Luffed, Muffed, and FOT’d

One of the critical skills needed for successful NVIS operation is determining which frequency or band to use when or if NVIS propagation is even possible.  Luckily, today there are online propagation prediction tools to help us.  In order to use these tools we need to understand a few terms.  Most of us know or have heard of the MUF and LUF, which stand for Maximum Usable Frequency and Lowest Usable Frequency.  What a lot of us don’t understand is the MUF and the LUF frequencies tell us that the path will be open at the LUF or the MUF only 50% of the time (NVIS). What we need is the FOT or the Frequency of Optimum Traffic.  This gives us a path reliability of 90%. It will frequently be about 50 to 85% of the MUF (NVIS).

To find the FOT, use a service like https://www.voacap.com/hf/ and input the transmitting and receiving stations. Then look for Best Frequency at the bottom of the map to find the FOT and viola! That and all sorts of other info are available. A couple of things to remember.  NVIS typically happens below 8 MHz.  If the FOT is above that, then NVIS propagation may not be possible. This website provides a lot of information and not all of it may be applicable to NVIS.  Software like VOACAP is designed for long-path propagation for broadcast stations.

Another good website is https://www.sws.bom.gov.au/HF_Systems/7/1. Try their Digital HF Prediction tab you can set things like elevation angle in the params tab.

When preparing a communications plan whether for a weekly net or emergency communications, a propagation study should be completed.  This will help determine which bands are best and when.  Today 11/22/23 at 1845 hrs UTC, the FOT is 27 MHZ.