High Frequency Communications
General Considerations
In the determination of a suitable HF communications link, the major concern is the signal to noise ratio of the received signal. The transmitting station produces an electromagnetic field radiated by the antenna. The amount of that energy detected by the receiving station depends on many factors, as indicated in this article. In addition, there is always an amount of noise impinging on the receiving antenna that occupies the same frequency spectrum as the transmitting station. Therefore, the noise is a form of interference. The ratio of received signal to received noise determines the relative quality of the communications path. For this article, noise is atmospheric noise or power line noise, not other signals (QRM).
The normal mode of HF communications depends on radiating a signal from an antenna at a particular elevation angle that will result in reflection of the signal from the ionosphere at a distance approximately midway between the transmitting and receiving antennas. For very long range communications, the signal will reflect off the ionosphere, then reflect off land or water, and again reflect off the ionosphere, performing what is commonly known as double hop communications. This concept is graphically illustrated in Figure HF-1. Although the height of the ionosphere varies from about 60 to 240 miles above the earth, the nominal height is 180 miles.
To obtain optimum communications, the ionosphere must be capable of reflecting the signal with minimum loss. The effectiveness of the ionosphere as a reflecting medium is highly dependent on solar radiation. The ionosphere is a region of rarefied air that has been ionized primarily by ultraviolet sunlight. In addition to daily variations, there is also an eleven (11) year variation as shown in Graph-1. The quantification of the effectiveness of the ionosphere to reflect radio waves is based on the number of sunspots visually observed.
During those years when there are a large number of sunspots, communications over long ranges are excellent at higher frequencies and use of lower frequencies tends to result in worse communications then would be expected when the sunspot numbers are low.
Above a critical frequency, which is a function of the effectiveness of the ionosphere, signals that are launched at a high angle, necessary for short range communications, will penetrate the ionosphere rather than be reflected. Due to this phenomena, short range communications must occur at low frequencies, typically less than 8 MHz. Long range communications requires longer path lengths, and associated higher losses. Since the noise levels are lower at the higher frequencies, the signal to noise ratio will be better at the higher frequencies when the ionospheric conditions will support this type of communications. This typically occurs at frequencies above 10 MHz during daylight hours. The reliability of the long range paths increases significantly with higher sunspot numbers.
During periods between sunset and sunrise, the ionosphere will have minimum effect from solar activity, and residual ionization will allow communications only at the lower frequencies. In addition, a sudden ionospheric disturbance may occur due to solar flares. These flares emit x-rays which cause an increase in absorption of the signal as it passes through the D layer of the ionosphere, greatly reducing the received signal level. The effect is very sudden with a gradual recovery to normal conditions. The primary reflecting layer of the ionosphere is known as the F layer, which is above the D layer.
In addition to the effects of the ionosphere, local radio noise is a major factor in communications. Except for manmade noise sources normally associated with centers of population, the major concern is atmospheric noise. This noise is the result of lightning that may occur anywhere in the world and be propagated in the same manner as the desired communications signal.
Mother Nature generates many Megawatts in Her transmitters, a small part in the audio spectrum known as Thunder, some in the visible spectrum we see as lightning, but the majority is propagated as energy in the RF spectrum. In addition to her big transmitters, Mother Nature also makes her own longwire antennas, some are horizontal and some are vertical. (Any time you pass current through a conductor, it will generate a magnetic field. The conductor in this case is ionized air.)
Electromagnetic noise resulting from lightning during local springtime thunder storms represents a worst case. This type of noise increases with decreasing frequency. At frequencies near 30 MHz, the majority of ambient noise is due to galactic noise, i.e. noise from stars that produce radio noise. Graph-2 is presented to indicate the general shape of the amplitude versus frequency curves for ambient noise.
The actual noise curve applicable to a particular location can be determined by measurement, using accurately calibrated equipment. The noise level is defined as the magnitude of the local noise when compared to the noise that would exist in a quite area. That is to say, where the only source of noise is that radiated by the temperature of the ground (thermal noise) in the local area, without atmospheric or galactic noise. (Any object that is warm will radiate noise.) The variation in atmospheric noise typically follows the daily and annual variations depicted in Graph-3.
Originally posted on the AntennaX Online Magazine by Ted Hart, W5QJR
Last Updated : 22nd April 2024