Using Signals of Opportunity to Experience and Understand HF Ionospheric Radio Propagation

Abstract

This paper will investigate and observe the use of signals of opportunity to enable undergraduate engineering students to experience and thereby better understand how the E and F layers of the ionosphere can enable intercontinental reception of high frequency (HF) radio signals and how the D-layer affects radio propagation. Signals of opportunity could be NIST's WWV at 2.5, 5, 10 and 15 MHz; Canada's CHU at 3.33, 7.85 and 14.67 MHz as well as the BBC and other international short wave broadcast stations. Unless deflected, wireless distances are line of sight on the order of 50 km. Ground wave, tropospheric, and other effects make possible communication distances beyond line of sight. However, HF signals that deflect off the ionosphere can enable reliable communication distances in excess of 4000 km, and with multiple hops can span the entire globe. In addition to answering the question of the how and why of ionospheric or sky wave propagation, we will also discuss and show how students can directly experience the various limitations such as what occurs during day light hours when signals at the lower portions of the HF band are absorbed by the D layer and thus do not propagate much beyond line of sight, and how solar events such as sunspot cycles and solar eclipses affect ionospheric or sky wave communication. Building on this knowledge, students will better understand radio propagation and explain why everyday AM broadcast, and FM radio signals coverage is local only, but each exhibit different properties. Furthermore, students will better appreciate the necessity of frequency diversity to achieve more reliable wireless communication.

The D and F Ionosphere Layers: Why are AM Broadcast Signals Mostly Local?

“…Our experimental apparatus is the same as used in a previous project [4] and consists of a Kenwood model TS-590 short-wave receiver, and an end-fed, 100-foot wire antenna strung around the woods of Old Lyme, Connecticut. The antenna was L-shaped, and oriented in both the north-south and east-west directions.…”

“…However, this is not the case with the closer 3.3 MHz CHU-Canada time signal. Crilly [4] reports that the 3.3 MHz CHU-Canada time signal can be received at Old Lyme 24/7 via ground-wave propagation. Note that the CHU transmitter power and distance to Old Lyme is 3 kW and 300 miles respectively.…”

“…This will be discussed shortly. In [4], it was observed that NIST's 10 MHz signal could be reliably heard over most of a 24 hour period. Skywave communication distances can be as much as 4000 km, but with multiple hops, the distances can be much greater.…”

“…Skywave communication distances can be as much as 4000 km, but with multiple hops, the distances can be much greater. Thus, relatively reliable global wireless communication via ionospheric propagation is possible, especially if frequency diversity is employed [4].…”

Using NIST’s Shortwave Broadcast Signals to Experience and Understand Ionospheric Radio Propagation

“…This is a relatively transient occurrence. See references [1] - [4] for more information on these means of deflecting radio waves. Generally, long distance communication for signals below 3 MHz are typically by the Norton Surface wave.…”