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G3B

RADIO WAVE PROPAGATION

- Maximum Usable Frequency; Lowest Usable Frequency; short path and long path propagation; determining propagation conditions; ionospheric refraction

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G3B011 of 12

What is a characteristic of skywave signals arriving at your location by both short-path and long-path propagation?

Why Radio waves travel about 300,000 km per second, so a trip around the world takes roughly 0.14 second. When the same signal reaches you both the short way and the long way around the Earth, the long-path copy arrives many tens of milliseconds later, which the ear hears as a hollow echo or ringing on the signal. This is a classic clue that you are hearing the station on both paths at once.
Watch out The ideas about cancellation or a 3 dB boost assume the two signals arrive in phase or out of phase, but the path difference is far too large for that kind of coherent addition at audio speeds; you get a time-delayed echo, not a fixed level change.
Long way round arrives late: the signature of short plus long path is an echo, about 0.14 s around the world.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B022 of 12

What factors affect the MUF?

Why Maximum Usable Frequency is the highest frequency that will still refract back to Earth over a given path, so it depends on both the ionization level of the ionosphere and the geometry of the path. Ionization is driven by solar radiation, which varies with time of day, season and the solar cycle, and can be upset by flares and geomagnetic storms. Path distance and location matter because a longer hop strikes the ionosphere at a shallower angle, which refracts higher frequencies back down, while a short, steep path has a much lower MUF. Since all of these change the MUF, every listed factor applies.
Watch out Each single choice is a real MUF factor, which is what makes them tempting, but none of them is the complete answer on its own.
MUF depends on sun, season, time and path geometry: when in doubt on MUF factors, pick all of them.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B033 of 12

Which frequency will have the least attenuation for long-distance skip propagation?

Why Ionospheric absorption, which happens mostly in the D layer, falls off roughly as the inverse square of frequency, so the higher you go the less signal you lose on the way up and back down. The MUF is the highest frequency the ionosphere will still refract back to earth on that path, so operating just under it gives the strongest signal with the least attenuation. Operators often aim for a band near 80 to 90 percent of the MUF for reliable, low-loss skip.
Watch out The choice just above the LUF is the opposite extreme: the Lowest Usable Frequency is set by absorption, so a signal barely above it is being heavily attenuated and barely makes it through. The critical frequency applies to signals sent straight up and is well below the MUF for a long, low-angle path, so those choices would waste the available refraction.
Absorption drops as 1/f^2, so ride high: just below the MUF.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B044 of 12

Which of the following is a way to determine current propagation on a desired band from your station?

Why Automated receiving networks such as the Reverse Beacon Network, PSK Reporter and WSPRnet post spots of signals they hear, along with frequency, time and signal strength. If you transmit a CW or digital signal and then look yourself up on one of those websites, you learn exactly which bands are open from your own station to which parts of the world right now. That is real, direct evidence of current propagation on your chosen band, not a forecast.
Watch out The A-index is a once-daily summary number describing geomagnetic disturbance over the previous 24 hours, so it is a general background indicator rather than a live band-by-band check, and listening for echoes of your own dots is an unreliable curiosity, not a practical test.
Skimmers and spotting networks show where YOU are heard; indices only hint at conditions.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B055 of 12

How does the ionosphere affect radio waves with frequencies below the MUF and above the LUF?

Why The MUF (Maximum Usable Frequency) is the highest frequency the ionosphere will bend back to Earth on a given path at a given time; the LUF (Lowest Usable Frequency) is the lowest frequency that survives D-layer absorption and still arrives with usable signal strength. Between those two limits is the usable window, where the wave is refracted enough to return to the ground and complete the hop. Above the MUF the bending is insufficient and the signal escapes into space; below the LUF the signal is absorbed before it can be heard.
Watch out Passing through the ionosphere is what happens above the MUF, not within the window, and the ionosphere is a passive medium that can only refract and attenuate, never amplify.
Between LUF and MUF is the sweet spot: too high and it escapes, too low and it is absorbed.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B066 of 12

What usually happens to radio waves with frequencies below the LUF?

Why The LUF, or lowest usable frequency, is the bottom end of the window for a given path: below it, absorption in the lower ionosphere (mainly the D layer, where collisions are frequent because the air is still relatively dense) soaks up the signal energy. The wave may still refract, but so little of it survives the trip that the signal is too weak to be useful at the far end. Absorption grows roughly as the inverse square of frequency, so the lower you go the worse it gets. The usable band for a path therefore lies between the LUF and the MUF.
Watch out Passing through the ionosphere is what happens above the MUF, at the high end of the window, not below the LUF, so that choice has the situation upside down.
MUF too high, signal escapes; LUF too low, signal gets eaten. Low frequency = high absorption.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B077 of 12

What does LUF stand for?

