A Quick Guide to HF Propagation Using Solar Indices

Understanding the influence of solar indices on HF radio propagation can help radio operators, amateurs, and professionals to find the best frequency to make a reliable conact.

1. What Are Solar Indices?

The solar indices provide insight into how the sun's radiation affects Earth's ionosphere, which, in turn, influences HF radio wave propagation.

The most important solar indices are:

1. SSN - Sunspot Number: Measures sunspots.
2. SFI - Solar Flux Index: Tracks solar radio emissions at a wavelength of 10.7 cm (2800 MHz).
   Table 1 illustrates the correlation between these indices and HF propagation conditions.
3. X-ray Flares: Table 2 categorizes solar flares which are potential for fadeouts of HF communication on Earth.
 

Conditions	BAD		Low		Average		Good		Better	Best
MUF (MHz)	<12	<15	> 21		> 24		> 28		> 50
SSN	0	25	50	75	100	125	150	175	200	250
SFI (sfu)	67	83	102	124	148	172	196	219	240	273

Table 1: The solar indices: SSN, SFI
Reference to the conversion between SSN and SFI🗗.

High SSN and SFI are good for daytime propagation on all HF bands. Best for higher frequency bands (20 meters and up). Provides better long-distance communication opportunities.

 

Solar  Flare  Class🗗	A	B	C	M		X
HF Propagation cond	Best			Average	Poor		BAD
Radio Blackout Scale🗗	R0			R1	R2	R3	R4	R5

Table 2: Solar flares may cause Radio blackout


Higher class of X-ray flares indicate potential for fadeouts of HF communication on Earth, in particular below 20 meters band.

 

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2. What Are Geomagnetic Indices?

The A and K indices refer to geomagnetic activity, characterized by disturbances in Earth’s magnetic field caused by solar wind and other solar phenomena. These disturbances can range from minor fluctuations to major geomagnetic storms.

1. A-Index represents geomagnetic activity, measuring disturbances in Earth's magnetic field caused by solar events like solar flares or coronal mass ejections (CMEs).
 
2. K-Index is another measure of geomagnetic activity, calculated from data recorded every 3 hours.

These indices track geomagnetic fluctuations influenced by solar activity, but they do not directly measure the solar phenomena themselves.

HF Propagation conditions		Best			Average			Poor			BAD
Geomagnetic activity index (linear)	A	0	4	7	15	27	48	80	132	207	400
Geomagnetic activity index (log-scale)	K	0	1	2	3	4	5	6	7	8	9

Table 3: The geomagnetic indices: A, K
Reference to the conversion between A and K🗗.

Low A-Index (0-7) and K-Index (0-2) indicate good geomagnetic conditions for HF communication and lower signal absorption and disruption.

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3. Tools for Monitoring Propagation Indices:
   Stay updated on solar indices using various online tools and resources:

Space Weather Prediction Center (SWPC) provides real-time solar and geomagnetic data, including SFI, A-Index, and K-Index.

DX Cluster Websites: Many DX cluster websites display current propagation conditions and include solar indices.

Propagation Prediction Software: Use dedicated software like VOACAP (Voice of America Coverage Analysis Program) for detailed HF propagation predictions based on solar indices.

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4. Adjust Your Operating Strategy:
   Based on the propagation indices, adapt your operating strategy:
   1. Select Appropriate Bands: Choose HF bands that match the current SFI and geomagnetic conditions.
   2. Optimal Times: Plan your HF operations during daylight hours when the SFI is higher.
   3. Monitor for Changes: Keep an eye on the indices, as they can change rapidly due to solar activity.
   4. Experiment: Try different bands and times to maximize your chances of successful HF communication.

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5. Continuous Learning:

HF propagation is a dynamic field, and solar indices can vary daily. Stay engaged with the amateur radio community and learn from experienced operators to refine your understanding and skills in using solar indices for HF propagation.

By grasping the basics of solar indices and their impact on HF propagation, you can enhance your ability to make effective long-distance HF contacts and enjoy the exciting world of amateur radio and shortwave communication.

Read more about Solar Indices & Ham Radio Propagation