Grounding conductors

Event Details

 Grounding ConductorsRF grounding: In the simplest view, radio frequency grounding provides a direct path to ground potential for electrical currents of radio frequency. So, any AC electrical currents with a frequency of alternation in the range of radio frequencies should run to ground potential via this path and not hang around on your station equipment to cause problems.ou’re probably wondering at this point, “From where do such stray and troublesome RF currents arise?” Any conductor attached to your station that is more than about 1/10 of a wavelength for your transmitting frequency will act as an antenna and pick up RF radiation, converting it into alternating electrical current of similar frequency. So, your antenna feedline, your equipment power cords, that computer interface cable, even the metal chassis or enclosures of your station components can pick up RF and create undesirable stray currents on your station. The safety ground on your electrical cable that connects to the safety ground of your shack’s power outlet will usually serve as an excellent undesired antenna for RF rather than a good RF ground. That safety ground will keep your 60 hertz power source grounded, but it was never designed to avoid picking up strong RF emissions from your transmitter! A separate and well-designed RF ground is needed in addition to the electrical safety ground to avoid stray currents on your gear.Now you should ask, “What’s so bad about having a few stray currents running around?” Experimenting with this can become a self-critiquing exercise! With strong currents you may get a sharp shock from your microphone as some of those currents find a path to lower potential through your body. Currents flowing among the various pieces of equipment in your station are called ground loops, and they can interfere with your radio electronics causing unpredictable operation of some equipment (like computer interfaces!), inject noise into your receiver (usually a constant hum), potentially cause audio distortion or even result in erroneous SWR readings. Ground loops are evil and you should take action to prevent them with a good RF grounding arrangement.So, you now question, “What constitutes a good RF grounding arrangement?” Several factors

  • Keep all wires and connections as short as possible to reduce RF effects
  • Connect the chassis (metal enclosure) of each piece of equipment to a common ground panel or bus with solid bonds; this keeps all equipment grounds at a common potential, avoiding current flow between them
  • Do not daisy chain separate grounds from equipment; each should be separately connected to the common panel or bus with a low impedance conductor
  • Connect the common panel or bus to an earth grounded rod or pipe using a very low impedance conductor such as wide copper strap
  • Keep the ground strap to earth ground as short as possible – if it approaches ¼ wavelength of your transmitting wavelength for any frequency it may resonate with the RF and create a hazard for RF burns if the conductor is touched (For the 10m band ¼ wavelength is only about 8 feet!)
  • If a relatively long ground conductor is necessary for your situation, try using more than one length of conductor to avoid resonances and RF hot spots on the ground conductor
  • In other challenging conditions try a piece of metal screen or wide foil/flashing placed underneath equipment and bonded to the common ground panel or bus. This can reduce RF hot spots with high RF voltages and minimize the chance of RF burn or shock from them

In many shacks it is difficult or impossible to comply perfectly with the ground connection length recommendation for all frequencies, especially the 10m band as noted above. Just do some simple calculations for wavelengths and be aware of the possibility of RF hot spots in your grounding system under some conditions.Finally, notice the highlighted recommendation to use wide copper strap for the RF grounding conductor. RF currents tend to flow on the surface of conductors. This is called the skin effect. The impedance of the conductor is reduced as the surface area of the conductor is increased. Thus, a wide flat strap will have lower impedance for RF currents than a relatively small round wire. Low impedance means the currents will more readily flow to the ground potential to which the conductor is attached.So, to very effectively rid your station of those pesky stray RF currents, provide them a short, low impedance, and direct route to ground potential with a wide, flat conductor, such as copper strap. 

Tower Grounding

Event Details

 Tower GroundingA typical lightning bolt may carry current of between 20,000 and 40,000 amps, half a billion to 500 billion joules of energy, and an electric potential of millions to hundreds of millions of volts. According to various sources a single strike could (with no claim of accuracy or testing of any of these):

  • Toast over 100,000 slices of bread
  • Light up 150,000 light bulbs
  • Power a home for a week
  • Electrocute a quarter million people
  • Power 3 million HT radio contacts, each one minute long

