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Theory of operation of Mobile HF NVIS Magnetic Loop Antenna ST-940B

In general ST940B antenna utilizes principle of a Very Small Closed Tuned Loop with aperture dimensions less than 0.1λ compared to wavelength and also referred to as a Magnetic Loop Antenna.

The loop radiator is effectively a single turn inductor continuously tuned by a variable capacitor. The radiation impedance and efficiency of the loop mainly depends on its surface conductivity in order to minimize resistance to quasi constant current, which creates a magnetic flux in the near field and an electromagnetic field in the far field. This type of antenna differs from open antennas (like whips, dipoles, log periodics, etc.) by its impedance, which is reactive and can be compensated by capacitor only. It has low, typically <10mΩ radiating resistance, which drops with frequency and, at the lowest frequency of the range, it can be less than 1.0mΩ. As the radiation efficiency is given by the ratio:

where Rr - radiating resistance, Rt - total resistance of the tuned circuit, it is necessary to minimize the radiating element resistance using a highly conductive conductor and low loss capacitor. If these conditions are fulfilled, the loop will deliver a high current and will have high selectivity with quality factor (Q-factor) typically 10 to 20 times higher that Q-factor of the traditional antennas. The RF currents and voltages present in such a loop must also be multiplied by Q-factor and, if fed by 100 watt radioset, can reach 30-50 Amperes and 5000-7000 Volts. The resulting intensity of magnetic field radiated by the loop in the near filed is thus nearly equal to 10000-20000 Watts. This requires involving of very high voltage low loss variable vacuum capacitor driven by precision stepper motor as tuning to resonance of high Q circuit is very sharp.

Once the mechanical issues are overcome, the narrow bandwidth of the tuned circuit turns to be advantageous feature as it helps to minimize unwanted harmonics in the emitted signal and, what is most important, to significantly increase a signal/noise ratio of the antenna when working on reception.

The tuned loop antenna, due to its low impedance closed circuit is almost insensitive to static and industrial noise. In fact, compared with any other antenna type, loops provide excellent and comfortable reception when reception with other antennas is noisy or completely impossible.

The ST940B Antenna System is a version of a Magnetic Loop Antenna (MLA) known as an Electro Magnetic Ground Plane Loop (EMGL) and sometimes called half loop antenna, because part of its loop is electrically included into a large ground plane. Simplified diagram of a full loop shown on the Figure 4.

Practically, the half loop is nearly the half size of a full loop that makes installation of such antenna possible on a small vehicle. Full loop equivalence is achieved by connecting the half loop ends with a highly conductive ground plane as shown on Fig. 5. The ground plane helps to form a special radiation pattern diagram in order to optimize overall antenna performance.

The full loop shown is excited by an inductively coupled smaller loop. The half loop on Fig. 5 is shunt fed by a wire connected to the loop at a specific point (so called γ-match - gamma match) that provides better and uniform impedance matching between radioset output and antenna input across frequency range as well as minimizes constructive difficulties. The modellization utilizing wire method of moments allows for precise definition of currents and voltages in the circuit and obtaining of required radiation pattern diagrams. The Figure 6 illustrates 3-D model of actual ST940B antenna built with aid of EZNEC 4 CAD calculation engine.

HF radio waves propagate in two paths simultaneously:
- ground wave
- sky wave

Ground wave

The ground wave travels near the ground for short distances, typically up to 70 km over land and 300 km over sea. The distance covered depends upon the operating frequency, transmission power and type of terrain. The lower soil conductivity, the less distance the ground wave will cover. Lossy soil, mountainous terrain or forests significantly reduce range of the ground wave communications.

Sky wave

The sky wave is the most important form of HF propagation. The radio wave is transmitted toward the sky and is reflected by the ionosphere layers to a distant receiver on earth. The reflective properties of the ionosphere layers change throughout the day, from season to season and yearly.

The extent to which a radio wave is reflected depends on the frequency that is used. If the frequency is too low the signal is absorbed by the ionosphere. If the frequency is too high the signal passes straight through the ionosphere. Within the HF band, low frequencies are generally considered to be in the range of 2 to 10 MHz. High frequencies are above 10 MHz.