Why LUF is the Lowest Usable Frequency, and like MUF it is defined for a specific path between two specific points at a given time. Below the LUF, absorption in the D layer (which increases as frequency decreases) attenuates the signal so much that it arrives too weak to be usable, even though the ionosphere would still refract it back. Usable HF operating frequencies for a path therefore lie in the window between the LUF and the MUF.
Watch out The choices tied to a 100-mile radius, a 24-hour period, or the past 60 minutes add conditions that are not part of the definition; LUF is path-specific and changes continuously with ionospheric conditions.
MUF and LUF are both point-to-point: work between them, too low gets absorbed, too high skips over.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B088 of 12

What does MUF stand for?

Why MUF is the Maximum Usable Frequency, the highest frequency that the ionosphere will refract back to Earth for a specific path between two points at a given time. Above the MUF signals punch through the ionized layer into space instead of bending back down. It is path-specific and changes continuously with time of day, season, and solar activity, so it is not a single value for a 24-hour period. The companion term LUF, Lowest Usable Frequency, sets the bottom of the window where absorption and noise still allow a usable signal.
Watch out The choices tying the figure to a 24-hour period are wrong because MUF is quoted for a particular path at a particular moment; and Minimum Usable Frequency confuses it with the LUF at the other end of the range.
MUF = Maximum, path between two points. Work just below the MUF for the best skip; above it, signals escape.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B099 of 12

What is the approximate maximum distance along the Earth's surface normally covered in one hop using the F2 region?

Why The F2 region is the highest ionospheric layer, typically 200 to 300 miles up, so a signal refracted from it can return to Earth roughly 2,500 miles from the transmitter. The maximum hop distance is set by geometry: the higher the reflecting layer and the lower the takeoff angle, the farther the skip. That is why worldwide contacts on HF usually require several F2 hops.
Watch out The choice around 1,200 miles is the classic single-hop distance for the lower E region, which is only about 70 miles up. The 12,000 mile figure is roughly halfway around the world, which takes multiple hops, not one.
E region low, about 1,200 miles; F2 region high, about 2,500 miles. Higher layer, longer hop.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B1010 of 12

What is the approximate maximum distance along the Earth's surface normally covered in one hop using the E region?

Why The E region sits roughly 60 to 70 miles up, so the geometry of a signal refracted there and returned to Earth limits one hop to about 1,200 miles. The higher F2 region, at 150 to 300 miles altitude, gives a longer hop of about 2,500 miles because the reflection point is farther above the ground and the take-off geometry reaches farther over the horizon.
Watch out The 2,500 mile figure is the classic single-hop F2 distance, not E region; 12,000 miles would require multiple hops or long path.
E is low so the hop is short: E = 1,200 miles, F2 = 2,500 miles. Higher layer, longer hop.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B1111 of 12

What happens to HF propagation when the LUF exceeds the MUF?

Why The MUF is the highest frequency the ionosphere will still refract back to earth on a given path, and the LUF is the lowest frequency that can get through D layer absorption with usable signal strength. Usable skywave frequencies lie in the window between them, LUF at the bottom and MUF at the top. If absorption rises (or ionization falls) until the LUF climbs above the MUF, that window closes and there is no frequency that both survives absorption and gets refracted back, so ordinary skywave contacts over that path fail.
Watch out The choices about enhanced propagation invert the idea: a wide spread between a low LUF and a high MUF is what makes conditions good, and nothing is enhanced when the two cross.
Usable band = LUF to MUF. When the floor rises above the ceiling, the window slams shut.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G3B1212 of 12

Which of the following is typical of the lower HF frequencies during the summer?

Why Summer brings frequent thunderstorm activity, and lightning discharges radiate broadband impulse noise that propagates well on 160, 80 and 40 meters. The result is a high static (QRN) floor on the lower HF bands, especially at night when those bands would otherwise be at their best. Weak signals get buried in the crashes, so summer operating on the low bands is usually noisier and shorter-range than winter operating.
Watch out Saying propagation is poor at any time of day overstates it: the ionosphere still supports the band, it is the received noise, not the refraction, that degrades summer low-band work. Worldwide daylight propagation on the low bands is blocked by D-layer absorption, and 'photon absorption' distortion is not a real effect.
Summer thunderstorms equal QRN: low bands get noisy, not dead. Winter is low-band DX season.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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