Calling upon the water analogy of electricity, a direct strike of lightning is sort of like having all of the current of Niagara Falls splashing down through a one inch diameter pipe under unimaginably extreme pressure. When it exits that pipe it is going to run and drive along every possible path, seeking low resistance routes to ground level voltage. It is prudent to not be in its path!So, any way you slice it – toast, bulbs, HT contacts, or waterfalls – that’s a lot of juice to handle. A radio tower needs to give all that energy a convenient path to ground in the case that Mother Nature comes to call with a friendly bolt or two. That’s what this question is getting at.With a direct lightning strike there is almost no way to avoid some damage to structure, cable, electronics, and almost anything nearby that is a conductor. Metals, such as option B’s ferrite-core RF choke, will melt like butter under extreme heat, and even the best lightning protection tubes will still allow a big spike of current to pass. Lightning is also fond of jumping from conductor to conductor, arcing through the air 4 feet or more, especially if some resistance is encountered in its path. So, you want to provide a short, direct, very low resistance path to ground that the bulk of the strike energy can travel along. A connection to water pipes, as suggested by option D, is sometimes OK for RF grounding of your station, but for lightning mitigation that approach could potentially conduct the energy from the tower right into nearby manmade structures. We need something better.While option A’s single four-foot ground rod is a nice thought, it is completely insufficient for Electric Niagara. As a minimum for a tower, separate eight-foot long ground rods for each tower leg are needed, bonded to the tower and to each other as described in option C. These deep ground rods will help to dissipate the lightning energy into the ground. Even better is an array of multiple eight-foot ground rods for each leg, bonded together for good conduction. With one or only a few rods, the earth in the vicinity around the rods can become saturated with charge and offer resistance, encouraging other paths or jumping of the lightning surge. Place ground rods so they have a free radius equivalent to their depth – about 8 feet – or a minimum of 16 feet between any two rods. This will provide sufficient dissipation of the surge without saturating the earth.A direct strike is dangerous and damaging, but most of the energy can be shunted into the earth with a properly designed mitigation system including rods as described here, and short, straight, wide strap conductors bonded among the tower and rods to create a dissipative network for the energy surge to travel away from the tower.Bonus Note: Nearby lightning strikes are less damaging than direct strikes, often creating waves of energy that roll outward from the strike zone through the ground near the surface. Imagine this like the circular ripples in a pool, spreading outward from the strike, weakening as they travel along. A ring of buried rods around the shack, interconnected with buried strap conductors just a few inches beneath the surface, can greatly reduce the effects of such ground waves entering the structure by routing the energy along the ring and into the deeply sunk rods to be dissipated. Such a construct should also be integrated with any rods/straps specific to the station’s tower or antenna. Granted, this may not be practical for many ham shack situations (and ham wallets), but it is very effective protection against Mother Nature’s occasional rages.
 

Electrical Shock Protection

Event Details

 It won’t shock you to realize that electrical safety is no joke in amateur radio. The 120 VAC that most home stations feed into a power supply is sufficient to kill you dead. And many advanced stations operate with 220 VAC. So, it behooves us amateurs to have at least the basics of electrical safety well understood. That’s why this question is in the pool, even though the answer may be obvious to anyone with the most fundamental comprehension of good safety practices with electricity. 

 Option A:  Yes! This is a great safety practice. Two of the wires in a three-wire cord and plug carry the electric current for powering an appliance (usually color coded white and either black or red). The third wire, usually color coded green, provides an electrical ground connection from the chassis of the device for safety. If any internal component of the device should contact the chassis and thereby produce a dangerous shock hazard for anyone coming in contact with the device, the safety ground connection shunts that current to electrical ground to reduce the possibility of electrical shock. 

 Option B:  Yes again! (OK, you see where this is going.) All AC powered “boxes” of your station should be connected to a common safety ground. The use of a single common ground point avoids any variation among the individual component ground level potentials (voltages). If some variation in ground level potential exists among different pieces of equipment, undesired currents can flow between components (“ground loops”) and offer the possibility of electrical shock to the human operator. 

 

Option C:  Oh yea! The old ground-fault interrupter (GFI) circuit is a great way to help avoid electrical shock from AC sources. Also known as the ground-fault circuit interrupter (GFCI), you can identify these electrical outlets by the inclusion of “Test” and “Reset” buttons on the faceplate.

