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Communications EW, part 2 -- HF propagation. (EW101).

This column is intended only to give general understanding of high-frequency (HF] propagation. HF propagation is very complex, depending on time of day, time of year, location, and conditions (such as sunspot activity) that impact the ionosphere. An excellent article by Richard Groller entitled "Single Station Location HF Direction Finding" (JED, June 1990, p. 58) is suggested as a starting point for further study. The next suggestion is a handbook such as Reference Data for Radio Engineers (RDRE), which includes typical curves for HF propagation. Finally, for specific ionospheric conditions, propagation parameters, etc., the US Federal Communication Commission (FCC] has a web site with loads of data (www.fcc.gov).

In this coverage, we will discuss the ionosphere, ionospheric reflection, HF-propagation paths, and single-site locator operation. The primary references for this column are Mr. Groller's article and the RDRE.

HF propagation can be line of sight, ground wave, or sky wave. Where line of sight exists, propagation is predicted by the formulas to be presented in next month's "EW 101" column. Ground wave, which follows the Earth, is a strong function of the quality of the surface along the path. The FCC web site has some curves for this propagation mode. Beyond about 160 kin, HF propagation depends on sky waves reflected from the ionosphere.

The Ionosphere

The ionosphere is a region of ionized gasses from about 50 to 500 km above the Earth's surface. Its primary interest here is that it reflects radio transmissions in the medium- and high-frequency ranges. As shown in Figure l, the ionosphere is divided into several layers:

* The D layer is from about 50 to 90 km above the Earth. It is an absorptive layer, with absorption decreasing with frequency. Its absorption peaks at noon and is minimal after sunset.

* The E layer is from about 90 to 130 km above the Earth. It reflects radio signals for short- and medium-range HF propagation during the daytime. Its intensity is a function of solar radiation and varies with the seasons and sunspot activity.

* Sporadic-E is a condition causing a short-term transient layer of ionization that is present in local summer, primarily in Southeast Asia and the South China Sea. It causes short-term changes in HF propagation.

* The F1 layer extends from about 175 to 250 km above the Earth. It exists only during daytime and is strongest during summer and periods of high sunspot activity. It is most prominent at the middle latitudes.

* The F2 layer extends from about 250 to 400 km above the Earth. It is permanent but extremely variable. It allows long-range and nighttime HF propagation.

Ionospheric Reflection

Reflection from the ionosphere is characterized by virtual height and critical frequency. The virtual height, as shown in Figure 2, is the apparent point of reflection of a signal from an ionspheric layer This is the height measured by sounders, which transmit vertically and measure the round-trip propagation time. As frequency is increased, the virtual height increases until the critical frequency is reached. At this frequency, the transmission passes through the ionospheric layer. If there is a higher layer, the virtual height increases to the higher layer.

The maximum frequency at which reflection can occur is also a function of the elevation angle (0 in Figure 1) and the critical frequency ([F.sub.CR]). The maximum usable frequency (MUF) is determined by the following formula:

MUF = [F.sub.CR] x sec([theta])

HF Propagation Paths

As shown in Figure 3, there can be several different transmission paths between a transmitter and a receiver, depending on ionospheric conditions. If the sky wave passes through one layer, it may be reflected from a higher layer. There can be one or more hops from the E layer, depending on the transmission distance. If the E layer is penetrated, one or more hops from the F layer can occur At night, this would be from the F2 layer, and in the daytime. from the F1 layer Depending on the local density of various layers, there can also be hops from the F layer to the E layer, back to the F layer, and finally to the Earth.

The received power from sky-wave propagation is predicted by the following formula:

[P.sub.R] = [P.sub.T]+[G.sub.T]+[G.sub.R] - ([L.sub.B]+[L.sub.i]+[L.sub.G]+[Y.sub.P] + [L.sub.F])

Where [P.sub.T] is the transmitter power, [G.sub.T] is the transmit-antenna gain, [G.sub.R] is the receiving-antenna gain, LB is the spreading loss, [L.sub.i] is the ionospheric-absorption loss, [L.sub.G] is the ground-reflection loss (for multiple hops), [Y.sub.P] is the miscellaneous loss (focusing, multipath, polarization, etc.), and [L.sub.F] is the fading loss.

Single Site Locators

A single site locator (SSL) determines the location of a HF emitter by measuring the azimuth and elevation of arriving signals. The measured elevation is the angle of reflection from the ionosphere. As shown in Figure 4, the elevation angles at the transmitter and receiver are the same, and the distance from the SSL to the emitter is given by the following formula:

D = 2R([phi]/2 - [B.sub.R] - [sin.sup.-1] [(R cos [B.sub.R]/{R+H})]

Where D is the surface distance from the SSL to the emitter; R is the radius of the Earth; [B.sub.R] is the elevation angle, measured at the receiver; and H is the virtual height of the ionospheric layer from which the signal is reflected.

What's Next

Next month we'll discuss VHF and UHF propagation. For your comments and suggestions, Dave Adamy can be reached at dave@lynxpub.com.
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Title Annotation:electronic warfare; high frequency
Author:Adamy, Dave
Publication:Journal of Electronic Defense
Geographic Code:1USA
Date:Jul 1, 2003
Words:944
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