An HF and lower VHF spectrum assessment system exploiting instantaneously wideband capture

We report on a spectral environment evaluation and recording (SEER) system, for instantaneously wideband spectral capture and characterization in the HF and lower VHF band, utilizing a direct digital receiver coupled to a data recorder. The system is designed to contend with a wide variety of electromagnetic environments and to provide accurately calibrated spectral characterization and display from very short (ms) to synoptic scales. The system incorporates a novel RF front end involving automated gain and equalization filter selection which provides an analogue frequency‐dependent gain characteristic that mitigates the high dynamic range found across the HF and lower VHF spectrum. The system accurately calibrates its own internal noise and automatically subtracts this from low variance, external spectral estimates, further extending the dynamic range over which robust characterization is possible. Laboratory and field experiments demonstrate that the implementation of these concepts has been effective. Sensitivity to varying antenna load impedance of the internal noise reduction process has been examined. Examples of software algorithms to provide extraction and visualization of spectral behavior over narrowband, wideband, short, and synoptic scales are provided. Application in HF noise spectral density monitoring, spectral signal strength assessment, and electromagnetic interference detection is possible with examples provided. The instantaneously full bandwidth collection provides some innovative applications, and this is demonstrated by the collection of discrete lightning emissions, which form fast ionograms called “flashagrams” in power‐delay‐frequency plots. Key Points This HF capture system uses a digital receiver to provide instantaneous wideband characterization of the environment to 45 MHz This HF capture system incorporates RF equalization and internal noise reduction techniques to robustly characterize the wideband spectrum under stressing high dynamic ranges Examples of high‐resolution time‐frequency collections and synoptic spectral collections are provided Plain Language Summary This work describes a receiver and recording system used to accurately and robustly measure the amount and nature of the radio‐frequency energy existing at a radio site. The measurement system is instantaneosly wideband which means it characterizes the entire spectrum between 2 and 45 MegaHertz simultaneously. Previous receiver systems that attempt instantaneously