The GFI outlet works by constantly monitoring the level of current flowing out of the outlet circuit and the level of current flowing back into the circuit. In normal circumstances the “outbound” current and “inbound” (return) current should be equivalent. But if an imbalance between the two levels occurs –  such as when a ham radio operator accidentally sticks his tongue to the unprotected 120VAC wire feeding the station power supply while also standing barefoot in a puddle of ale recently spilled on the floor of the shack –  well, you get the idea.

 The imbalance of monitored currents that results due to some of the supplied current flowing through tongue-body-ale-ground instead of back into the GFI outlet causes the GFI circuit to be interrupted, tripping much like an overloaded circuit breaker and terminating the flow of electric current in the circuit. So, short out a GFI circuit anywhere and it will trip the interrupter and possibly save your life. 



RF Exposure

Event Details

 Radio frequency exposure is usually not a significant hazard for most amateur radio stations. However, you should be familiar with the FCC recommended Maximum Permissible Exposure (MPE) limits, know how to evaluate your station for RF exposure limits, and know how to keep exposure levels safe by making adjustments to your station. 

 

What makes excessive RF exposure dangerous? Radio frequency radiation is not ionizing radiation. This means that RF does not strip electrons from atoms leaving electrically charged particles and it does not alter DNA genetic molecules. Rather, RF energy is absorbed by our body’s tissues and the result is heat. Like that leftover chunk of ham in the microwave oven, our body tissues can get warmed up by absorbing RF energy. And some parts of our bodies are more susceptible to this kind of heating than others because of a relatively reduced capacity to efficiently carry away the heat energy. Our eyes are one notable example. Too much RF for too long of a period can result in enough heating that some tissues may be heat damaged.

Our body absorbed some radio frequencies more readily than others. The most easily absorbed frequencies are right smack in the Technician privileges, too – the VHF range, from 30 – 300 MHz. This includes the very popular 2-meter and 6-meter amateur bands. So, the lowest values for recommended MPE are associated with the VHF bands. So, for equal power levels you’ll get a bit warmer due VHF exposure than from HF or UHF exposure. Frequency matters!

Still, for most stations these effects from VHF are not that big of a worry. In fact, your station may radiate up to 50 watts (PEP at the antenna) in the VHF frequencies before an exposure evaluation is required. While you still don’t want to straddle your antenna while it’s transmitting or gaze lovingly at it from mere inches, you are not likely to receive significant tissue warming in typical operating cases such as an elevated exterior antenna, a mag-mount car antenna, or even a cookie-sheet mounting antenna inside your house up on a bookshelf. The 5 watts of your HT is perfectly safe, even at very close range. But, if you’re pumping out more than 50 watts, you are required to evaluate your station’s exposure. Power matters!

The distance of a human from the radiating antenna significantly affects exposure. The intensity of RF radiation falls off as the square of the distance from the radiating element. So, double your distance from the antenna and you have reduced your exposure to ¼ the previous level. Relocating an antenna is one of the most common actions to take to prevent exposure to RF radiation in excess of FCC-supplied limits. Distance matters!

The gain of an antenna can also impact exposure. Reference Chapter 7, Antennas, to learn how directional or “beam” antennas direct most of the radiated energy in one direction, thereby changing the radiation pattern as compared to an antenna that radiates equally in all directions (isometric antenna). When you collect all that RF and push it in one direction the RF intensity in that aimed direction is increased, and this gain in signal strength must be considered in evaluating your station’s situation. Radiation patterns matter!

So, to reduce the RF exposure to humans caused by your station you may want to do any of the following:

  • Relocate your antenna away from people or extend it higher
  • Reduce your average transmitter power, especially in the VHF bands (lower PEP and/or duty cycle)
  • Use frequencies outside of the VHF band
  • Point your directional antenna away from people

I Got My License, Now What?

Credit Bob KØNR

Event Details


 

It seems that when people are studying for their ham radio Technician license exam, they understandably get very focused on learning the material and passing the FCC exam. Suddenly, the Volunteer Examiner tells them “you passed” and the thrill of success bursts forth!

This is sometimes followed by the question: I got my license, now what?

