It’s All In The Layers
Someone told me they considered the “magic” of ham radio was the radio signal, but in my opinion the magic is really the radio wave skip propagation that allows world-wide communication without satellites.
You have probably heard another amateur operator mention at one time or another something about propagation being “terrible” or just the opposite, it was “excellent”. What does this all really mean? I want to take a few minutes to revisit radio propagation outside of what can be found in the usual study guides and perhaps pique your interest in researching further on your own.
Depending on the band your using you may not even use any form of “skip” or atmospheric benefits. An example of this would be line of sight or ground waves used by VHF. Typical repeater use will all but bypass what we often think of as radio propagation. It is true that the signals are “propagating” through the air but not necessarily skipping along the atmosphere to the benefit of either sender/receiver. Actual VHF propagation can still occur to our benefit and allow for distant radio communication in the form of tropospheric propagation and ducting. This is when under just the right weather conditions a sandwich of moist hot air is trapped between two layers of cold dry air.
But how can we visualize VHF/UHF propagation or know when it’s happening? There is a tropospheric forecast available online http://www.dxinfocentre.com/tropo_wam.html that visually shows predictions like weather charts show us hot and cold fronts and storms.
A prediction is only as good as the data the science model shows. How can it really be used by a radio operator? By noticing a storm pattern or simply that the conditions are favourable could mean it’s time to experiment.
However to really visualize real time propagation reports on VHF we turn to APRS. Automatic Packet Reporting System is used by a variety of amateur radio operators to report their location and for messaging via packet radio. APRS equipped VHF radios send out beacon packets on a National frequency of 144.39 MHz and these packets include the sender’s GPS location. All APRS receivers are able to read the beacon packets can record the information. Some of the receivers are Internet enabled and catalogue all the beacons to popular websites like https://aprs.fi. This information can then be used to show real time VHF propagation reports because we know both the sender and receiver’s precise location and can calculate exactly the distance between each station.
An example of this is the website http://aprs.mountainlake.k12.mn.us/ which adds visual aides to show the direction and distances between sites that have recently been heard.
Digital modes are far better at being received at long distances but the theory is that if packet radio is working, then voice will see a benefit as well. The contacts displayed that are yellow or red are a great distance for VHF and not necessarily line of site. Without much elevation gain 2-way VHF is limited by the curvature of the earth. Only 6 feet above sea level antennas the distance about 3 miles. Most of the time for VHF use, one or more of the transmitters is well above 6 feet and aides in the transmission beyond the curvature of the earth.
The National VHF calling frequency of 146.520 Mhz FM or 144.200 Mhz SSB can be used during times with good VHF propagation and, if conditions are right, make some very long distant contacts on 2m. The ARRL confirmed in 2015 that the VHF calling frequency could also be used for contests. http://www.arrl.org/news/use-of-146-52-mhz-fm-simplex-frequency-cleared-for-arrl-contests. This means that with the right conditions a contest on VHF could be great fun.
Larry Shaunce WD0AKX in Minnesota produced a video of VHF ducting https://www.youtube.com/watch?v=gBJQ0Ha9ORM
One of the first methods to help measure HF propagation is to listen on WWV. https://www.nist.gov/time-and-frequency-services/nist-radio-stations/wwv. This is a time and frequency checking station located near Fort Collins Colorado. It broadcasts the local time 24/7 up to 10,000 watts of power on 5, 10, 15 Mhz and 2500 watts on 2.5 and 20Mhz. One way of looking at it might be if you can’t hear this station clearly it could indicate issues running QRP or 100 watts on the nearest band. It can also be used simply to test your antenna and rig since it’s a strong signal running 24 hours a day. http://tf.nist.gov/stations/iform.html
Unrelated to propagation, ARRL has a contest each year that contestants try to measure an exact frequency. FMT (Frequency Measuring Test) can also be used to determine propagation shifts. WWV can be used as warm up for those wanting to begin entering the FMT contest world.
Beyond the use of WWV and FMT there are the International Beacon Project . This consists of several key locations around the world each in perfect time synchronization. They rotate their signals and every few seconds a different location can be heard. All you must do is park your radio on the nearest band you wish to operate and listen.
The callsign of the station is sent by CW at 100watts, then long dashes after consisting of gradually less power, 10watts, 1watt, 100milliwatts. You could somewhat predict how well propagation is on the band by listening for key stations in your region. Find it at URL: http://www.ncdxf.org/beacon/
Long term predictors of radio propagation is the use of tracking space weather specifically the sunspot activity. This is an entire subject in of itself that I personally would like to learn more about.
We are at the bottom after an 11 year cycle of “good” solar activity.
Recently I came across an article about the idea of the US Air Force wanting to spread plasma bombs in the sky to improve radio communication. Multiple research teams have been contracted to study this and the current thinking is that some form of small targeted plasma bombs in space could be used to improve radio communication. Perhaps some day we will all be looking at the targeted plasma propagation website?
~ Jeremy VE7TMY
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