A frequency chosen for daytime transmission may not necessarily be suitable for night time use. During the day the layers of the ionosphere are thick and absorb lower frequencies and reflect higher frequencies. At night, the ionosphere becomes very thin. The low frequencies that were absorbed during the day are reflected and the high frequencies that were reflected during the day pass straight through to the space. The height of the reflective layers is varies during the day that causes change of reflection angle and therefore distance at which the reflected wave is returned to earth is also varies. Sometimes sky wave can be reflected to the area where ground wave is also present. In this case the sky wave will travel longer distance and may arrive to earth in different phase resulting in interference effect known as fading, e.g. consequent lowering and increasing of the resulting signal strength.

Summer HF communications usually operate on higher frequencies than those used in winter over the same distance. Solar activity varies over an 11 year cycle. Higher frequencies need to be used during periods of peak activity.

It is important to remember that you may need to change the frequency you are using to achieve the best communication. The general rules for HF communications are:

• the higher the sun, the higher the frequency
• the longer the distance, the higher the frequency

The Chart 1 on the page 4-7 illustrates typical mobile 5 m tuned vertical whip performance at distances up to 1500 km versus used frequency. The ground wave emitted by such antenna propagates in accordance with the principle highlighted in the chapter 4.3 and is getting inaudible at approximately 70-80 km distance. The sky wave emitted by the whip is reflected back to the earth at approximately 300 km. Due to low, typical 30º take off angle of radiation maxima in the elevation plane of this antenna there are no waves that strike the ionosphere at angles required to reflect them at distances below 300 km. The skip zone is the region consisting of areas of the earth's surface which are outside the radius the ground wave will reach, and yet not far enough away to receive reflections of sky waves. Therefore, the gap in coverage between 80 and 300km will occur. This effect is also known as silent or dead zone. Within typical distances of 80 to 300 km reliable HF communications is impossible without use of special antenna types.

To eliminate the skip zone the transmitted sky waves must strike the ionosphere at high, typically 60-90? angles to be returned back to earth at similar angles in order to fill 0-300 km distance of skip zone. It is like taking hose and spray water into a ceiling. At high angles of hose water will cover spot under it. Once the angle is lowered and jet strikes the ceiling at shallow angle, the water will fall quite far away leaving dry floor. The Nearly Vertical Incidence Skywave (NVIS) antennas have been developed for fixed HF radio stations such as dipoles installed at lowered, typically 0.25λ height, Inverted Vees or dipoles with additional grounded reflector wire installed beneath it. The main purpose of those modifications is to maximize the upward radiation towards the vertical (or zenith) and minimize low-angle sky and ground waves.

However, the NVIS turns to be a difficult task for mobile applications. Due to lack of space there are no NVIS antennas were possible for mobile HF communications. The bent whip, frequently called NVIS whip cannot change situation effectively as its radiation effectiveness very much depends on the soil conductivity while usually lossy soil is a bad reflector. The half loop and conductive reflector combination is the only real NVIS antenna available for mobile HF communications.

NVIS is the mode of operation that is most suitable for small to medium range of communications, where all radiosets in the network are located within 1000-1500 km radius. This is the range that can be covered by a vehicle and is well within a typical range for any mobile communications requirement. This is a local range. Despite the same equipment is sufficient for intercontinental or even worldwide communications, those are more exclusions than practice. Once your network is considered local it must provide reliable communications between all fixed and mobile stations registered in your local network.

To achieve maximum benefits from NVIS propagation it is important that all radiosets in the network, either mobile or fixed, are equipped with NVIS antennas. Falling to do with so will result in poor communications where two way conversation becomes one way, which is little better than no way, as it would be with all non-NVIS radiosets in the network.

General rules for proper frequency planning for sky wave operation as highlighted in chapter 4.4 are valid for the NVIS mode. However, when NVIS propagation becomes a target mode, a better study of ionosphere properties is essential.

The ionosphere is a high altitude region of the earth's atmosphere composed of gaseous atoms which have broken into ions. The sun is the source of the ionizing energy, so the condition of the ionosphere varies with time of the day, season of the year, the 11-year sunspot cycle, and the 27-day rotation of the sun. Ionosphere continuously fluctuates in height and thickness. The layers of the atmosphere that effect radio propagation are the D, E, F1 and F2 layers (refer Chart 1, 2). In a nutshell, it is the F2 layer which is usually involved in reflecting sky wave back to earth, while the D layer absorbs signals. The E-layer can either help, or hinder. It is established that all frequencies which can be reflected by the F2 layer lie within 2-14 MHz range. It must be noted that higher frequencies of the range emitted and reflected at regular shallow angle sky wave frequently tend to penetrate the F2 layer if were transmitted at higher angles. Therefore, NVIS mode requires more accurate real time knowledge of ionosphere condition when choosing frequency. The critical frequency foF2 is the frequency up to which a return can be obtained from a sky wave directed vertically at the F2 layer of the ionosphere is the most important parameter to know. A good ?working? frequency for NVIS will often be between 10 - 15% below, i.e. 85% of the foF2.