The most general answer to this question is “find something you are interested in doing and do it.” For many new hams, this is easy— they just need to think about what got them interested in ham radio and follow that path. But other folks have this basic idea that they “want to do ham radio” but may not be sure how to actually get started. This article is to give you some ideas on what to do, assuming you have a Tech license and some basic 2m or 70 cm radio equipment.

 If you haven’t already connected up with some local radio hams, give that a try. Having someone to talk to about various ham radio activities can really help. If you have a radio club in the area, be sure to connect up with them and attend a meeting.  

 Here are some ideas for radio activity to help get you started (in no particular order): 

 

Public Service And More

Often people get interested in amateur radio to provide a service to the community. There are many opportunities to get involved in helping out with events such as walkathons, marathons, bike races, etc. Communications support may be provided by a ham radio club or, more likely, the local Amateur Radio Emergency Service (ARES) group.

  Often I hear new hams say they are interested in emergency communications or as the ARRL says When All Else Fails. They’ve heard about or experienced landline and mobile phones getting overloaded during blizzards, hurricanes and wildfires and want to have alternative communications. 

 Being prepared for emergencies boils down to two basic questions: 1) what are the conditions that you are preparing for? 2) who do you want to communicate with? Most likely, you need to be ready for a power outage of some duration, which implies the use of battery backup or a gasoline generator to power your radio equipment. Who you want to communicate with varies from just your immediate family over short distances to being able to contact other hams much further away. Thinking through the answers to these two questions will get you started on creating the desired communication capability. 

 Another way to connect with the local amateur radio community is via VHF/UHF repeaters. These things are the utility mode for communicating locally. 

 Many hams start out with a VHF/UHF handheld transceiver (HT), which gets them on the air quickly. This really is a ham shack in your hand, which is useful for many activities. By itself, the HT has limited range, so many hams are interested in extending its range. One thing you can do is attached an external antenna to the HT to give it greater radio coverage 

 This will increase your simplex range and allow you to hit more distant repeaters. Another thing to consider is establishing a VHF/UHF home base station  which provides more output power to increase coverage. 

 While the majority of VHF operating is using FM, there is a whole ‘nuther world out there in the weak-signal operating modes. We call this “weak signal” since we are often pulling signals out of the noise to make a contact. Signal Sideband (SSB) is the preferred voice mode when signals are weak since FM performs poorly when the signal level drops. You’ll also find quite a bit of Morse Code CW (Continuous Wave) communication used since it is even better than SSB when the signals are weak. 

 To play with SSB, you need an all-mode transceiver that operates on VHF such as the Yaesu FT857D or FT817 D   You’ll also need to get a suitable antenna, one that is horizontally polarized and probably a yagi antenna with gain. 

 The 6m band is known as The Magic Band because it can suddenly come alive with signals bouncing off sporadic-e clouds in the ionosphere. On most days, 6 meters acts like any other VHF band with mostly local propagation. But when the sporadic-e hits (very common in the summer months), you can talk across North America.  When the normal sunspot cycle is strong, we can also get F2 propagation, which allows contacts to be made into Europe, South America and Asia. 

 Another great use of the 2m and 70 cm bands is to contact outer space. The International Space Station (ISS) has a ham radio station on board and most of the astronauts have their amateur radio license 

  The primary use of this station is for contacts with schools as part of NASA education outreach mission. However, the astronauts sometimes decide to make contacts on their own time. It really depends on the interests of the astronaut and a few of them have really gotten into making random ham radio contacts. Also, very often there is a packet radio station transmitting from the ISS such that you can “digipeat” through the station to contact other hams on earth. It is even a fun exercise to see if you can successfully track the ISS and then hear the packet station transmitting. The ISS is in low earth orbit (LEO), so it is usually overhead for only 10 minutes or so, depending on the pass. 

 Another type of space operation is using OSCAR (Orbiting Satellite Carrying  Amateur Radio) satellites, which are basically repeaters in the sky.  These satellites are also in LEO so you repeat through them to contact other hams while you both have the satellite within range. Some of these satellites use FM, so you can work through them using just a dualband (2m/70cm) HT and a small yagi antenna. It does take a bit of study and practice to track the satellites, figure out the right frequency, point the antenna and adjust for doppler shift. But that is what makes it a fun learning experience and radio challenge. 

