Mobile HF Loop Antennas - Part 3
Radio Wave Propagation Radio waves are named differently depending on how they propagate away from a transmitting antenna, as follows:
Groundwaves Groundwaves travel near the ground. In the HF spectrum groundwaves generally are useful for communicating only over relatively short distances ranging up to about 70 km over land or 300 km over sea, because of rapid near-earth signal attenuation. Actual groundwave communication distances vary widely and depend on the operating frequency, transmitting power, transmitting and receiving antenna characteristics, types of terrain, types of soil, presence of salt water, receiver characteristics, and the strengths of interfering signals and noise. The lower the soil conductivity, the less distance over which groundwave signal strengths generally will be useful. Ground surface obstructions such as mountains, hills, large buildings, or densely-forested areas also tend to attenuate groundwave signals and reduce groundwave communication distances.
Skywaves Skywave radiation generally is the most important radiation from an HF transmitting antenna, because most HF communication distances exceed distances where groundwave signals are strong enough to be useful. Radio waves with frequencies below the ionosphere's current MUF (Maximum Usable Frequency) that radiate toward the sky at suitable vertical angles are refracted back down toward distant locations on earth by one or more ionized regions of the ionosphere. However, the refractive properties of those regions change constantly throughout each day, from season to season, and from year to year, causing received signal strengths at distant locations to vary widely at different times. In many cases transmitting frequencies can be changed as ionospheric conditions change to maintain communications with a distant location. However, that can be done only if both the radio equipment and the antennas on each end of the circuit can be retuned easily to new frequencies. One advantage of the small (compared to a wavelength) loop antenna described in this article is that it can be quickly tuned over a wide frequency range by merely changing the capacitance of a single tuning capacitor.
More about Skywave Signal Frequencies The extent to which radio waves are refracted from the ionosphere to a given point on earth depends in part on the frequency being used. If the frequency is too low a radio signal will be absorbed by ionospheric attenuation. If the frequency is too high the signal will pass through the ionosphere and into space without returning to earth. If the frequency is between those two frequencies the signal will return to earth, but not necessarily where someone wants to communicate. Where HF communication is possible between two points on earth at a given time it generally is possible only within one or more narrower bands of frequencies. The limits of the frequency band or bands that can be used change constantly and especially tend to be different between day and night.
The ionized regions of the ionosphere are thick during the daytime due to ionization caused by intense electromagnetic energy and particles radiated from the sun. Thick ionized regions absorb lower frequency signals and refract signals of higher frequency. During nighttime the ionized regions become much thinner because electromagnetic radiation and particles from the sun are shielded from the ionosphere by the earth. Low frequency signals that were absorbed during daytime are refracted at night when the ionization thins and higher frequencies that were refracted and returned to distance points on earth during the day pass through the thinner ionosphere and into space.
The height of ionized regions also varies at different times of day and night and that changes the distances out to areas where signals are returned back to earth at a given frequency. Under some conditions skywave signals return nearby into areas where groundwave signals also can be received (even back to the transmitting site). Because of the different signal path lengths via the ionosphere and over the earth the skywave and groundwave signals may arrive at a receiving site in-phase, out-of-phase, or at any phase angle between those extremes. If they arrive out-of-phase they tend cancel so that the resulting signal is equal only to their strength difference. If they arrive in-phase they add so that the resulting signal is equal to the sum of their strengths. Because the height and thickness of the ionosphere constantly change the skywave signal phase is always changing. As a result, the net sum of the skywave and groundwave signals tends to continually oscillate in strength unpredictably between weaker and stronger.
The Eleven-Year Solar Cycle Ionospheric ionization is not only affected by rotation of the earth, as explained above. It also is affected by the 11-year solar cycle. During periods of greater sunspot activity more radiation and particles arrive from the sun, there is more ionization of the ionosphere, and higher transmitting frequencies generally must be used to communicate between two distant points on earth. The reverse is true during periods of low sunspot activity.
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