High frequency (HF) radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters (ten to one hundred metres). Frequencies immediately below HF are denoted Medium-frequency (MF), and the next higher frequencies are known as Very high frequency (VHF). Shortwave (2.310 - 25.820 MHz) overlaps and is slightly lower than HF.

[edit] Propagation characteristics

Since the ionosphere often refracts HF radio waves quite well (a phenomenon known as skywave propagation), this range is extensively used for medium and long range radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:

• Sunlight/darkness at site of transmission and reception
• Transmitter/receiver proximity to terminator
• Season
• Sunspot cycle
• Solar activity
• Polar aurora

These and other factors contribute, at each point in time for a given communication path, to a

• Maximum usable frequency (MUF)
• Lowest usable high frequency (LUF) and a
• Frequency of optimum transmission (FOT)

[edit] Exploitation of, and limits imposed by, these characteristics

When all factors are at their optimum, worldwide communication is possible on HF. At many other times it is possible to make contact across and between continents or oceans. At worst, when a band is 'dead', no communication beyond the limited groundwave paths is possible no matter what powers, antennas or other technologies are brought to bear. When a transcontinental or worldwide path is open on a particular frequency, digital, SSB and CW communication is possible using surprisingly low transmission powers, often of the order of tens of watts, provided suitable antennas are in use at both ends and that there is little or no man-made or natural interference.^[1] On such an open band, interference originating over a wide area affects many potential users. These issues are significant to military, safety^[2] and amateur radio users of the HF bands.

[edit] Uses

A modern Icom M700Pro two-way radio for marine HF radio communications.

The high frequency band is very popular with amateur radio operators, who can take advantage of direct, long-distance (often inter-continental) communications and the "thrill factor" resulting from making contacts in variable conditions. International shortwave broadcasting utilizes this set of frequencies, as well as a seemingly declining number of "utility" users (marine, aviation, military, and diplomatic interests), who have, in recent years, been swayed over to less volatile means of communication (for example, via satellites), but may maintain HF stations after switch-over for back-up purposes. However, the development of Automatic Link Establishment technology based on MIL-STD-188-141A and MIL-STD-188-141B for automated connectivity and frequency selection, along with the high costs of satellite usage, have led to a renaissance in HF usage among these communities. The development of higher speed modems such as those conforming to MIL-STD-188-110B which support data rates up to 9600 bit/s has also increased the usability of HF for data communications. Other standards development such as STANAG 5066 provides for error free data communications through the use of ARQ protocols.

CB radios operate in the higher portion of the range (around 27 MHz), as do some studio-to-transmitter (STL) radio links. Some modes of communication, such as continuous wave morse code transmissions (especially by amateur radio operators) and single sideband voice transmissions are more common in the HF range than on other frequencies, because of their bandwidth-conserving nature, but broadband modes, such as TV transmissions, are generally prohibited by HF's relatively small chunk of electromagnetic spectrum space.

Noise, especially man-made interference from electronic devices, tends to have a great effect on the HF bands. In recent years, concerns have risen among certain users of the HF spectrum over "broadband over power lines" (BPL) Internet access, which is believed to have an almost destructive effect on HF communications. This is due to the frequencies on which BPL operates (typically corresponding with the HF band) and the tendency for the BPL "signal" to leak from power lines. Some BPL providers have installed "notch filters" to block out certain portions of the spectrum (namely the amateur radio bands), but a great amount of controversy over the deployment of this access method remains.

Some radio frequency identification (RFID) tags utilize HF. These tags are commonly known as HFID's or HighFID's (High Frequency Identification).

[edit] See also

• Low frequency
• High Frequency Internet Protocol
• AM radio
• High Frequency Active Auroral Research Program
• Radio propagation
• Space weather

[edit] References

1. ^ Paul Harden (2005). "Solar Activity & HF Propagation". QRP Amateur Radio Club International. http://www.qrparci.org/content/view/58/118/. Retrieved 2009-02-22. 
2. ^ "Amateur Radio Emergency Communication". American Radio Relay League, Inc.. 2008. http://www.arrl.org/pio/emergen1.html. Retrieved 2009-02-22. 

[edit] Further reading

• Maslin, N.M. "HF Communications - A Systems Approach". ISBN 0-273-02675-5, Taylor & Francis Ltd, 1987
• Johnson, E.E., et al., "Advanced High-Frequency Radio Communications". ISBN 0-89006-815-1, Artech House, 1997
• V. Narayanamurti, et al., "Selective Transmission of High-Frequency Phonons by a Superlattice: The "Dielectric" Phonon Filter". Phys. Rev. Lett. 43, 2012–2016 (Issue 27 – 31 December 1979).
• Boulos-Paul Bejjani, et al., "Transient Acute Depression Induced by High-Frequency Deep-Brain Stimulation". New England Journal of Medicine, Volume 340:1476-1480 May 13, 1999 Number 19. Massachusetts Medical Society.
• H. C. Liu, "Analytical model of high-frequency resonant tunneling: The first-order ac current response". Phys. Rev. B 43, 12538–12548 (Issue 15 – 15 May 1991).
• Sipila, M., et al., "High-frequency periodic time-domain waveform measurement system". IEEE Transactions on Microwave Theory and Techniques, Volume 36, Issue 10, pg. 1397-1405, Oct 1988. ISSN 0018-9480 INSPEC 3291255 DOI 10.1109/22.6087
• Morched, A., et al., "A high frequency transformer model for the EMTP". IEEE Transactions on Power Delivery, Volume 8, Issue 3, pg. 1615-1626, Jul 1993. ISSN 0885-8977 INSPEC 4581865 DOI 10.1109/61.252688

[edit] External links

• Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
• Douglas C. Smith, High Frequency Measurements Web Page; Index and Technical Tidbits. D. C. Smith Consultants, Los Gatos, CA.
• High Frequency Propagation Models, its.bldrdoc.gov.
• High Frequency Wave Propagation, cscamm.umd.edu.
• "Grounding for Low- and High-Frequency Circuits" (PDF)
• "High frequency noise" (PDF)
• "Advantages of HF Radio" Codan