 The  Summits  on the Air (SOTA) program is a great combination of hiking and portable ham radio operating. The basic idea of SOTA is to operate from a designated list of summits or to work other radio operators when they activate the summits. The designated summits are assigned scoring points based on elevation with scoring systems for both activators (radio operators on a summit) and chasers (radio operators working someone on a summit). 

 A basic VHF SOTA station is a handheld FM transceiver with a ½-wave telescoping antenna. The standard rubber duck on a handheld transceiver (HT) is generally a poor radiator so using a ½-wave antenna is a huge improvement. Just stuff the HT and antenna in a backpack along with the usual hiking essentials and head for the summit.  

 Some new hams are interested in digital communications via amateur radio. This is a great way to blend computer technology and radio communications. 

 

I’ve mostly given examples of VHF/UHF operating, but a Technician license does give you some useful operating privileges on the High Frequency (HF) bands. In particular, Techs have voice privileges on 10 meters (28.3 to 28.5 MHz). When the sunspots are active, 10m is an awesome worldwide DX band. You literally can talk around the world. To do this, you’ll need a transceiver capable of SSB on the 10m band and a suitable antenna. The antenna does not have to be exotic — a simple dipole or 1/4-wave vertical can do well.

If you get hooked on the fun of HF DX, then you’ll want to start working on your General Class License. But that is a topic for another day.


Credit Bob KØNR

Build A 2M Yagi for Emergencies (and Fun!)

Event Details

  Build a usable Yagi style antenna for the 2 Meter (2M - 144MHz) amateur radio band.  The challenge part is that I wanted to use stuff I had lying around, and I wanted to build it using the fewest possible tools.  Definitely no electric powered tools, so that this could be replicated in an emergency, no-power situation.  It's a PVC boom and uses standard 12GA electrical circuit wire for the elements.  Everything is assembled using wire ties, and cut to length based on measurements - no antenna analyzer. 

How does a J-pole antenna work

How does a J-pole antenna work?

Event Details

How does a J-pole antenna work?

 Find out how a J-pole amateur radio antenna works! 

How To Build A Dipole Antenna

Event Details

HF Propagation & Sunspots

Event Details

 

HF Radio Propagation and Sunspots

- an overview of the basics of the effects that sunspots have on HF radio propagation and radio communications in general including two way radio communications, maritime mobile radio communications, mobile radio, radio broadcasting and amateur radio communications.

 

As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.

HF radio communications of various forms including two way radio communications, maritime mobile radio communications, radio broadcasting, amateur radio communications, and in fact any form of radio communications that uses the HF bands and ionospheric radio propagation is very dependent upon the state of the ionosphere. The higher the levels of radiation received from the Sun, the greater the levels of ionisation in the ionosphere and in general this brings better propagation conditions for HF radio communications.

It is found that the number of sunspots on the Sun has a considerable effect on the levels of radiation emitted and hence impacting on the ionosphere. In turn this has a marked effect on radio communications of all forms. Sunspots are therefore of great interest to anyone involved in HF radio communications, as it affects the radio propagation conditions so significantly.

 

What are sunspots?

If the sun is viewed by projecting its image onto a screen, dark areas can be seen from time to time. These can last anything from a few hours right up to several weeks. These spots are cool areas (relatively speaking) on the surface of the sun. The temperature is around only 3000°C against a sizzling 6000°C for the rest of the surface. It is much hotter under the surface reaching temperatures in excess of a million degrees Celsius.

A word of warning

Under no circumstances should the sun be viewed directly, even though dark glasses. In the past many people have had their sight damaged by doing this.

These sunspots are areas where there is intense magnetic activity. The fields in these areas are enormous and as a result the surface of the sun is disrupted. In these areas the surface cools dramatically causing a darker region to be perceived.

Around the sunspot there is an area called a plage. This is slightly brighter than the surrounding area and it is a large radiator of cosmic rays, ultra-violet light and X-rays. In fact it results in the overall level of radiation coming from the sun to increase. In turn this increased radiation level from around the sunspots causes the ionosphere to become ionised to a greater extent. This means that higher frequencies can be reflected from the ionosphere.

