2-meter band

From Wikipedia, the free encyclopedia - View original article

 
  (Redirected from 2 meters)
Jump to: navigation, search

The 2-meter amateur radio band is a portion of the VHF radio spectrum, comprising frequencies stretching from 144.000 MHz to 148.000 MHz in International Telecommunication Union region (ITU) Regions 2 (North and South America plus Hawaii) and 3 (Asia and Oceania)[1] and from 144.000 MHz to 146.000 MHz in ITU Region 1 (Europe, Africa, and Russia).[2][3][4] The license privileges of amateur radio operators include the use of frequencies within this band for telecommunication, usually conducted locally within a range of about 100 miles (160 km).

Contents

History

Amateur radio

Operating on the 2-meter band

A 2-meter, handheld transceiver.

Because it is local and reliable, and because the licensing requirements to transmit on the 2-meter band are easy to meet in many parts of the world,[5] this band is one of the most popular non-HF ham bands. This popularity, the compact size of needed radios and antennas, and this band's ability to provide easy reliable local communications also means that it is also the most used band for local emergency communications efforts, such as providing communications between Red Cross shelters and local authorities.[6] In the US, that role in emergency communications is furthered by the fact that most amateur-radio operators have a 2-meter handheld transceiver (HT), handie-talkie or walkie-talkie.

Repeaters and the use of FM on the 2-meter band

Much of 2-meter FM operation uses a radio repeater, a radio receiver and transmitter that instantly retransmits a received signal over a longer distance and usually on the same antenna at the same time. Most repeaters are normally located in high locations such as skyscrapers or mountain tops overlooking great expanses of territory. On VHF frequencies such as 2-meters, height greatly influences how far one can talk. Typical and reliable repeater range is about 20–30 miles. Some repeaters in unusually high locations can be reliable out to 50–75 miles. Reliable range is very dependent on the height of the repeater antenna and also on the height and/or location of the handheld unit or mobile unit attempting to gain access to the repeater. Line of sight would be the ultimate in reliability. The typical two meter FM operator has about 50 watts of transmit power, and has a simple vertical antenna mounted on their rooftop. This type of station setup normally provides good signals into his or her favorite local repeater.

However, even without repeaters available, the 2-meter band provides reliable crosstown communications throughout smaller towns, making it ideal for emergency communications. Antennas for repeater work are almost always vertically polarized since 2-meter antennas on cars are usually vertically polarized allowing for maximum signal coupling which equates to stronger signals in both directions. Simple radios for FM repeater operation have become plentiful and inexpensive in recent years.

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). Mode "B" or "V/U" 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. On some amateur radio satellites, amateurs using these modes can expect reception distances of up to around 3,000 miles (4,800 km). Most amateur satellites are Low Earth Orbit satellites, or LEO's as they are affectionately known, and generally are about 450 miles high. 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 far beyond the reach of the LEO's from any two points on the earth that have line of sight of the satellite at the same time. Or what is commonly referred to in satellite techno speak, the satellite "Footprint". Satellites are basically orbiting repeaters.

A set of two long Yagi antennas for the 2-meter band fed in phase to obtain more gain and narrow main lobe of radiation (WA6PY)

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.

Los Angeles County Statute

Los Angeles County has a statute (which dates from 1944) concerning mounting a "shortwave receiver" in a motor vehicle. While the statute specifically states one of the forbidden bands as 150 to 160 MHz, most two meter transceivers can tune into this portion of the spectrum at least as receivers, and are therefore unlawful to mount in a motor vehicle in Los Angeles County. While arrest rarely happens, the statute is still on the books. There are also California Penal Code statutes covering similar activities. Recently however with new legislation in various states licensed ham radio operators are exempt from these prohibitions including exemptions from using a radio while driving. These prohibitions and/or exemptions vary from state to state.

Note: Federal Law preempts many Local Ordinances and State Laws which may prohibit a licensed Amateur Radio Operator from possessing an amateur radio based on its factory ability to receive frequencies outside of ham bands. See PR 91-36 Which is also known as FCC 93-410.

References

  1. ^ US Amateur Radio Frequency Allocations. http://www.arrl.org/FandES/field/regulations/allocate.html accessed 12 May 2008.
  2. ^ RSGB (UK) band plan
  3. ^ RSGB (UK) band plan Accessed 14 May 2008
  4. ^ Sharing spectrum with other services a ham radio reality. The ARRL Letter, Vol. 20, No. 2. http://www.arrl.org/arrlletter/01/1102/ Accessed 14 May 2008
  5. ^ http://www.rsgb.org/getlicence/#foundation RSGB licensing guide
  6. ^ http://www.rsgb.org/emergency/ RSGB Radio Emergency & Public Service Communications website

External links