2-meter Band - Long Distance Communications 75 Miles and Beyond

Long Distance Communications 75 Miles and Beyond

While the 2-meter band is best known as a local band using the FM Mode, there are many opportunities for long distance (DX) communications using other modes. The typical 2 meter station using CW (Morse code) or SSB (single side band) modes consists of an exciter (radio) driving a linear amplifier which generates about 200-500 watts of RF power. This power is normally fed to a multi-element horizontally polarized, directional beam antenna knowns as a Yagi as opposed to a vertical omnidirectional antenna. The DX advantage goes to stations that are located in relatively high locations compared to the surrounding terrain with views clear to the horizon. Such stations are able to communicate 100–300 miles consistently and it is not unusual to be heard at distances much further. These distances can be traversed on a daily basis without any noticeable help from known "Signal Enhancements". However, when coupled with these well known signal enhancements, astonishing distances can be bridged.

To traverse these distances, directional Yagi antennas are almost essential and are generally horizontally polarized. These antennas provide huge signal gains over a dipole or simple vertical and make short work of several hundred miles providing reliable communications. Meteor scatter, Sporadic E, and Tropospheric Ducting are the most common forms of VHF signal enhancement and are described further below.

Tropospheric Ducting:

Occasionally, signal bending in the atmosphere's troposphere known as tropospheric ducting can allow 2-meter signals to carry hundreds or even thousands of miles as evidenced by the occasional 2-meter contact between the west coast of the United States and the Hawaiian Islands, the northeast region to the Florida coast and across the Gulf of Mexico. These "Openings" as they are known, are generally first spotted by amateurs operating SSB and CW modes since amateurs using these modes are always alert for ducting or signal enhancement events. Completion of contacts using these weak signal modes involves the exchange of signal level reports and location by grid square which is known as the Maidenhead Locator System. Two way ducting contacts can have very strong signals and are often made with moderate power, small antennas and other types of modes. Long distance ducting contacts do occur using FM modes as well but for the most part go unnoticed by many FM operators.

Sporadic "E":

Another VHF propagation event called, Sporadic E propagation; is a phenomenon involving radio reflections off highly ionized segments of the ionosphere which can bring contacts well over 1,000 miles (1,600 km) with very strong signals received on both ends of the conversation. Unlike some other long distance modes, high power and large antennas are not required to "Work" some distant stations via a sporadic "E" event. Low power, even as low as one watt can sustain a two-way conversation over hundreds of miles or more. Sporadic-E events can last for hours or can last for minutes. Sporadic-E is a rare and completely random propagation phenomenon.

Satellite Communications:

The 2-meter band is also utilized in conjunction with the 70-centimeter band, or the 10 meter band and various microwave bands via orbiting amateur radio satellites. This is known as cross band repeating. On board software defines what mode or band is in use at any particular time and this is determined by amateurs at so-called earth stations who control or instruct the satellite behavior. Amateurs know what mode is in use via published internet schedules. For instance, a favorite mode is Mode "B" or "V/U" which simply indicates the uplink and downlink frequencies or bands the satellite is currently using. In this example, V/U means VHF/UHF or VHF uplink with UHF downlink. Most amateur satellites are Low Earth Orbit satellites, or LEO's as they are affectionately known, and generally are about 450 miles high. At that height amateurs can expect reception distances of up to around 3,000 miles (4,800 km). However, there are a few amateur satellites that have very high elliptical orbits. These satellites can reach altitudes of 30,000 miles above the earth where an entire hemisphere is visible providing outstanding communications capabilities from any two points on the earth within line of sight of the satellite; distances that are far beyond the reach of the LEO's. Satellites are basically orbiting repeaters.

Trans-equatorial Propagation:

Trans-equatorial propagation also known as (TEP) is a regular daytime occurrence on the 2-meter band over the equatorial regions and is common in the temperate latitudes in late spring, early summer and, to a lesser degree, in early winter. For receiving stations located within +/− 10 degrees of the geomagnetic equator, equatorial E-skip can be expected on most days throughout the year, peaking around midday local time.

Meteor Burst:

By speeding up Morse code using an audio tape recorder (this is an obsolete method), or using a computer and digital modes such as JT6M or FSK441, very short high speed bursts of digital data can be bounced off the ionized gas trail of meteor showers. The speed required to confirm a two way contact via a short lived ionized meteor trail can only be performed by fast computers on both ends with very little human interaction. One computer will send a request for contact and if successfully received by a distant station, a reply will be sent by the receiving stations computer usually via the same ionized meteor trail to confirm the contact. If nothing is received after the request, a new request is transmitted. This continues until a reply is received to confirm the contact or until no contact can be made and no new requests are sent. Using this high speed digital mode, a full two way contact, can be completed in one second or less and can only be validated using a computer. Depending on the intensity of the ionized meteor trail, multiple contacts from multiple stations can be made off the same trail until it dissipates and can no longer reflect VHF signals with sufficient strength. This mode is often called burst transmission and can yield communication distances similar to sporadic "E" as described above.

Auroral Communications:

Another phenomenon that produces upper atmosphere ionization suitable for 2-meter DXing are the auroras. Since the ionization persists much longer than meteor trails, voice modulated radio signals can be used (sometimes), but the constant movement of the ionized gas leads to heavy distortion of the signals causing the audio to sound 'ghostly' and whispered. In most instances using auroral reflections on 2-meters, audio or voice is totally unintelligible and ham operators wishing to make contacts via aurora, must resort to CW (Morse code). CW signals returning from an auroral reflection have no distinct sound or tone but simply sound like a swishing or whooshing noise. An exception to this phenomenon would be the 6-meter band which is significantly lower in frequency than the 2-meter band by 94 MHz. In many instances 6-meter voice modes are readable but with varying degrees of difficulty when reflected off an aurora. Therefore, when using an auroral event as a radio signal reflector, the reflected signal strength and signal intelligibility decreases with increasing transmitting frequency.

Moon Bounce (EME):

To communicate over the longest distances hams use moon bounce. VHF signals normally escape the Earth's atmosphere, so using the moon as a target is quite practical. Due to the distance involved and the very high path loss getting a readable signal bounced off the moon involves high power ~ 1000 watts and steerable high gain antennas. Receiving these very weak return signals, again involves the use of high gain antennas (usually the same ones used to transmit the signal) and a very low noise front end RF amplifier and a frequency stable receiver. However, new and recent technological advances in weak signal detection has allowed the successful reception of signals off the moon using much smaller or less well equipped stations allowing reception of signals that are "in the noise" and not audible to the human ear. One of these modes is JT65 which is a digital mode. Due to the delay of the signal traveling to the moon and back (travel time approx. 2.5 seconds), a person transmitting may hear the end of his own transmission returning.