NVIS is a propagation mode that utilises high angle radiation to send a signal straight upwards to be reflected back to earth via the ionosphere for effective short to medium range communication. The antenna has to be designed to radiate signals vertically, the frequency must be chosen to utilise a frequency below the critical frequency to facilitate reliable omni-directional communications over a radius of 200 miles at most. This mode of operation makes NVIS ideal for National and localised communication during disasters or other emergency situations. Military services have used NVIS for decades to provide short haul communication with other units on the ground. Before entering into NVIS communications; a little theory.
Propagation of a Radio Wave
Line of Sight - where two antennas are in line of sight and visible to each other. This mode is generally used by VHF and UHF stations.
Surface Mode or Ground Wave, is generally used where frequencies below 3.8 MHz are used. These signals follow the curvature of the earth and distance is dependent on ground conductivity. The greater the conductivity, the less the attenuation of the transmitted signal. See Fig.1
Fig.1 Relationship between frequency and distance travelled over earth’s surface
The graph in Fig.1 shows the relationship between frequency and distance travelled using ground wave. This distance would be determined by the conductivity of the path
between two stations - the lower the frequency, the less the resultant
attenuation over a defined path.
Ionospheric Propagation - where the transmitted signal is refracted back
to earth via the ionospheric layers such as E, F, or F1 and F2 Layers.
Fig.2 Illustrates the ground wave distance and the signal propagated back to earth via the Ionosphere
Fig.2 shows a dead or skip zone where no signal will be received from the point where the Ground Wave signal disappears and the signal from the Ionosphere returns to earth.
DX operation dictates the necessity for a signal to be radiated at the
lowest possible angle to gain the greatest distance travelled through
space and back to the earth’s surface shown by Fig.3. Generally a simple
rule of thumb for the dipole antenna is to mount it at a minimum of a
half wave above ground to achieve the lowest angle of radiation - See
Fig 3.
Fig. 3. Radiation pattern of dipole at a half wavelength above ground
Many operators are unable to string the dipole antenna at a half wave or
greater above the ground and therefore experience much higher angles of
radiation and consequent reduction of a distance covered.
Using NVIS Successfully
To achieve a reliable communication path in a circle, with a radius of 250 miles, one has to be able to radiate a signal, from the antenna, at a typical angle of 60 – 90 degrees and thus returning from the ionosphere at a similar angle thereby covering 0 – 250 miles. This will fill in the skip or dead zone Fig. 4. Ground wave has to be minimised to avoid interference with the returning wave. Imagine pointing a hose at the ceiling and observing the way in which the water returns to the ground. If pointed vertically it will fall within a certain area as opposed to pointing it at an angle where it will fall some distance further away.
Fig.4 the effect of radiating a signal at typical angles from 60 – 90 degrees
Obviously there will be a signal received in the immediate ground wave vicinity of the transmitting station.
Choosing the Correct Frequency
It should be noted that the closer the operation is to the equator, the higher the frequency that may be used but, for practical purposes at our latitudes, the bands normally considered would be 160, 80, 60 and 40 metres. A “higher” frequency, such as 40 metres, would be used during the day, a “middle frequency”, such as 60 metres, during afternoon and evening and a “lower frequency, such as 80 metres or even 160 metres, during the night. Frequencies used at given times would be dependent on seasonal variations and period of the sunspot cycle. The Critical frequency is the key to successful NVIS working.
Critical frequency (Fo) is the highest magnitude of frequency above which the radio wave penetrates the ionosphere and below which the radio waves are reflected back to earth from the ionosphere. Its value is not fixed and it depends upon electron density of ionosphere at any point in time.
The Critical Frequency of the F2 Layer is the highest frequency that a radio wave transmitted vertically will be returned to earth and anything above this will be transmitted into space.
The Ionogram is the best tool for determining the state of the Ionosphere at any given time of the day. A transmitted signal is swept across a frequency range and the time taken for it to return to earth determines the height of the layer in question. Fig. 5. shows a basic Ionogram.
Fig.5 The basic Ionogram showing both heights and Critical Frequencies of the Ionosphere at a given time of day
As a wave approaches the reflection point, its group velocity approaches zero and this increases the time-of-flight of the radio signal. Eventually, a frequency is reached that enables the radio wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma frequency of the layer. In the case of the extraordinary wave, the magnetic field has an additional effect, and reflection occurs at a frequency that is higher than the ordinary wave by half the electron gyro-frequency. A bit off topic but will explain why refraction back to earth occurs at a slightly higher frequency than the FoF2.
Fig. 6 Shows an Ionogram taken at 15:20 on the 14th July 2015 from http://www.ukssdc.ac.uk/ionosondes/view_latest.html
The Scale on the Y (vertical) axis is Distance in Km and the X (horizontal) axis is Frequency in MHz. The Ionogram in Fig. 6 shows that the ideal frequency of operation for NVIS would be around 5.625 MHz. The heights of the ionospheric layers are shown in the left hand column. The E-Layer is 100 Km, the F-Layer 200 Km and the F2 Layer 361 Km. The FoF2 (Critical Frequency) is 5.625 MHz. The parameters on the bottom left of the Ionogram, denote the communication distance for a given MUF. For example, over a path distance of 600 Km, the ideal frequency would be 7 MHz. Note. For NVIS operation, the optimum frequency is generally 10% lower than the Critical Frequency FoF2.
Choice of Antenna
As previously mentioned, the ideal requirement of a dipole antenna for DX is to mount it at least a half wavelength above ground. By lowering the antenna, the radiation angle increases until an optimum point is reached where the angle of radiation is almost vertical. When the height of the dipole is raised above a half wavelength the angle of radiation is lowered but by reducing the height of the dipole between a quarter wave length to an eighth wavelength, the angle radiation increases seen inFig 7.
