Mario Filippi a regular contributor to our blog has recently written in with another article of his. This time he’s submitted an interesting article about ionosondes and how he listens to and watches them with an RTL-SDR dongle and upconverter. We present his article below.

Chirp Sounders and Those Ear-Jarring “Zwoops”

Written by Mario Filippi (N2HUN) – (All photos courtesy of author)

Have you ever experienced a loud disconcerting “zwoop” sound quickly passing through your headphones while listening to the HF or shortwave bands? Surely many of us have, and for years these odd sounding transmissions were a mystery, but the conundrum was unraveled one day when using my RTL-SDR (software defined radio) dongle for some HF (high frequency, 2MHz – 30MHz) listening. The HF band is populated by an array of non-voice (digital) signals from familiar modes such as CW, RTTY, and FAX to more contemporary modes such as ALE, PSK-31, and JT65, to name a few. Many different modes and sounds, both man-made and from Mother Nature, some familiar, some mysterious, inhabit the breadth of the HF band. These frequently heard “zwoops,” on different portions of the band definitely were in the “mysterious” category.

Over the past several years these high-pitched “zwoops” passing through my headset at lightning speed disturbed the calm of a normal evening spent listening to shortwave with my venerable boat anchor-like Yaesu FRG-7 receiver. However, further investigation using a RTL-SDR dongle (from www.rtl-sdr.com), Nooelec HamItUp upconverter, and SDR# software visualized these signals emanating from ionosondes. Their transmissions appear on the waterfall image as pulsed lines traveling up (and sometimes down) different segments of the HF band. Their purpose is helping to assess the ionosphere’s propagation status.

In short, ionosondes, or ionospheric sounders, sometimes referred to as “chirp sounders” are transmitters that send out a radio signal across a specific frequency range, only to be heard by receivers at distant locations that analyze what the propagation characteristics are. Armed with this information, these analyses are an aid in two-way radio communications, such as determining the best frequencies to use at a given time by radio operators around the world. So what do these ionosonde transmissions appear like using the RTL-SDR and SDR# software? See some examples below.

Chirp sounder transmissions appear randomly as one navigates the HF bands and in the author’s experience are a hit and miss affair, but with the advent of software defined radios with real-time spectral displays of two megahertz or more in width, one can increase the possibility of hearing and seeing them more regularly. Note that ionosonde tracings on a waterfall can take many different shapes; I have shown only a few examples. The speed at which the ionosonde transmits up or down the band varies with the setup, but it’s an amusing signal to watch as it gracefully and speedily streaks across the band’s waterfall image with its’ meteor-like trail.

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