VHF and higher radio communications depend on “direct line of sight” for stations to hear each other. That’s why hams use repeaters in high places to relay their signals around mountains and over much longer distances than their actual signals can travel. Weak signal operators have various modes we can take advantage of to extend our signals far beyond “line of sight”. Sometimes REALLY beyond line of sight.
Yes, THAT moon! Whenever the moon is above the horizon it is in our line of sight. It is so far away that it’s line of sight for about half of the planet at any time. So if a ham in South Africa points his antenna at the moon before his moonset, and a ham in Arizona does the same after moonrise here, they can have a QSO by reflecting a signal off the moon’s surface to one another. OK, it’s more complicated than that, alot more, but it is possible! That is a contact that’s 480,000 miles long on average! Talk about DX!
Tropospheric scatter aka Tropo
This is a more “down to earth” mode. There is enough curvature to the earth that even from Tucson to Phoenix, there is no line of sight. So we rely on weather patterns in the troposphere to propagate our signals beyond what we can usually expect. Sometimes up to 300 to 400 miles. These patterns vary during the day and by season and frequency. A temperature inversion is a type of weather pattern that acts like a duct to carry our signals over mountains for many miles. Microwave stations can use rain scatter to enhance communication distances.
This is another method of communication where we use the ionized trails of meteors flashing overhead to reflect our signals up to 1200 miles or so. These ionized patches of atmosphere only last a few seconds or sometimes just a fraction of a second. Many hams use digital software to send high speed data signals off the meteor trails to a receiving station. Six meters is the easiest band to use and very modest stations can make many contacts there with this mode. Two meters is the second most popular band for meteor scatter. The bands above 144 MHz are progressively harder to use for meteor scatter and thus require more extensive antennas and power output.
If there aren’t any meteors handy, and the moon has set, then we can always use air scatter. The larger and higher the plane is the better. A 747 cargo jet can reflect your signal to a distant station up to 400 to 600 miles away if it is high enough and about halfway between the stations. Contact can be made on SSB, CW, or digital modes. There is a free software package, AircraftScatterSharp by W3SZ which is incredible. It plots the aircraft in the desired “sweet spot” between the stations and alerts you when the plane is in position to make a contact. It WORKS!! Flight Radar 24 is another free app that you can use on your phone to reliably predict airplane signal enhancement.
Recently AMSAT, the group that coordinates amateur radio satellites worldwide, designated the latest hamsat OSCAR-100. Hard to believe we hams have had 100 satellites in orbit over the years. Think of a satellite as a fast moving repeater in the sky. You can use some of them with only a dualband HT. Others need a more elaborate station using computerized antenna tracking and SSB or CW. Others just downlink pictures or data about the spacecraft’s systems. China sent one into orbit around the moon that hams have decoded pictures from. AMSAT members usually have a table at most of the hamfests where they do live demonstrations of satellite communications with handheld gear and simple antennas.
Sporadic E or Es
This is another propagation mode like tropo, but where the reflective layer of the atmosphere is much higher up. This allows signals on 6 and 2 meters as well as occasionally 222 MHz signals to be reflected back down to earth a thousand or more miles away. These layers are large in area and occur in patches, especially during the summer. If these patches line up favorably then our signals can “bounce” off of successive patches to travel even 2500 miles! This is known as multihop. So much for “line of sight”. This is very rare on 144 MHz and really gets your heart thumping when it happens. There is a website called DX MAPS that does a good job of helping alert us to these openings.
This is a mode that we use for worldwide communication during the sunspot cycle peak years on 6 meters only. The band gets crazy with signals from Europe to Australia during the peak years. You don’t need much to work many countries on SSB during the good times. It does not happen very often.. but when it does – LOOK OUT!
FT-8. The “new” digital mode
FT8 is one of the WSJT suite of modes where you can work dx stations when conditions are poor. Many do not know that FT8 was written to be used on VHF, especially situations when signals come in strong quickly then fade out minutes later. You also don’t need a “superstation” to have fun and work lots of DX. This software can copy weak signals buried in the noise so far that the human ear can’t hear them! You don’t get to chat with the other station so it is impersonal at best, but when you have a small station you just can’t beat it. Even large stations use this mode during contests to increase their scores. More on contesting later.
There are other modes as well, but the above ones give you a brief idea of the kinds of things we “weak signal guys” use on a daily basis. It’s fun to get our stations working to the best performance possible. There is considerable “tinkering” involved to keep things going and you find yourself always thinking of ways to make it work better. It could be as simple as raising your antenna a bit more, upgrading your feedline to something better, or adding an amplifier to put out a more powerful signal. Putting a receive amplifier in line to boost the signal strength of other stations is another great upgrade.