Fig.7 Comparison of NVIS Dipole antenna at 1/8 wavelength above ground against Dipole at ½ wavelength or greater above ground.
NVIS antennas are always horizontal as it is not possible to obtain a radiation angle of 90 degrees or thereabouts from a vertical antenna.
The ideal height of the NVIS antenna is around a quarter wave length above ground although it will work if lowered further but efficiency may be sacrificed although noise levels will be reduced. The placement of a counterpoise underneath the antenna may enhance the efficiency of the antenna if the conductivity of the earth is poor.
The dipole and the Inverted-Vee antenna can provide an excellent radiation pattern for NVIS and short skip conditions. To achieve this characteristic the antenna should be no higher than 0.3 wavelengths above ground. A useful version of such an antenna for 80, 60, and 40 metres would be to have links to connect each LF section - Fig. 8
Fig. 8 Multiband Dipole with Jumpers
With the required height of the dipole being close to the ground, it is easy enough to change the jumpers for the desired band.
The Horizontal Loop Antenna
The G4HOL Loop (see Technical Topics on www.galwayvhfgroup.blogspot.com) with a circumference of 283 ft or in a horizontal square or rectangular configuration strung 20ft above ground will provide excellent NVIS coverage of Ireland and indeed into the UK. This antenna gave excellent results across Ireland where a dipole at full height heard weak watery signals. This antenna will tune on all bands from 80 to 10 metres if mounted at 20 ft it will favour NVIS on 80 and 40 metres. The Radiation pattern of such an antenna is shown in Fig. 9
Fig. 9 NVIS characteristics of the G4HOL Loop antenna strung 20ft above ground
Other antennas that may be of historical interest are the Shirley antenna and the Jamaica antenna which were used during the WWII with impressive results. These antennas were basically two phased dipoles strung 20-30ft above ground. Fig 10 shows the configuration of the Shirley antenna.
Fig. 10 The Shirley NVIS Antenna
Portable Operation is easily achieved due to the fact that the antennas are not strung high above ground and it is possible for a one man operation to erect the antenna. Another choice of antenna could be the use of a slanted 8 metre fibreglass pole with 33 ft of wire wrapped around it and fed at the bottom via an ATU (Fig. 11.) and a set of counterpoise wires 5% longer than the wavelength in use added beneath the antenna. Conor, EI4JN, reported good results and noted an enhancement in signal strength from semi-local stations whilst using this arrangement.
Fig. 11 Portable operation with 33ft of wire wrapped around a 8m fibreglass pole tuned with an ATU
Mobile operation using NVIS antennas during WWII was widely used as its potential for communicating with troops across a local area had been realised.
Many will have seen military vehicles with the antenna pulled diagonally across the main body of the vehicle and using the ground plane of the vehicle body to force the radiation of the antenna skyward. Other systems employ a loop antenna using the framework of the roof rack of the vehicle as a ground plane. Barrett Communications and South Midlands Communications supply roof mounted loop antennas although these antennas are costly and not within the reach of the average radio amateur. This type of antenna is widely used by voluntary aid vehicles Africa. See Fig. 12
Fig. 12 Mobile NVIS Loop antenna supplied by Barrett Communications
Among the many advantages of NVIS :
* NVIS covers the area which is normally in the skip zone, that is, which is normally too far away to receive ground wave signals, but not yet far enough away to receive sky waves reflected from the ionosphere.
* NVIS requires no infrastructure such as repeaters or satellites. Two stations employing NVIS techniques can establish reliable communications without the support of any third party.
* Pure NVIS propagation is relatively free from fading.
* Antennas optimized for NVIS are usually low. Simple dipoles work very well. A good NVIS antenna can be erected easily, in a short amount of time, by a small team (or just one person).
* Low areas and valleys are no problem for NVIS propagation.
* The path to and from the ionosphere is short and direct, resulting in lower path losses due to factors such as absorption by the D layer.
* NVIS techniques can dramatically reduce noise and interference, resulting in an improved signal/noise ratio.
* With its improved signal/noise ratio and low path loss, NVIS works well with low power.
Disadvantages of NVIS operation include:
* For best results, both stations should be optimized for NVIS operation. If one station's antenna emphasizes ground wave propagation, while another's emphasizes NVIS propagation, the results may be poor. Some stations do have antennas which are good for NVIS (such as relatively low dipoles) but many do not.
* NVIS doesn't work on all HF frequencies. Care must be exercised to pick an appropriate frequency, and the frequencies which are best for NVIS are the frequencies where atmospheric noise is a problem, antenna lengths are long, and bandwidths are relatively small for digital transmissions.
* Due to differences between daytime and night time propagation, a minimum of two different frequencies must be used to ensure reliable around-the-clock communications.
On many occasions, the Galway VHF Group has used 80 metres ground wave to achieve coverage in mountainous areas of Connemara or the Burren. Whilst the results have been impressive in a relatively small area using ground wave at lower frequencies, attenuation has been noticed and also interference apparent from further afield. NVIS may eliminate this problem and improve and expand the general coverage locally and reduce distant continental interference.
For National coverage NVIS is the obvious choice linking most areas of the Nation together on the one Net within a 250 mile radius of the Control Station. Some individuals may be inadvertently using NVIS due to the fact that their antennas are not mounted too high above the ground.
The “G4HOL loop antenna” had been used for many years by EI5DD giving in excellent results with the use of just 5 watts over paths across Ireland and also into the UK. The 40 metre IRTS news, from Dublin, was received at Strength 9+ in Galway 99% of the time throughout the year. .
Ongoing experimentation will be carried out over the next year on fixed, portable and mobile operations using NVIS techniques 160, 80, 60 and 40 metres.
