The Practicalities of VHF and UHF

Overview

The VHF and UHF bands are generally the first bands that radio amateurs encounter, owing to their popularity, their reliability for local communications, and their versatility. Generally speaking, entry-level amateur radio licensees start out with very wide privileges on VHF and UHF and relatively limited HF privileges. Equipment for the VHF and UHF bands is also more affordable, further increasing its popularity.

Since propagation on upper VHF and UHF is generally line-of-sight, the bands are simple and reliable for local communications. Repeaters are often used to extend this range by taking advantage of mountains, antenna towers, and Internet or RF links to other repeaters. VHF and UHF bands are also wide enough for higher-fidelity, higher-bandwidth modes like narrowband FM (with a deviation of either 2.5 or 5 kHz) and ≥1200 baud packet (which includes AX.25, FX.25, Winlink, and APRS). Several types of operation are common on VHF and UHF, including FM simplex, repeater, satellite, terrestrial weak-signal, and EME.

Due to the limited reach of VHF and UHF bands, band plans and frequency allocations often vary substantially between different places. It is thus critical for VHF/UHF operators to learn the band plans that apply to their region.

VHF

Frequencies from 30-300 MHz are considered VHF (Very High Frequency), a range which includes broadcast FM, a sizable chunk of the commercial land mobile spectrum, and the 6 meter, 2 meter, and 1.25 meter amateur radio bands. Propagation on VHF is usually by line-of-sight, though the lower VHF bands also experience sporadic E propagation via the ionosphere. Sporadic E propagation is more likely to occur on lower-frequency bands, and is more common in the summer.

By far, the most popular VHF band is 2 meters, owing to network effects and inexpensive equipment, and most of this band is dedicated to repeaters. The most common mode on these channels is FM with a deviation of ±5 kHz. When repeaters are used, their transmit and receive frequencies are separated by ±0.6 MHz. In North America, 146.520 MHz is used as a simplex calling frequency and 144.390 MHz is used for APRS.

UHF

Frequencies from 300 to 3 000 MHz are considered UHF (Ultra High Frequency). Due to the extremely large width of this band, it hosts many radio services. The most commonly-used ISM bands are in the UHF range, named for the industrial, scientific, and medical equipment that used the bands when they were first established to provide a place for equipment that used RF in its operation but not for communications purposes (like microwave ovens) but now used in addition by electronic equipment for short-range communications. Most cellular networks use UHF bands, as do many commercial land-mobile radio systems. Many of the upper UHF amateur radio bands are close to ISM bands, and equipment that uses the ISM bands can often be modified to operate using amateur radio bands and regulations. This is used in AREDN mesh networks, among other places.

In most of the United States, amateurs have access to the 440 MHz (70 cm), 902 MHz (33 cm), 1.2 GHz (23 cm), 2.3 GHz, and 2.4 GHz bands. For UHF and above, amateur radio bands are by convention denoted using their frequencies rather than their wavelengths; one may thus say "let's talk on 440" when they mean the band with a wavelength of 73 cm and a range of 420 to 450 MHz in most of the US. Indeed, 440 MHz is the most popular UHF band; repeaters are common, equipment is cheap, and its shorter wavelength makes it more compatible with the narrow features of dense urban buildings, making it better in those environments.^{[citation needed]} The 70cm band allocations vary wildly around the world. For instance, Canada allocates 430-450 MHz, the US allocates 420-450 MHz, and Mexico allocates 430-440 MHz.

In the US and Canada, the portion from 440 MHz to 450 MHz is often used for repeater and simplex communications. The US/Canada UHF calling frequency is 446 MHz, around which most simplex activity is centered. Repeaters operate above and below this; their outputs are lower in the band and their inputs are almost always 5 MHz above their outputs.

Simplex operation

"Simplex" describes FM or digital voice operation on a single frequency without a repeater. Procedures are very similar to SSB operation on HF. Instead of activity occurring in a range of frequencies, calling frequencies are instead used because of the width of the VHF and UHF bands. To initiate a contact, call CQ as normal; when someone responds, either QSY (change frequency) from the calling frequency to another simplex channel (to free up the calling frequency) or continue the conversation.

