Sky Wave and Ionospheric Propagation Questions and Answers

Q: Why does a solar flare affect only the sunlit side of the earth?

Electromagnetic radiation illuminates the hemisphere facing the sun and nothing else, so the enhanced D layer absorption is confined to daylight paths. Circuits crossing the terminator are partly affected. The night side has no D layer to enhance, magnetic shielding is not the mechanism for photons, and the arrival is immediate rather than delayed. The contrast with the particle events that arrive a day later and concentrate at high latitude is worth holding alongside.

Q: How does the solar cycle affect high frequency propagation?

Greater solar activity means more ionising radiation, higher F layer electron density and critical frequencies perhaps twice as high at solar maximum as at minimum, which opens the higher bands for long distance working. Absorption also rises, and the ionosphere persists throughout the cycle. Both D and F regions vary. An eleven year cycle in a propagation medium means that operating practice learned in one part of it can be quite wrong in another.

Q: What is the sunspot number used as?

The smoothed sunspot number is the traditional index against which ionospheric prediction programs are calibrated, since it correlates well with the ultraviolet flux that ionises the F layer. Solar radio flux at ten point seven centimetres serves the same purpose and is measured more objectively. It is a proxy rather than a direct measurement of any ionospheric or radiative quantity. Using an easily observed index as a proxy for a hard to measure driver is common wherever a long historical record is valuable.

Q: What is a multi-hop path?

Long circuits are served by successive ionospheric refractions with intervening ground reflections, each pair being one hop of perhaps three or four thousand kilometres. Each ground reflection costs several decibels, more over land than over sea. Chordal hops between layers without touching ground do occur and are more efficient, and ground relay is a quite different arrangement. Counting hops estimates both the loss and the likely arrival angle, which is what a path prediction reports.

Q: Why is a sea path better than a land path for multi-hop propagation?

Each intermediate reflection loses less energy from a highly conducting sea surface than from dry ground, and over several hops the difference amounts to many decibels. That is why transoceanic circuits often outperform continental ones of the same length. The ionosphere does not depend on what lies beneath it, propagation speed is unchanged, and distance is a matter of geography. The same conductivity that governs surface wave range reappears here in a different role.

Q: What is a chordal hop?

Where the ionosphere is tilted, a wave can be refracted from one region to another without an intervening ground reflection, saving the several decibels each such reflection would cost. It helps explain occasional very long distance reception at unexpectedly high signal strengths. The other options describe geometry, ordinary multi-hop and magnetic ducting, which is a further distinct mechanism. Avoiding a lossy interface by exploiting a gradient in the medium is an idea that appears in optics as well.

Q: Why does a low takeoff angle favour long distance high frequency working?

A shallow angle both lengthens each hop and raises the secant factor, so a higher frequency is returned and fewer hops are needed. That is why antenna height above ground, which controls the elevation pattern, matters so much on a high frequency circuit. Absorption depends chiefly on frequency, the skip zone grows rather than vanishes, and mounting height is the means rather than the end. Two benefits from one geometric choice make low angle radiation the central goal of high frequency antenna design.

Q: How does antenna height above ground affect the takeoff angle?

Ground reflection interferes with the direct wave, and the first lobe of a horizontal antenna falls to lower elevation as the height in wavelengths increases, which is why high frequency antennas for long distance work are mounted as high as practicable. At half a wavelength the lobe is near thirty degrees and at one wavelength near fifteen. Height is an electrical parameter of the installation rather than a mechanical convenience, and it often matters more than the choice of antenna.

31.

Why does a solar flare affect only the sunlit side of the earth?

A.The X radiation travels in straight lines and cannot reach the night side
B.The night side ionosphere is too weak to be affected by any radiation
C.The magnetic field shields the night side from solar radiation
D.The flare's effect is delayed by hours and arrives after sunset

Answer: A

Electromagnetic radiation illuminates the hemisphere facing the sun and nothing else, so the enhanced D layer absorption is confined to daylight paths. Circuits crossing the terminator are partly affected. The night side has no D layer to enhance, magnetic shielding is not the mechanism for photons, and the arrival is immediate rather than delayed. The contrast with the particle events that arrive a day later and concentrate at high latitude is worth holding alongside.

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32.

What is an ionospheric storm?

A.The daily variation of the ionosphere between day and night
B.A disturbance following a solar particle event, depressing F layer density
C.A period of unusually intense sporadic E over a wide area
D.A sudden increase in D layer absorption caused by a solar flare

Answer: B

Charged particles arriving a day or more after a solar event disturb the F region, depressing its critical frequency and raising absorption at high latitudes for days, so maximum usable frequencies fall and circuits become unreliable. Sporadic E and flare absorption are distinct phenomena on different timescales, and the diurnal cycle is normal behaviour. Sorting solar terrestrial effects by their delay after the solar event identifies which mechanism carried them and how long they will last.

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33.

How does the solar cycle affect high frequency propagation?

