Friday, October 30, 2015

5 MHz - Sometimes Everything Comes Together!

Following a couple of weeks receiving WSPR transmissions at the EI5DD QTH, it became apparent that 5MHz had a few little secrets often missed as a result of retiring to bed before Midnight. At around 01:00 - 04:00,  it appears that the band was opening into South America and South Africa. It was time to see if it were possible to have a phone contact.

 All revealed on the map below



 Reception Window of the WSPR Receiver showing DX between 01:40 and 04:28 UTC


I had the surprise of my life on the 29th of October when I heard Z35BY from Macedonia Calling CQ on 5403.5KHz at 00:39 UTC; there was also some CW activity from a LB station. Shortly after the QSO I thought I heard some voices from the States. White noise sometimes plays tricks like that but I called a few times only to be pleasantly surprised by WP3UX from Puerto Rico followed a little later by K1CP and W2YC. I received an Email from KB8VAO informing me that he could hear me in Ohio but his calls did not seem to be getting through. Indeed it it could have been his call as there were odd bursts of voice in the noise but nothing discernible.

Finally, on the 31st of October, following a brief QSO with OY1OF from the Faroe Islands, a CQ call resulted in a call from America well down in the noise. After a couple of tries, the station rose out of the noise and it was KB8VAO at a good 55. The QSO was just long enough to pass information and a report from both sides before everything faded back into the noise again. 

See Logs and Ionogram Below:


Extract from Log 29th - 31st October


Ionogram from around the time of the QSOs on 31st October 2015


The Icom IC-M700
 
No doubt the DX can only get better as the winter progresses, but the first ever DX on the band is always the best. Always listen to the white noise on the band.Sometimes there are minor changes that indicate that things are going to happen. WSPR reception has shown that the band does open into South America and South Africa in the early hours of the morning so it may be worth while just staying up that little bit longer.

Subsequent Log from the 31st - !st of November of October


A couple of nights later and there were definite signs of good conditions on the way. A QSO into LA resulted in a "fluttery" QSO, on 5403.5KHz, with good signal strength. In the background, there was evidence of voice in the noise. Quite a few stations on from America but the best was VE1YX who was coming in with a strength 9+. Another late night!

Prefixes Worked to Date:

C31, EA, EI, G, GB, GD, GI, GM, GW, IZ, K1, K2, LA, LB, M, MM, MW, MX, N1, OK7, OY, OZ, TF, VE1, VE2, W2, W3, WP & Z35

Monday, October 12, 2015

NVIS Operation

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. 

Operating on the 5MHz Band

History

Irish Radio Amateurs first received permission to operate, experimentally on a secondary user basis, on specific spot frequencies located within the 5 MHz Band in October 2008. There were 3 spot frequencies allocated and one beacon frequency. More recently an additional 3 spot frequencies were issued. The modes of operation  allowed are J3E (SSB), G1B (Phased Modulation - PSK and a maximum power level of 23dBW 200 watts.  The spot frequencies are issued on a Secondary User, Non-Interference basis, the Primary users being Military and Aviation. A special application has to be made and this is renewed on an annual basis.

As radio amateurs are secondary users, there is no formal band plan for 5 MHz as each country has a different allocation. Strong debate on this subject is heard among UK operators but they basically do not call the shots as secondary users!

Operating on 5 MHz

Many tranceivers are able to transmit on 5MHz if they have the "Alaska Frequency" faciltiy and it is often just a case of opening or closing a link to gain access to the whole of the 5MHz band. it is essential to to set up the tranceiver correctly to ensure that the transmission only operates within the allocated 3KHz wide spot frequency. Upper side-band is the standard mode of voice operation. One should use a SSB filter no wider than 2.5KHz. The transmission should be set 1.5KHz below the centre of the channel frequency and with a typical voice band pass of 300Hz to 2800HZ the signal will just fit inside the channel allocation. See Fig 1.

Fig. 1. showing set up of transmitter for SSB operation

The table shown in Fig. 2 illustrates the centre frequency of the allocated channel, the appropriate carrier frequency, and the VFO dial frequency required. The dial frequency for the Clansman PRC 320 is also shown. It is probably best to place the dial frequencies into memory and select them as required but do remember to have USB programmed in as well. Note that the examples here are the Irish allocations which are comprised of only 6 channels.

