Amateur Radio VHF/UHF Communications - SARCNET

This activity introduces the concept of VHF and UHF communications and provides practice in its use.

Very High Frequency (VHF) defines a segment of the Radio Frequency (RF) spectrum, located between 30MHz and 300MHz (with a wavelength of 10 metres to 1 metre, respectively) and Ultra High Frequency (UHF) defines a segment of the Radio Frequency (RF) spectrum, located between 300MHz and 3000MHz, or 3GHz, (with a wavelength of 1 metre to 100 centimetres, respectively).

To simplify the use of VHF/UHF radios, tuning has been made much easier. Each band is divided into a fixed number of channels. It is then possible to select a channel by simply turning a rotary switch, instead of carefully tuning in a station using a dial knob. Channel numbers can then be assigned for different purposes. It is very important to check with your local Amateur Radio associations to find out which channels may be used. Some channels may be allocated for emergency use, or repeaters, or satellites. Using these channels for a chat will often bring a swift rebuke from the operators monitoring them. Check your local Band Plan and stick to the agreed channels! An example band plan, from the Wireless Institute of Australia, is shown below. Look for calling, simplex and chat frequencies you can use (shown in bold), but also check for any specific exceptions.

144.000 - 144.700 NARROW BAND MODES

To ensure clear, virtually noise-free, voice communications for critical applications, VHF/UHF radios typically use high quality Frequency Modulation (Although Amplitude Modulation is still used for aeronautical radios, for a different reason). Increasingly, VHF/UHF radios are now supporting digital modulation modes, like P25 and DMR. These modes enable many other digital features such as talk groups and data communication.

Traditional, Single Side Band (SSB) modulation is also supported on some VHF/UHF Amateur Radios. This mode can be used to exploit unusual propagation modes such as Sporadic-E, Tropospheric Ducting and Aircraft Assisted (so not all VHF/UHF communications is LOS). SSB is also used for satellite and moon-bounce communications.

The continuous snap, crackle and pop of shortwave radio is usually not present in VHF/UHF radios, due to the use of a squelch circuit. This ensures that only signals, which exceed a minimum, workable, threshold, can be heard in the speaker. A squelch control can be used to the adjust the threshold, which can be useful to eliminate local interference, keeping the speaker quiet when not in use.   

As mentioned earlier VHF/UHF antennas are much smaller than HF antennas. This means that they can also be made with higher gain, such as a colinear antenna, or be made quite directional, like a Yagi antenna.  

The thermal noise present in the RF stages of a VHF/UHF receiver is typically higher than the band noise floor. So connecting an antenna to a VHF/UHF receiver hardly increases the noise level at all. Except perhaps for man-made Radio Frequency Interference (RFI), which is unfortunately present on all frequencies. A low-noise pre-amplifier and extra front-end filtering can therefore be very effective at VHF/UHF.

When using hand-held VHF/UHF radios, always select the lowest power required for the other station to hear you. This will greatly extend the battery life. Always keep the batteries charged. They will last longer and you never know when you might need the radio in an emergency. Always have two radios with you, to test them and in case you need them. The radio comes with a small rubber-ducky antenna. A longer, higher-gain, colinear antenna, fitted as an after-market accessory, will give much better range. You can build your own portable Yagi antenna, for amazing results when working from mountain tops.