As sunspots appear in groups, especially the larger ones a sunspot number was devised. This is not the number of sunspots that are observed but a number indicating the level of sunspot activity. The number is very closely related to the actual amount radiation received from the Sun. In this way it is a good measure of solar activity. The daily readings are smoothed mathematically to take out the erratic variations to give the Smoothed Sunspot Number. Sometimes the abbreviation SSN is seen, and it is this smoothed sunspot number that it refers to.

Eleven year cycle

The number of sunspots on the surface of the sun varies with time. At times very few or even none may be visible, whereas at other times the number is very much greater. Although the number varies greatly over short periods of time as the sun rotates, careful analysis using the SSN reveals a longer term trend. It is found that over a period of approximately eleven years over which the sunspots vary. At the peak of this cycle conditions on the bands at the top of the short wave spectrum are very good. Low power stations can be heard over remarkably long distances. At the bottom of the cycle bands around 30 MHz will not usually support normal propagation via the ionosphere.

Sunspots have been observed by the Chinese since before the birth of Christ. However it was not until the mid-eighteenth century that astronomers started to keep records of sunspot numbers. By looking at these over the years it is possible to see the trend since then, and the cycles which have occurred since then. Cycle number 22 officially started in September 1986. It started with a sunspot number of 12 and rose rapidly over the following 33 months to reach a peak of 158. From its peak the sunspot number fell slightly and rose again to give a second, smaller peak before falling to bring the cycle to an end in 1996

Summary

The sunspot activity is of great importance to anyone involved in HF radio communications. Whether two way radio communications, maritime mobile communications, general mobile communications, point to point radio links, amateur radio communications, radio broadcasting or whatever form of radio communications. The level of sunspot activity has an enormous effect on the ionosphere and hence on HF radio propagation conditions. Accordingly even a superficial understanding is advantageous.

Solar Flares and Disturbances

Event Details

 

Solar Flares and Disturbances for Radio Propagation

An overview of the basics of solar flares and other disturbances including CMEs and how they affect ionospheric HF radio propagation conditions for two way radio communications, maritime mobile radio communications, point to point radio communications and radio broadcasting.

 

As electromagnetic waves, and in this case, radio signals travel, they interact with objects and the media in which they travel. As they do this the radio signals can be reflected, refracted or diffracted. These interactions cause the radio signals to change direction, and to reach areas which would not be possible if the radio signals travelled in a direct line.

The condition of the Sun has a major impact on ionospheric radio propagation. Accordingly it affects a variety of forms of HF radio communications including two way radio communications, maritime mobile radio communications, general mobile radio communications using the HF bands, point to point radio communications, radio broadcasting and amateur radio communications.

As the Sun provides the radiation that governs the state of the ionosphere and hence HF radio propagation, any flares or other disturbances are of great importance. Under some circumstances these can enhance radio communications and the HF radio propagation conditions. Under other circumstances they can disrupt radio communications on the HF bands, while at the same time providing some radio propagation conditions that can be used at VHF by radio amateurs.

there are a number of types of disturbance that are of particular interest for radio communications. Flares are one of the most obvious. However, apart from solar flares there are other disturbances that occur. One is the coronal mass ejection, and there are also coronal holes.

Solar flares

Solar flares are enormous explosions that occur on the surface of the Sun. They result in the emission of colossal mounts of energy. In addition to this, the larger solar flares also eject large amounts of material mainly in the form of protons.

Flares erupt in just a few minutes with apparently no warning. When they occur the material is heated to millions of degrees Celsius and it leaves the surface of the Sun in a huge arch, returning some time later. The flares normally occur near sunspots, often along the dividing line between them where there are oppositely directed magnetic forces.

It is the magnetic fields appear to be responsible for the solar flares. When the magnetic field between the sunspots becomes twisted and sheared the magnetic field lines may cross and reconnect with enormous explosive energy. When this occurs an eruption of gases takes place through the solar surface, and it extends several tens of thousands of miles out from the surface of the Sun and follow the magnetic lines of force to form a solar flare. The gases from within the sun start to rise and the area becomes heated even more and this causes the level of visible radiation and other forms of radiation to increase.

 

During the first stages of the solar flare, high velocity protons are ejected. These travel at around a third the speed of light. Then, about five minutes into the solar flare, lower energy particles follow. This material follows the arc of the magnetic lines of force and returns to the Sun, although some material is ejected into outer space especially during the larger flares.