A.The ionosphere disappears entirely at solar minimum
B.Only the D layer varies with the solar cycle, not the F layer
C.Maximum usable frequencies rise with sunspot number
D.Absorption falls to zero at the peak of the sunspot cycle

Answer: C

Greater solar activity means more ionising radiation, higher F layer electron density and critical frequencies perhaps twice as high at solar maximum as at minimum, which opens the higher bands for long distance working. Absorption also rises, and the ionosphere persists throughout the cycle. Both D and F regions vary. An eleven year cycle in a propagation medium means that operating practice learned in one part of it can be quite wrong in another.

Discuss in Forum

34.

What is the sunspot number used as?

A.A direct measure of the ionospheric critical frequency at any hour
B.A count of the flares expected during the following month
C.A measure of the total radiation received by the upper atmosphere
D.An index of solar activity for ionospheric prediction

Answer: D

The smoothed sunspot number is the traditional index against which ionospheric prediction programs are calibrated, since it correlates well with the ultraviolet flux that ionises the F layer. Solar radio flux at ten point seven centimetres serves the same purpose and is measured more objectively. It is a proxy rather than a direct measurement of any ionospheric or radiative quantity. Using an easily observed index as a proxy for a hard to measure driver is common wherever a long historical record is valuable.

Discuss in Forum

35.

What is a multi-hop path?

A.A signal returned by the ionosphere and the ground several times
B.A signal returned by two different ionospheric layers in succession
C.A signal that travels between two ionospheric layers without descending
D.A signal relayed by several ground stations along the route

Answer: A

Long circuits are served by successive ionospheric refractions with intervening ground reflections, each pair being one hop of perhaps three or four thousand kilometres. Each ground reflection costs several decibels, more over land than over sea. Chordal hops between layers without touching ground do occur and are more efficient, and ground relay is a quite different arrangement. Counting hops estimates both the loss and the likely arrival angle, which is what a path prediction reports.

Discuss in Forum

36.

Why is a sea path better than a land path for multi-hop propagation?

A.Sea paths are shorter than land paths between the same points
B.Ground reflection loss is much lower over sea water
C.The ionosphere is more heavily ionised above the oceans
D.The wave travels faster over sea and suffers less absorption

Answer: B

Each intermediate reflection loses less energy from a highly conducting sea surface than from dry ground, and over several hops the difference amounts to many decibels. That is why transoceanic circuits often outperform continental ones of the same length. The ionosphere does not depend on what lies beneath it, propagation speed is unchanged, and distance is a matter of geography. The same conductivity that governs surface wave range reappears here in a different role.

Discuss in Forum

37.

What is a chordal hop?

A.A path reflected once from the ionosphere and once from the sea
B.A path along which the wave is guided by the earth's magnetic field
C.A path refracted between ionospheric layers without touching the ground
D.A path that follows the chord of the earth rather than its surface

Answer: C

Where the ionosphere is tilted, a wave can be refracted from one region to another without an intervening ground reflection, saving the several decibels each such reflection would cost. It helps explain occasional very long distance reception at unexpectedly high signal strengths. The other options describe geometry, ordinary multi-hop and magnetic ducting, which is a further distinct mechanism. Avoiding a lossy interface by exploiting a gradient in the medium is an idea that appears in optics as well.

Discuss in Forum

38.

What is the maximum range of a single F layer hop?

A.About one thousand kilometres
B.About twenty thousand kilometres, half way around the world
C.About two hundred kilometres, similar to a line of sight path
D.About four thousand kilometres

Answer: D

With the F layer at around three hundred kilometres and the ray leaving at a low angle, geometry limits a single hop to roughly four thousand kilometres. Longer circuits need several hops or chordal paths. The E layer, being lower, gives a shorter maximum of about two thousand. Knowing the single hop limit lets the number of hops on a circuit be estimated at once, which in turn gives the expected loss and the arrival angle.

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39.

Why does a low takeoff angle favour long distance high frequency working?

A.It gives the longest hop and the highest usable frequency
B.It reduces the absorption suffered in the D layer on the way up
C.It avoids the skip zone entirely for all frequencies used
D.It allows the antenna to be mounted closer to the ground

Answer: A

A shallow angle both lengthens each hop and raises the secant factor, so a higher frequency is returned and fewer hops are needed. That is why antenna height above ground, which controls the elevation pattern, matters so much on a high frequency circuit. Absorption depends chiefly on frequency, the skip zone grows rather than vanishes, and mounting height is the means rather than the end. Two benefits from one geometric choice make low angle radiation the central goal of high frequency antenna design.

Discuss in Forum

40.

How does antenna height above ground affect the takeoff angle?

A.The takeoff angle depends only on the antenna type used
B.Greater height lowers the angle of the first elevation lobe
C.Greater height raises the angle so more energy goes upward
D.Height affects only the gain and not the elevation pattern

Answer: B

Ground reflection interferes with the direct wave, and the first lobe of a horizontal antenna falls to lower elevation as the height in wavelengths increases, which is why high frequency antennas for long distance work are mounted as high as practicable. At half a wavelength the lobe is near thirty degrees and at one wavelength near fifteen. Height is an electrical parameter of the installation rather than a mechanical convenience, and it often matters more than the choice of antenna.

Discuss in Forum

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