Simplex operation is relatively uncommon because of the use of repeaters on VHF/UHF, but it is often used during contests, as a backup for when a repeater is down, and while driving. During contests, activity occurs throughout the simplex portion of many bands. When a repeater is down, emergency plans will sometimes dictate that a repeater output frequency be used on simplex as a backup, with no offset or tone. When driving in North America, especially when traveling long distances, simplex is a common method of communication between cars on the freeway. Often, drivers with VHF equipment in their car will monitor 146.52 MHz; sometimes, they will use bumper stickers or the presence of amateur radio license plates or antennas to telegraph this.^{[citation needed]}

A special type of simplex operation is possible with APRS, called Voice Alert. With Voice Alert, packets are transmitted with a special CTCSS tone of 100 Hz (for North America). This indicates that the person transmitting the beacon is monitoring the APRS frequency for voice messages transmitted using the same CTCSS tone, and allows for people to listen for attended stations by setting their radio to mute transmissions without the tone. To initiate a simplex contact with a station using Voice Alert, use the APRS channel with the right CTCSS tone to request that they QSY (change frequency) to an open simplex channel. For instance, if KK6YRB hears a packet on 144.39 MHz with a tone of 100 Hz and sees that it is coming from KN6LID, they would transmit "KN6LID, QSY to 146.415; this is KK6YRB", also on 144.39 MHz and with a 100 Hz tone. It is critically important that voice transmissions be kept brief, since they are tying up the entire APRS channel.

FM repeater operation

What makes VHF and UHF bands so powerful for local and regional communications is the usage of repeaters. Repeaters simultaneously receive and retransmit weak or low-coverage signals from an input and retransmit them on one or more strong or high-coverage outputs. To do this effectively, they are often located in high places (where local topography and geography permit), have high-gain antennas, transmit with high power, and are located in the middle of a population center.

FM repeaters almost always use duplex operation on frequency pairs that are standardized within a particular area. Every repeater output frequency almost always has a corresponding input frequency located at a standard offset above or below the output frequency. To prevent the repeater from being activated by noise, and to allow for a greater degree of frequency-sharing, FM repeaters will almost always use either CTCSS (also known as tone squelch) or DCS. To use a repeater, your transceiver must be set to the correct CTCSS or DCS code.

Networks

Many repeaters are part of repeater networks, where several repeaters function together in one of several ways. Sometimes, this can just involve multiple receiver sites linked to one transmitter site; a device selects receive site with the strongest signal for transmission. Other times, repeaters can be part of wide-area networks that use RF links, hubs, and many repeaters; these can sometimes span entire states. An example of the latter type, covering the US state of California, is the CARLA network.

Operating procedures for repeater networks are necessarily different from those of single repeaters. The first principle is that repeater networks should not be used when single repeaters will reliably substitute, since using a repeater network will often tie up the whole network and make it unavailable to every other user. (Likewise, don't use a repeater when simplex will work reliably!) Because of this, it is especially important to allow emergency traffic to interrupt and to yield the network to them (and provide any other assistance) when requested. Since repeater networks often use RF or Internet links that take time to bring up, wait between pressing the push-to-talk switch and speaking. Otherwise, your speech will be cut off. Repeater networks will often have their own specific operating rules and procedures, and you must be aware of them (as well as the setup of the network more broadly) before operating.

Repeaters on some networks will indicate whether transmissions are being received locally or via the network, and can sometimes also be taken off the network temporarily to avoid tying up the network with local communications.

Linking

Several systems have been developed to use the Internet to allow repeaters to be linked together and to allow amateur radio operators to connect to repeaters using the Internet. While some repeater networks are built using Internet links, it is vastly more common for repeaters to be linked temporarily by individual hams. Internet links also allow amateur radio operators to have conversations or join nets on distant repeaters and to stay in contact with old communities while on vacation, in college, or after having moved.

There are three major Internet linking systems: IRLP, Echolink, and AllStarLink.

Inbound connections from an Internet node (either an amateur radio operator on their computer or a repeater) to a repeater are usually no more complicated than just connecting. However, certain repeaters will not accept inbound connections at particular times or from nodes not approved by the owners or control operators of the repeater.

There is also no standard etiquette or procedure for outbound connections from a repeater to a node. Some repeaters will allow users to bring up and tear down links with DTMF commands; on others this is restricted to club members or control operators only.

Autopatch

Autopatch is a feature of some repeaters that allows one to place PSTN (public switched telephone network) calls through a repeater by using DTMF tones to dial a special code and the telephone number to be called. Once the call is initiated, the repeater will send audio from its input frequency to the called party and the output frequency; when it is not receiving anything on its input, it will transmit audio from the called party on the output frequency. Another code can then be sent to hang up the call. On autopatch, the rules of amateur radio and the limitations of half-duplex communication still apply.

By and large, cellphones have rendered autopatch obsolete, and so it is no longer set up on many repeaters. Regardless, autopatch can be useful in a limited number of situations.

Weak-signal

Terrestrial

HF modes, such as SSB, CW, and FT8, are often used for weak-signal communications on VHF and UHF. Most VHF/UHF transceivers are FM-only, so the use of special all-mode VHF/UHF transceivers, HF/VHF/UHF transceivers, or transverters are required to operate using these modes. Unlike FM simplex or repeater operation, weak-signal antennas are horizontally polarized by convention.