A very rough guide is to take the higher frequencies (say 7-12 MHz) for daytime communications, with the lower (say 2-4 MHz) for nighttime use. In practice, to maintain NVIS communications over a 24 hours period, effectively 3 different frequencies are required;

• a day frequency (the highest of the 3)
• a night frequency (lowest of the 3)
• a transition frequency, somewhere between the other two.

A more accurate method is to follow weekly propagation bulletins or use propagation-prediction programs available (Miniprop, for example) on the Internet. There is access to various propagation information sites, which either provide real-time indications or detailed recent history records of the critical frequency presented in form of ionospheric maps at no cost.

Those map help to determine the frequencies that will always be returned to the earth. Transmitted frequencies higher than the indicated contours (which are given in MHz) may penetrate the ionosphere, resulting in lost power to space. Frequencies lower than the indicated contours will never penetrate the ionosphere. Lower foF2 values indicate a weaker ionosphere and correspond to regions with lower Maximum Usable Frequencies (MUF). Higher foF2 values indicate a stronger ionosphere and correspond to regions with higher MUFs. Below are some useful links to websites that offer regularly updated maps online:
http://www.wdc.rl.ac.uk/ionosondes/view_latest.html
http://www.ips.gov.au/HF_Systems/4/3
http://www.hfpack.com/

NVIS can be viewed more as a "Systems Concept" and not just what antenna to use. The concept of NVIS is to have reliable communications anywhere within a 1000 km radius through use of special antenna systems in conjunction with such techniques as frequency planning and network management. A great help in achieving of benefits from NVIS concept in recent times is ALE - Automatic Link Establishment.

In the commercial and military world, the problems of changing propagation conditions, plus the fact that skilled radio operators are getting lesser in number, led to the development of ALE.

ALE scans and tests authorized frequencies or channels for a particular path until it finds a frequency that will support communications over a path. Each radioset in an ALE network constantly broadcasts a sounding signal and listens for other sounding signals generated by other network members. A quality analysis of these signals by an on-board processor determines the best frequency for communications and this frequency is then selected automatically for operations. This has dramatically increased the efficiency of HF Communications and is obviously highly useful for NVIS mode.

Many NVIS antennas are designed to work only within NVIS frequency ranges, which usually limited by 2.0-14.0 MHz range, while most HF radiosets can cover full HF band (1.6 ? 30.0 MHz). For correct functioning of NVIS in conjunction with ALE featured radiosets, all channels or frequencies programmed in your radioset must correspond with these limits.

The Chart 2 illustrates overall performance of the ST940B NVIS antenna system. Both the charts are drawn upon results derived from field tests of two ST940B NVIS antennas in comparison with two 5m autotune whip antennas being installed at the same vehicles. The comparison is self-explanatory. Tests were held on the move trough desert dunes, humid forests, under rain, at day and night, measuring S/N ratio in highly industrialized towns, under high voltage lines.

Based on those data, it was established that two 100 W radiosets equipped with two ST-940B half loops having -11 to +5dBi typical gain figure from 3 to 12 MHz would insure reliable voice and data communications at any distance from 0 to 1000 km at least.

This mission cannot be fulfilled by any 3 to 10 m vertical or bent whip antenna on a poor or medium soil even in association with a 1kW radio set as it does not transmit and receive enough energy to cover the typical 70-300 km skip zone.

There is standard procedure for communicating over HF radio. Before you begin transmitting, listen to the channel that you are going to use and ensure that there is no voice or data communication taking place. You may need to wait until the channel is clear or select another channel. Even the channel is seemed to be clear, it is always good to ask several times: Is the frequency in use? When you first establish communication with another station, it is customary to state their call sign and then your own using the phonetic alphabet.

Always maintain polite and friendly style of conversations. Swearing or foul language should not be used ? heavy penalties can apply. Keep communication as short as possible.