‘Short Skip' propagation and NVIS
During our nets, the ability to hear relatively close or nearby stations is often referred to as ‘short skip” propagation band conditions. Short skip propagation is in reality NVIS (near vertical incidence skywave) propagation, and this is a path that provides useable signals in the range between ground wave and conventional skywave distances, and has a range usually of 0-200 (+/- 100) miles. The performance of this NVIS propagation is dependant on the critical frequency foF2.
Critical Frequency foF2
The foF2 frequency is the critical frequency of the F2 layer of the ionosphere. This cricital frequency (fo, in Mhz) is associated with the electron density of the F2 layer, and is measured as the highest frequency of radio signals that reflects back from the F2 layer at near vertical incidence.
Global plots and maps of foF2 are available on the internet and are usually taken and published on 15 minute intervals. One source of these maps is the Australian Space Weather Services. Their maps are created using data supplied from many other data provider services scattered around the globe. One of these data providers is a radiosonde station located at Point Arguello.
For our nets to have good ‘Short Skip' conditions where we can hear reasonably close stations, we would like to have the foF2 frequency to be above, or close to, our band frequency of 7.272 MHz. Additionally, ‘Long Skip” band propagation has, as a rule-of-thumb, a usable maximum frequency of about 3 times the usable foF2 NVIS frequency.
URL Links for obtaining foF2 Data Plots
Two links are provided here. The first is a link to the current global foF2 map from the Australian Space Weather Services website.
• Click for Global foF2 map
The second link, selected because it provides foF2 measurements very local to a significant geographical area of our daily nets, is to the Point Arguello data website.
• Click for Pt. Arguello foF2 data
After opening the Pt. Arguello link, get an foF2 reading and data plot by selecting the desired year; then the month, day, and time-of-day (UTC).
