Radio Communications Systems During Crisis Situations (Frequencies)

What are the best frequencies to use?

• This section deals with long distance propagation via the ionosphere and does not apply to local line of sight/point to point communications. Except during an extreme solar storm line of sight comms should work with no issues. We're also talking reliable communications that are available 90% of the time or better.  Other bands may open up during atmospheric events but it varies too much to be considered reliable.
• The trick to choosing a frequency is to know which ones are going to work at certain times.  The times can be divided into day/night/twilight, solar minimum/maximum, all four seasons, and even location as in high latitudes (near the poles) mid latitude, and equatorial latitudes (near the equator). Even where (how far) you are trying to communicate with will dictate what the best frequency to use is.
• A combination of these factors will determine what the Lowest Usable Frequency (LUF) and Maximum Usable Frequency (MUF) will be for a given situation.  It can be quite complicated when taking in all the parameters that go into calculating the LUF/MUF needed to talk to another region on the earth.
• In general radio waves between the LUF and MUF reflect of the ionosphere. Radio waves below the LUF are absorbed by the ionosphere and radio waves above the MUF pass through the ionosphere into space. The trick is to know what the LUF and MUF are. But in a SHTF situation that information may not be available to you.  Even the parameters used to calculate the LUF and MUF won’t be available to you.  So how do we figure out what frequencies will work and what ones won’t?  We don’t want to waste precious battery power transmitting on useless frequencies.
• So how does all this frequency stuff work anyway?  The sun produces ultraviolet (UV) light and that UV light is captured/filtered/absorbed by the upper levels of the atmosphere or ionosphere.  The UV light hits the upper atmosphere and the atoms up there temporarily loose an electron and then recombine with that electron later. The separating and combining of electrons is a continual process as long as the UV light is present.  These free electrons are called ions, hence the name ionosphere. During the day (or more precisely on the sunlit side of the earth) the UV light forms more ions. During the night or on the shadow side of the earth there is no UV light so there are fewer ions. During periods of low solar activity the UV light levels are even lower and the density of free electrons is also low.  During periods of high solar activity the UV levels are quite high and the free electron density is high.  The more ions present in the atmosphere the higher the MUF will be.  Think of the ion density like a net, the more ions the smaller the openings in the net.  The smaller openings keep the higher frequencies (due to their smaller wavelength) from passing though into space and they bounce back to earth.
• As the UV light excites the atmosphere on the sunlit side of the earth the ions created separate into layers.  On the shadow side of the earth the few ions that are there combine into fewer layers.  The upper layers (F1 and F2) during the day make long distance communications possible on the higher HF frequencies. Again the more free ions the higher the MUF will be. During daylight hours the LUF is set by the lowest of the layers called the D layer (remember D for Day) that actually absorbs all the frequencies below the LUF.  So during the day ionosphere propagation of the low frequency waves is impossible, no matter how many watts of power you force into it.  After sunset the D layer quickly (thankfully) dissipates so long distance communications is now possible on the lower frequencies. The F1 and F2 layers combine into a single F layer.
• In the region of the atmosphere where the sunlit and shadow areas meet (twilight/sunset/sunrise on earth) there is a collapsed combination of all the layers and they form a little conduit.  This little conduit traps the radio waves and sends them all along the conduit bouncing them to the earth all along the way.  This little conduit is also called the grayline.  So what happens is anyone passing through the grayline can communicate with someone else that is also in the grayline at another location on earth. So communication is possible during sunrise on one side of the earth to somewhere during sunset on the other side of the earth.  So don’t rule out twilight hours for long distance communications.
• Also remember even after dark when the D layer dissipates, the LUF will be higher towards the west (sunlit areas) that to the east (areas already in the dark). This is because the D layer still exists for the areas in the daylight and will absorb the lower frequencies while the D layer is gone for the areas in the dark.  So when trying to talk to areas still in sunlight a higher band may be needed while talking to areas already in the dark maybe possible on a lower band.
• The peak for this solar cycle is from 2013 to 2016, a good 3 years of higher frequency band openings. Right now in 2011 we are about ¼ of the way to the peak.  The downslope of the solar cycle is usually less steep than the upslope which is a good thing. It means the peak will arrive sooner and linger longer.
• A few caveats I need to mention during solar maximum.  Once the sun really begins to pick up activity there can be solar storms from X-ray Flares or UV Coronal Mass Ejections (CME) that can wipe out all communications for a period of time, usually just a few hours but sometimes days. These storms can be heard as really loud white noise or static on the radio. 10 meters really gets noisy during a CME and you can actually hear the wave waver in intensity as the CME passes over the earth.  Also a large X-ray or CME burst can cause inducted power spikes on really long antennas.  This is what happens to our power grid during a solar storm.  Also these storms can wipe out satellite communications and even damage satellites because they are not protected by the earth’s electromagnetic field.
• So in saying all that we really haven’t discussed what frequencies to use when only to determine higher frequencies work better in the day and lower frequencies work better at night.
• Long Distance Comms (HF):
• During solar minimum 20 meters is about the highest frequency that can be used during the day and usually only with high reliable modes like CW and PSK.  During solar maximum up to 10 meter and even 6 meters can be used during the day and since it takes a little while for the ions to recombine 20 meters can be used for a while after dark. SSB voice can be used easily during solar maximum.
• At night during solar minimum the lower frequencies work well up to 40 meters. During solar maximum all the low frequencies through 20 meters work well even 10 meters in the early evening. Noise may wipe out 160 and 80 meters at times.  Electrical storms (lightning strikes) can cause very loud crashes of static, so be careful if you are wearing head phones and listening to the lower frequencies. It’s quite startling and painful.
• So you can see that the more viable bands are 40 and 20 meters. That’s why they are called workhorse bands. If you could only have two HF bands these would be the ones to have.  And these bands are where I’d invest more in my antennas.
• At a minimum these antennas should be at least ½ wavelength high to get the take-off angle below the critical takeoff angle so it will skip.  They do not need to be higher than 1 wavelength and trying to doing so will only use valuable resources that could be used elsewhere.
• Local Comms (HF):
• Using a Near Vertical Incidence Skywave (NVIS) antenna (covered under the Antennas section)
• I’d use 80 meters at night and 40 meters during the day.  During solar minimum 80 meters is not absorbed as much during the day. During solar max 80 meters is wiped out during the day. 40 meters is open at night most of the time. 
• To sum it all up:
• 80 meters and below work during the night.
• 40 meters and up work during the day. 15 and 10 meters need some solar activity to open up; dead during solar minimum for the most part and dead at night. The current solar peak is slated from 2013 to 2015.
• The more active the sun is (further into the solar max peak) the higher the frequencies we can use. Up to 6 meters is possible.
• Local point to point or line of sight communications are not affected by solar activity except during solar storms.
• Now the final part to this is what specific frequency should I transmit on?  
• Other than listening up and down the band for someone to talk to there are specific frequencies on each band call calling frequencies.  There are calling frequencies for different modes.  A good source can be found here. Once you establish a contact on the calling frequency you should move from that frequency to carry on the conversation.  
• During a crisis situation "networks" or nets are established on the bands to handle emergency traffic.  There are also established nets like the Maritime Mobile Service Network (MMSN)on the 20 meter band at 14.300MHz that run from 1600 to 0200 UTC or on 21.412MHz that run from 2200 to 2400 UTC. The MMSN is for ships that need to check in to give position reports or report emergencies. All kinds of Nets and their times and frequencies can be found here.