Precipitation only scatters or absorbs radio energy when the raindrops are a significant fraction of a wavelength, which happens at microwave frequencies (roughly 10 GHz and above). At 28 MHz and 50 MHz the wavelength is measured in meters, so raindrops and fog droplets are millions of times smaller and essentially invisible to the wave. That is why weather-related rain scatter and rain fade are microwave concerns, not 10-meter or 6-meter concerns.
Watch out Absorption is the tempting pick because it is real, but it applies to microwave bands and to the ionospheric D layer, not to raindrops affecting HF and low VHF.
Raindrops are millimeters; 10 m and 6 m waves are meters. Too big to notice the drops.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
Icom IC-7300 HF/50MHz Base TransceiverSponsored · View on Amazon →Powerwerx SS-30DV 30A DC Power SupplySponsored · View on Amazon →
← T2C All groups T3B →