More On Propagation...

Back to Basics

From the Canadian Basic Question Bank

Back To Basics is a regular column in the SARC Communicator Newsletter, available on this blogsite.

B-007-003-002
What is the maximum distance along the Earth's surface that is normally covered in one hop using the F2 region?

A. 2000 km (1250 miles) 
B. 300 km (190 miles) 
C. 4000 km (2500 miles)
D. None, the F2 region does not support radio-wave propagation

There are at least a dozen questions in the Canadian Basic Question Bank that touch on propagation, this is just one of them. The science of RF propagation can take volumes to explain, let’s see if we can summarize the basics.

Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering.

Radio propagation is affected by the daily changes of water vapor in the troposphere and ionization in the upper atmosphere influenced by the Sun. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for Amateur Radio contacts, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

Several different types of propagation are used in practical radio transmission systems. Line-of-sight propagation means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through building walls and foliage. Line of sight transmission is used in short to medium range radio transmission such as garage door openers, cell phones, cordless phones, handheld transceivers, wireless networks, FM radio and television broadcasting and radar, and satellite communication, such as satellite television. Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, about 40 miles. It is the only propagation method possible at microwave frequencies and above. At microwave frequencies moisture in the atmosphere (rain fade) can degrade transmission.

At lower frequencies in the MF, LF, and VLF bands, due to diffraction radio waves, can bend over obstacles like hills, and travel beyond the horizon as surface waves which follow the contour of the Earth. These are called ground waves. AM broadcasting stations use ground waves to cover their listening areas. As the frequency gets lower the attenuation with distance decreases, so very low frequency (VLF) and extremely low frequency (ELF) ground waves can be used to communicate worldwide. VLF and ELF waves can penetrate significant distances through water and earth, and these frequencies are used for mine communication and military communication with submerged submarines.

At medium wave and shortwave frequencies (MF and HF bands) radio waves can reflect or refract from a layer of charged particles (ions) high in the atmosphere, called the ionosphere. So radio waves transmitted at an angle into the sky can be reflected back to Earth beyond the horizon, at great distances, even transcontinental distances. This is called skywave or "skip" propagation. It is used by amateur radio operators to talk to other countries, for diplomatic communications, and by international shortwave broadcasting stations. Skywave communication is variable, dependent on conditions in the upper atmosphere, and can be disrupted by events like solar flares, it is most reliable at night and in the winter. Due to its changing nature, since the advent of communication satellites in the 1960s many long range communication needs that previously used skywaves now use satellites.

Solar activity has a cycle of approximately 11 years. During this period, sunspot activity rises to a peak and gradually falls again to a low level. 

The current prediction for Sunspot Cycle 24 gave a smoothed sunspot number maximum of about 69 in the late Summer of 2013. The smoothed sunspot number reached 68.9 in August 2013, the official maximum. 

We are currently over 7.5 years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906.

When sunspot activity increases, the reflecting capabilities of the F1 layer surrounding earth enable high frequency short-wave communications. The highest-reflecting layer, the F2 layer, which is approximately 200 miles (320 km) above earth, receives ultraviolet radiation from the sun, causing ionization of the gases within this layer. During the daytime when sunspot activity is at a maximum, the F2 layer can become intensely ionized due to radiation from the sun. When solar activity is sufficiently high, the MUF (Maximum Usable Frequency) rises, hence the ionization density is sufficient to reflect signals well into the 30 – 50 MHz VHF spectrum. Since the MUF progressively increases, F2 reception on lower frequencies can support potential low band amateur radio paths. A rising MUF will initially affect the 27 MHz CB band, and the amateur 28 MHz 10 meter band before reaching 45-55 MHz TV and the 6 Meter amateur band. The F2 MUF generally increases at a slower rate compared to the Es MUF.

Since the height of the F2 layer is some 200 miles (320 km), it follows that single-hop F2 signals will be received at thousands rather than hundreds of miles. A single-hop F2 signal will usually be around 2,000 miles (3,200 km) minimum. A maximum F2 single-hop can reach up to approximately 2,500 miles (4,000 km). Multi-hop F2 propagation has enabled low-band VHF reception to over 11,000 miles (17,700 km).

The correct answer to our question therefore is (C) 4,000 Km (2,500 miles) 

~ John VE7TI