Fig.2. Showing the relationship of carrier and dial frequency to the allocated spot frequencies

If setting up for PSK or other digital keyboarding Mode, the dial frequency should be set up the same as for USB voice and the PSK centre audio frequency should be at 1500Hz exactly - see Fig. 3. Operators should also monitor USB voice to ensure that no interference is caused.

Fig. 3 setting up for PSK31 or other keyboard modes.

CW should be set up in exactly the same manner ensuring that the CW carrier is transmitted at the centre of the channel only. Likewise one should monitor USB voice to avoid interference with other users. 

It should be noted that Amateur operators are Secondary users and should immediately vacate the channel if a Primary user should appear on channel. Irish Radio Amateurs are permitted to communicate with Licenced Radio operators only and do not have the facility or permission to enter into communication with Primary users of the band .

Typical Antennas for 5 MHz

The dipole shown in Fig. 4. is probably the easiest option for 5 MHz. by stringing it at a lower height e.g. a 1/4 wave or less above ground, one can take advantage of NVIS techniques. Placing it at a 1/2 wave or greater will take advantage of a lower angle of radiation and enable DX operation. 

Fig. 4. Standard Dipole for 5 MHz.

Many other antennas such as the Inverted-Vee and better still the G4HOL antenna  >> click here << strung just 200 ft above ground could be used. See the Technical Topics section of the Galway VHF Group Blogspot.

Propagation on 5MHz

Regular checks of the Chilton Ionogram will show the state of the bands during the day. Ideally the FoF2 should be around 5.5 - 5.6 considering that the optimum working Critical frequency should be 10% less. Checking the RAF Volmet on 5450 KHz and the Shannon Volmet on 5505KHz will give a good indication of the state of the 5 MHz band at any given time of day - that is whether more distant into the UK or just local in Ireland or both.

The 5 MHz band is an interesting "in between" band as it has both 80 metre and 40 metre properties. As the 80 metre band begins to fade out so the 5 MHz band becomes more useful until heading towards solar noon where the band slowly fades out in favour of the 40 metre band. As the 40 metre band opens more into Europe during the late afternoon so the 5MHz band begins to favour EI and UK operation being at is best at sunset. As the evening progresses towards midnight, the 5 MHz band begins to open up into Europe and becomes more "DXy". Quite predictable really.

WSPR Mode

Using WSPR (Weak Signal Propagation Reporter) it is possible to establish where the propagation is taking the signal to. In the case of Ireland we do not have the facility to actually transmit in this mode, due to frequency limitations, but it is possible to use the system on receive only. As it is in receive only it can be left running 24 hours a day. Here are some results of recent reception in Fig. 5. The Hidden secrets of 5 MHz  revealed! Note what happens between 01:00 and 04:00 UTC - a rather nice opening to South Africa and South America.


Fig. 5. Screen shots of WSPR Reception on 5 MHz at 01:00 - 04:08 UTC


Regular nets are held at sunrise and sunset around 5398.5 KHz and the RSGB News may be heard transmitted on 5 MHz on Sunday afternoons at 4pm.

To get around the problem of many who may not have applied for the 5 MHz allocation it is possible to hold a cross-band QSO where you, the 5 MHz licencee, transmit on the 5 MHz frequency and the other station transmits on a 40 or 80 metre frequency. In the case of countries with allocations outside of the Irish Allocation it is also possible to transmit on an Irish frequency allocation and receive on the other operators frequency and example - EA7JVZ set up a QSO as follows for the UK/Irish ops:

Transmit Frequency: 5278.5 kHz USB (UK/Ireland)
Receive Frequency:  5428.5 kHz USB (Spanish TX Frequency)

Unfortunately the Spanish 5MHz allocation does not coincide with the UK or Irish system at present.

The future of 5 MHz frequency allocations will be decided at the WARC 2015 conference where a more unilateral allocation on 5 MHz may be possible. Naturally there are a number of Administrations/Primary users who do not want to give up their possession of the 5MHz spectrum.