Down the (NVIS) Rabbit Hole

Here is another short entry I wrote some time ago. I am not sure when I wrote it nor why I never posted it. But it turned up when i did a search for draft articles. It is complete, so here it is.

It all started so innocuously. The May edition of the SNARS Crackin’ Static arrived in my email inbox. I started reading it and came across an article about Near Vertical Incident Skywave (NVIS) antennas.

NVIS is very useful for short range high-frequency radio communications. The military uses it for their communications and the Amateur Radio Emergency Service (ARES) uses it for emergency auxiliary communications for their served agencies.

It is also useful for contacting close stations when activating a park (POTA) or a summit (SOTA) when the stations have intervening topography or are beyond ground wave propagation. Ground wave works out to about 50 miles or so (depending on frequency). The first skip is out somewhere over 500 miles or so (depending on both frequency and ionospheric conditions). So there is a gap in between the 50–500-mile range.

This is where NVIS comes into play. The trick is to choose an operating frequency such that the ionosphere will refract near vertical radio waves back to ground. If the frequency is too low, then it is absorbed by the ionosphere. If it is too high, no refraction occurs.

So, the Goldilocks principle applies — the just right frequency is needed.

And so off I go down the rabbit hole, reading about ionosondes (radio stations that test the ionospheric conditions) and the data they produce.

And here I go learning about Lowest Usable Frequency, , and Maximum Usable Frequency plus a host of associated data and technology.

I love looking at graphs of data as well and learning how to interpret what I see.

This research will probably result in a short note on how to read the data and the charts. If one is a radio operator, then this is important material… provided you want your signal to get through.