HF Systems
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HomeHF SystemsSection InformationHF Communications Problem Page Tuesday, Jul 16 2024 14:53 UT
Section Information

HF Communications Problem Page

This page is intended to assist our customers in determining the nature of their HF communications problem. There can be many reasons why an HF signal may not reach the intended recipient, including solar flares, ionospheric storms, geomagnetic storms, equipment faults and the choice of frequency. Whilst ASWFC cannot determine the state of your HF equipment, we can provide you with an estimate of the state of the HF environment by using the information listed below. It is suggested that environmental factors are checked first then your equipment. When checking your equipment particular attention should be paid to your receiver location (ie a radio frequency quiet location, away from computers and other electrical equipment) and the receive antenna's suitability to the angle of arrival of the radio signal you wish to hear. Ensuring the connections between the antenna and the receiver are impedance matched and in good condition are also important factors in receiving a good HF signal.

Below is a list of icons and their purpose which may be useful to check if you are having problems with HF communication. A red icon indicates that the current environment is disturbed or expected to be disturbed.

If you require historical information on HF conditions please view the ASWFC Weekly Propagation report archive.

For a quick check on HF disturbances you can view the plot of the ASWFC ionospheric T index for the last 30 days.

An ASWFC HF Warning is issued by the ASWFC Duty Forecaster when disturbed ionospheric conditions are expected. For mid-latitudes this usually means lower than normal maximum usable frequencies. The Warning will contain the period when the disturbance is expected. During the warning period lower than normal frequencies may only be usable, and/or the quality of HF communication reduced. If you are listening to a HF broadcast that is available on several frequencies, switching to the next lowest frequency to the one you usually use may be required at times during the warning interval.

During a solar flare, increased ionisation in the D-layer of the ionosphere can result in reduced signal strengths. If this icon is red a shortwave fadeout is in progress. Shortwave fadeouts effect the sunlit ionosphere and are strongest where the Sun is overhead. By clicking on the icon a map is displayed centred on the longitude where the Sun is overhead. If your HF circuit has a reflection point near this sub-solar longitude, it may be disrupted by the solar flare/fadeout. Fadeouts are also visible on ASWFC ionograms via the ASWFC ionogram Viewer for Australian/New Zealand stations as an increase in the lowest vertically reflected frequency (usually around 1 to 2 Mhz) in the ionogram. During a flare the absorption can be strong, resulting in no return of the radio signal from the ionosphere, indicated by a blank ionogram.

An ASWFC HF Fadeout Warning is issued by the ASWFC Duty Forecaster when increased solar flare activity, and shortwave fadeouts are expected. IF the fadeout warning is correct then the HF fadeout icon will be red at times during the duration of the HF Fadeout warning. You can see the area of the Earth effected by the fadeout by clicking on the fadeout icon.

Polar regions can be affected by high energy solar protons from the Sun, usually following a strong solar flare. If your HF circuit has a reflection point close to the polar cap, then it may be disrupted by the polar cap absorption (PCA) event. This icon will turn red when a PCA is in progress.

A K index is a three hourly index of "geomagnetic activity", or variations in the Earth's magnetic field, relative to an assumed quiet day curve for the recording site. K index values range from 0 (quiet) up to 9 (extremely disturbed). The ionospheres response to geomagnetic activity is not always to become depressed. Even if maximum usable frequencies are not reduced by geomagnetic activity, the quality of HF radio communication is likely to be degraded when geomagnetic activity has increased. A K index of 4 or more may indicate degraded HF communications quality. During periods of increased geogmagnetic activity ionograms can show "spread F". "Spread F" is a term used to describe an increase in the thickness (both in height and frequency) of the ionogram trace, and may indicate reduced HF communications quality near this station location.

Current Level of Ionospheric Support for HF communications

The icons below indicate the level of ionospheric support in the Australian region and the world. The images show the difference between the current observed hourly conditions and the predicted monthly conditions. The colours blue, green, yellow, red, correspond to "enhanced", "normal", "midly depressed", and "depressed" conditions. Depressions and enhancements are with respect to the ASWFC predicted monthly T index for the current month. If a large red area is displayed in the image for your region a lower than normal frequency may be required for HF communication whilst depressed ionospheric conditions persist.

Is your choice of frequency correct?

ASWFC provides suggested frequency of operation information in a variety of formats, including the option of producing your own prediction. It is recommended that the frequency that you are trying to use is compared to an ASWFC frequency prediction for that circuit. You can do your own HF prediction at the ASWFC online prediction tools page.

The following explanatory documents are also available:

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