Tuesday, June 12, 2007

Propagation and EMCOMM

An EM advanced studies training module - by D. W. Thorne, K6SOJ

A few years ago I was involved in a Search and Rescue operation in extremely rugged country in the far NW corner of California. The primary search area consisted of two very deep and steep canyons that are separated by a 2,000 ft. ridge. Before the search was over about a dozen SAR units from as far away as 300 miles were called in to assist. There was no cell phone coverage and only one Sheriff's Department repeater was accessible. The IC (who was from an adjacent county) said the local Sheriff wanted to keep their SO frequency clear of the SAR traffic and asked that it be used (by SAR) for emergency traffic only.Around 2200 local time, one other emcomm volunteer (a trainee) and myself arrived at the SAR CP/base camp positioned in a deep canyon and we were asked immediately to establish contact with the Sheriff's Office the IC's home county. My first thought was about setting up a NVIS* antenna, and establishing an HF link either on 40 or 75 meters with one of several HF stations that had been previously alerted and were monitoring some previously designated frequencies and that could relay traffic to and from the Sheriff's Dispatch Center via telephone.
I knew there was a VHF amateur repeater located on a mountain top about 20 miles to the north...in another state! I thought, why not give it try? I switched to the repeater frequency, keyed the mic, heard the identifier, and then identified myself. Immediately, I heard a familiar voice reply...it was my wife! She was at our home station over 125 miles away, but by using out tower mounted 13 element Yagi she had solid contact with the repeater. Needless to say, the IC, who was watching, was very impressed!The search went on for about a week before finally being called off. The missing person (or his remains) were never found. Most of the searchers were non-hams, so all tactical communications were on VHF public service simplex frequencies (NASAR, CLEMARS, etc.). By stationing a radio relay team (the young trainee and myself) on the ridge that separated the two deep canyons, effective communications were maintained. Every message between the two canyons was through our relay.
A portable repeater may have worked, but there are very few (if any) used by public service agencies and there are very few "spare" public service "frequency pairs" available for portable/field operations. Plus mutual aid responders may not be able to program the radios to an "new" pair. Frequencies such as NASAR, CLEMARS, NALEMARS and other SOA (scene of action) simplex frequencies should be in all SAR transceivers.(NOTE: Typical amateur radio gear is not FCC "type accepted" transmitting on PS channels. Listen only. Hams who are active in SAR, fire, EMS, or other public service, should consider buying commercial radio gear that can be legally operated on both public safety and amateur services.)Most local amateur emcomm (and nearly all public service communications) are handled on VHF, UHF, or higher frequencies. Which are line-of-sight whether direct or via a repeater (if available).One of the great advantages that we as radio amateur have is that we have a wider range of frequencies and modes option that just about anyone! With all the new emcomm volunteers now entering the world of HF, it is advantageous to know some basic and practical aspects of HF radio propagation.40 and 80 meters are the "Workhorse Bands" for Regional Emcomm:
While most local or tactical emcomm can easily be handled on VHF or UHF frequencies, most regional traffic (50-300 or more miles) is handled on the 40 or 75-80 meter bands. (The 160 meter band and the 60 meter band should not be ruled out, but by and large the 40 and 80 meter bands are the workhorse bands most used for emcomm networks.

I am not a physicist are these comments an attempt to explain and define all the intricacies and nuances of HF radio signal propagation. There are many excellent books available that can adequately explain that area of science that is wrought with multiples and rarely understood variables. As one ham friend of mine says, "It's all Voodoo!"

The SEA and the SUNMost of us who have studied the basics of radio know that the earth is surrounded by layers of ionized particles. The ionosphere is in a constant state of flux. It is affected primarily by the sun, and it varies immensely with the time of day, the time of year, the solar cycle, geomagnetic storms, and other factors. The ocean tides on the earth are influenced by the gravitational pull of the sun and the moon and to a small degree, the other planets, and is compounded by the winds. The ionosphere (envision a canopy above the earth), is ever expanding, contracting, fluctuating in the amount of ionization level, and possibly other factors that scientists may not have even discovered.

