A 'skip zone' is

The medium which reflects high frequency radio waves back to the earth's surface is called the

The highest frequency that will be reflected back to the earth at any given time is known as the

All communications frequencies throughout the spectrum are affected in varying degrees by the

Solar cycles have an average length of

The 'skywave' is another name for the

The polarisation of an electromagnetic wave is defined by the direction of

That portion of HF radiation which is directly affected by the surface of the earth is called

Radio wave energy on frequencies below 4 MHz during daylight hours is almost completely absorbed by this ionospheric layer

Because of high absorption levels at frequencies below 4 MHz during daylight hours, only high angle signals are normally reflected back by this layer

Scattered patches of high ionisation developed seasonally at the height of one of the layers is called

For long distance propagation, the radiation angle of energy from the antenna should be

The path radio waves normally follow from a transmitting antenna to a receiving antenna at VHF and higher frequencies is a

A radio wave may follow two or more different paths during propagation and produce slowly-changing phase differences between signals at the receiver resulting in a phenomenon called

The distance from the far end of the ground wave to the nearest point where the sky wave returns to the earth is called the

High Frequency long-distance propagation is most dependent on

The layer of the ionosphere mainly responsible for long distance communication is

The ionisation level of the ionosphere reaches its minimum

One of the ionospheric layers splits into two parts during the day called

Signal fadeouts resulting from an 'ionospheric storm' or 'sudden ionospheric disturbance' are usually attributed to

The 80 metre band is useful for working

The skip distance of radio signals is determined by the

Three recognised layers of the ionosphere that affect radio propagation are

Propagation on 80 metres during the summer daylight hours is limited to relatively short distances because of

The distance from the transmitter to the nearest point where the sky wave returns to the earth is called the

A variation in received signal strength caused by slowly changing differences in path lengths is called

VHF and UHF bands are frequently used for satellite communication because

The 'critical frequency' is defined as the

The speed of a radio wave

The MUF for a given radio path is the

The position of the E layer in the ionosphere is

A distant amplitude-modulated station is heard quite loudly but the modulation is at times severely distorted. A similar local station is not affected. The probable cause of this is

Skip distance is a term associated with signals through the ionosphere. Skip effects are due to

The type of atmospheric layers which will best return signals to earth are

The ionosphere

The skip distance of a sky wave will be greatest when the

If the height of the reflecting layer of the ionosphere increases, the skip distance of a high frequency transmission

If the frequency of a transmitted signal is so high that we no longer receive a reflection from the ionosphere, the signal frequency is above the

A 'line of sight' transmission between two stations uses mainly the

The distance travelled by ground waves in air

The radio wave from the transmitter to the ionosphere and back to earth is correctly known as the

Reception of high frequency radio waves beyond 4000 km normally occurs by the

A 28 MHz radio signal is more likely to be heard over great distances

The number of high frequency bands open to long distance communication at any time depends on

Regular changes in the ionosphere occur approximately every 11

When a HF transmitted radio signal reaches a receiver, small changes in the ionosphere can cause

The usual effect of ionospheric storms is to

Changes in received signal strength when sky wave propagation is used are called

Although high frequency signals may be received from a distant station by a sky wave at a certain time, it may not be possible to hear them an hour later. This may be due to

VHF or UHF signals transmitted towards a tall building are often received at a more distant point in another direction because