Friday, December 11, 2015
HF Radio Propagation Modeling
Today's antenna modeling programs, like EZNEC and 4nec2, use the decades-old NEC program code to calculate the electric and magnetic fields produced by an antenna in 3 dimensions. (They also model parameters like impedance to assess the power efficiency of the antenna and transmission line combination.) We normally look at the radiation pattern results for the far field, considered to be greater than 2 wavelengths away from the antenna. NEC-based software is not equipped to model the fields at a great distance. The software does not model what happens to the electromagnetic energy as it propagates over a long distance.
To complete the picture of radio communication between two HF transceivers we need to look at radio propagation tools. Here are two links that list some of the available resources:
VOACAP, which I became aware of while using 4nec2. I have not yet tried to use it, but I saw that VOACAP has both online and downloadable versions. 4nec2 has a utility for exporting an antenna design in the format required by VOACAP to model the propagation. Common pre-built antenna models can also be used as inputs to VOACAP. For point-to-point propagation modeling the design of the antenna at each end should be known. The first link above has a nice primer on VOACAP and a quick guide with lots of information.
The second link above also lists a program HFWIN32 which is said to contain 32-bit versions of three published propagation modeling tools: ICEPAC, VOACAP and REC533. Some more recent instructions and links are available at this web site.
A newer, freeware program HamCAP is designed to interface with VOACAP to predict HF radio propagation.
Playing with these propagation models may contribute to a better understanding of which antenna types should work best for a given frequency, time of day, geographic locations and solar conditions.