Effect of solar flares:   For most solar flares, the main effect felt on Earth is an increase in the level of solar radiation. This radiation covers the whole electromagnetic spectrum and elements such as the ultra-violet, X-rays and the like will affect the levels of ionisation in the ionosphere and hence it has an effect on radio communications via the ionosphere. Often an enhancement in ionospheric HF propagation is noticed as the higher layers of the ionosphere have increased levels of iononisation. However if the levels of ionisation in the lower elvels start to rise then this can result in higher levels of attenuation of the radio communications signals and poor conditions may be experienced. Additionally an increase in the level of background noise at VHF can also be detected easily.

Flares generally only last for about an hour, after which the surface of the Sun returns to normal although some Post Flare Loops remain for some time afterwards. The flares affect radio propagation and radio communications on Earth and the effects may be noticed for some time afterwards.

Solar Flare Classifications:   Flares are classified by their intensity at X-ray wavelengths, i.e. wavelengths between 1 - 8 Angstroms. The X-Ray intensity from the Sun is continually monitored by the National Oceanic and Atmospheric Administration (NOAA) using detectors on some of its satellites. Using this data it is possible to classify the flares. The largest flares are termed X-Class flares. M-Class flares are smaller, having a tenth the X-Ray intensity of the X-Class ones. C-Class flares then have a tenth the intensity of the M-Class ones.

It is found that the occurrence of these flares correlate well with the sunspot cycle, increasing in number towards the peak of the sunspot cycle.

CMEs

Coronal mass ejections, CMEs, are another form of disturbance that can affect radio communications. Although much greater than flares in many respects, CMEs were not discovered until spacecraft could observe the Sun from space. The reason for this is that Coronal Mass Ejections, CMEs can only be viewed by looking at the corona of the Sun, and until the space age this could only be achieved during an eclipse. As eclipses occur very infrequently and only last for a few minutes. Using a space craft the corona could be seen when viewing through a coronagraph, a specialised telescope with what is termed an occulting disk enabling it to cut out the main area of the Sun and only view the corona. This enabled the corona to be viewed.

Although ground based coronagraphs are available, they are only able to view the very bright innermost area of the corona. Space based ones are able to gain a very much better view of the corona extending out to very large distances from the Sun and in this way see far more of the activity in this region, and hence view CMEs.

Coronal Mass Ejections, CMEs are huge bubbles of gas that are threaded with magnetic field lines, and the bubbles are ejected over the space of several hours. For many years it was thought that solar flares were responsible for ejecting the masses of particles that gave rise to the auroral disturbances that are experienced on earth. Now it is understood that CMEs are the primary cause.

It is now understood that CMEs disrupt the steady flow of the solar wind producing a large increase in the flow. This may result in large disturbances that might strike the Earth if they leave the Sun in the direction of the Earth.

Coronal Mass Ejections, CMEs are often associated with solar flares eruptions but they can also occur on their own. Like solar flares their frequency varies according to the position in the sunspot cycle, peaking around the sunspot maximum, and falling around the minimum. At solar minimum there may be about one each week whilst at the peak two or three may be observed each day. Fortunately they do not all affect the Earth. Material is thrown out from the Sun in one general direction and only if this is on an intersecting trajectory will it affect the Earth.

CMEs can give rise to ionospheric storms. These can provide a short lived enhancement to ionospheric radio propagation conditions but before long this can result in a black out to radio communications via the ionosphere.

Coronal Holes

Coronal holes are another important feature of solar activity. They are regions where the corona appears dark. They were first discovered after X-ray telescopes were first launched into space and being above the Earth's atmosphere they were able to study the structure of the corona across the solar disc. Coronal holes are associated with "open" magnetic field lines and are often although not exclusively found at the Sun's poles. The high-speed solar wind is known to originate from them and this has an impact on ionospheric radio propagation conditions and hence on all HF radio communications.

Summary

Solar disturbances are responsible for many of the major changes in the ionosphere. The effects of both CMEs and solar flares can cause major changes to ionospheric radio propagation, often disrupting them for hours or sometimes days. As a result a knowledge of when they are happening, and their size can help in predicting what ionospheric radio conditions may be like.