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Disadvantages of VHF Radio Frequencies to Public Safety

Catastrophic events such the 7.0 magnitude earthquake in Haiti highlight how seemingly reliable communication methods can fail. For this reason, emergency management organizations such as FEMA (Federal Emergency Management Agency) rely upon the efforts of amateur radio. But, even amateur radio that transmits on VHF frequencies has disadvantages that can predispose it to fail as well. For a hobbyist, a failed VHF transmission is a learning opportunity; but, for a community in crisis, communication failings can compromise public safety.

  1. Propagation

    • In amateur radio circles, VHF bands are frequently referred to as point-to-point or line-of-sight (LOS) transmission. This means that the receiver or repeater (a receiver that amplifies and retransmits a signal) must be within sight of the transmission or just beyond the horizon. VHF is a popular radio frequency band because it's simply designed and can work at low power. As a reliable emergency form of communication, it does have potential failings. Transmission range is generally a couple of hundred miles; where signal transmission must span a large body of water, the range is beyond VHF capability.

    Vulnerability

    • During catastrophic disasters such as earthquakes and tsunamis, repeaters, the mainstay of VHF communication, are vulnerable. Severe weather, natural disasters and military attacks can render them ineffective. In war-torn regions such as Gaza and Afghanistan where communication with outside humanitarian agencies is essential, the repeater vulnerability directly impacts public safety of those most in need. One option is to utilize HF transmission (high frequency). Lower in frequency energy than VHF, it's more effective for long-distance transmission. Because it transmits through buildings and obstacles, it's a preferred frequency band for submarine use. Additionally, HF frequencies can utilize the ionosphere, the highest layer in Earth's atmosphere, as a refractory medium in sustaining wave propagation. Because of higher energy levels in VHF, waves, rather than bending, shoot through the ionosphere into space.

    Signal Jamming

    • In World War I, German forces effectively used signal "jamming" as a method of interrupting Allied communication. Jamming introduces an interfering signal into the receiver along with the original signal. Explained in terms of "Wave Theory," jamming can occur if two waves, both in-phase, change the amplitude of a frequency wave transmission; a jamming signal is destructive if the interfering signal cancels out the transmitted message. Either way, VHF transmission is easily affected by intentional and unintentional interference, both of which can cause a loss of information in the message.

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