The D layer (closest to the earth) is only a factor present during the daylight hours and is responsible for the absorbing most MW and HF radio signals. This is why MW BCB signals do not propagate (over any great distance) during daylight hours. Then there is "sporadic E", which some liken to clouds of ions which come and go with the seasons often only lasting a few minutes or hours. Radio hams who enjoy the six meter band (50-54 MHz) love it when the "E layer comes to life!" The most commonly relied upon layer for HF radio is the highest...the F layer. To further confuse the issue, the F layer divides into two levels during the daytime. F1 and F-2. One or the other will refract (bounce a signal back to earth) from a point of refraction depending upon: 1) the frequency; and 2) the angle at which a given signal hits that refraction point.Most of us knowledgeable hams who want to be able to maximize their ability to communicate by bouncing radio signals off the ionosphere, have learned by experience what works and what doesn't work. Often by much trial and error. (This is what is known as experience!) They have learned and also realize that what works today, may not work tomorrow, but it may work again the day-after-tomorrow. Even at the same time and on the same frequency! In fact...what works now, may not work five minutes from now!

Most of us have played pool or billiards. The object in those games is to bounce (or ricochet') a ball off of the opposite bumper. The more direct, or acute the angle that a ball hits the bumper, the closer it will return to it starting point. (E.g. - the side pocket near to you.) If you "glance the ball" off the bumper at an obtuse angle, it will "land" farther way from the starting point. (Hopefully, in the corner pocket.)

Radio signals behave in much the same way. Where they go, depends (in part) at what angle they are directed towards the ionosphere. NVIS (near vertical) go up, and down, land closer to the transmitting station, and may not interfere with distant stations. Low angle (aimed at the horizon) will land a long, long way away, but may not be heard by who you want to talk to.

Now, if the ionosphere was a straight edged surface like the bumper of a billiard table, it would be easier to calculate just where a signal might "bounce to" or land. (This is actually done using solid passive reflectors on mountains for micro wave communications.) But the ionosphere is curved and it consistently varies in thickness. Imagine that you are playing pool on a circular table! Imagine also the cushion is constantly changing in its softness. Now imagine that the table is constantly changing it's circumference. (Like the iris of the human eye or a camera.) That would make for a very challenging game of pool!

The ionosphere is constantly changing in all of these physical characteristics. Therefore, so does the refraction point (distance above the earth) vary for any given frequency. And...just as in billiards...the angle at which a signal "hits" that refraction point will determine how far it will "skip" or return to earth. To further complicate tings, the layer varies in thickness and intensity. If it didn't, the radio signals would be very specific as to where they land. When propagation is marginal, signal paths may actually be very selective. When band conditions are is optimal, signals on many frequencies may propagate well and be received over a wide footprint. This is often called signal scatter.

A few generalities to keep in mind:

1. 40 meters usually provides a better signal path during daylight hours for communications in the 100 to 800 mile range.2. 75-80 meters is usually better during daylight hours for communications in the 30-200 mile range.3. During daylight hours, when the MUF* is below 7 MHz, or when the 40 meter band "goes long", 75 meters may work.
4. 75-80 meters is usually better during nighttime hours. (40 meters tends to "go long" at night.)5. On 160, 80/75, and 40 meters, lower (30 ft. or less) horizontal antennas (NVIS**) are usually better for closer ranges.
6. Normally, the higher any antenna is (above ground) the lower the angle of radiation. (Good for DX...but not as good for NVIS.)
7. A vertical antenna has low angle of radiation, and probably will not get your signal "up and out" of a deep canyon or over another obstruction.

*MUF = Maximum Useable Frequency
** NVIS = Near Vertical Incident Signal http://www.emcomm.org/antenna/nvis.htm

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