HF spectrum monitoring using a PC-based scanning receiver

1. HF spectrum monitoring using a PC-based scanning receiver M.R. Hyde IPS Radio and Space Services

2. Abstract • Real-time HF spectrum monitoring using scanning receivers has traditionally required expensive radio equipment with a hardware computer interface, dedicated logging equipment, and custom software. These diverse components can require considerable expertise to integrate into a working system sufficiently reliable to deploy at remote sites. This presentation describes a simple, relatively low-cost setup using one of the new PC based scanning receivers. The system consists only of a PC with installed receiver card, and an antenna. There is a freely available form of interpreted BASIC programming language for these receivers, which includes commands to control receiver functions. Commands are also available to control data logging and produce simple on-screen graphics. This small instruction-set language makes monitoring acquisition and control program development quick and easy, even for novice or non-programmers. Given a stable mains power supply, the system is sufficiently reliable to operate unattended for many months. Some HF spectrum data from scans acquired during field trials on a range of antennas are presented. A near-real-time short-wave fadeout monitoring application is also discussed and some sample field data presented.

3. HF channel occupancy monitoring • The scanning receiver used in these trials is a WinRadio model WR-1550. We are using the internal version, which occupies a single ISA-bus card slot. An external (serial interface) version is also available. IPS is primarily concerned with radio propagation in the HF spectrum, so the trials were limited to a 2 - 20 MHz frequency range, although the receiver has a very wide RF bandwidth extending to 1.5 GHz. A newer series is now available from WinRadio, the WR-G303/313, which is restricted in maximum frequency to the HF range, but has greater flexibility and functionality than the 1550 series. Internal/external configurations for this newer series are PCI-card/USB-bus format respectively. • The HF spectra below show channel occupancy in the 2-20 MHz region. At present, the logging program is set up to maintain a continuous record of all scans and a separate record of the most recent 24 hours of data at half-hourly intervals. In a projected service application, the 24-hour record can be accessed half-hourly and used to update a running plot of near-real-time channel usage, to be displayed on the IPS public website. • The present program scans the range 2-20 MHz in 5 kHz steps (=3600 steps/scan), taking an average of each 50 steps, making 72 data points per scan. The minimum time the receiver must sit on each frequency is 100 mS, so a scan takes 6 - 7 minutes to complete. At the end of each spectrum sweep, loops in the program perform some other measurement functions including the short-wave fadeout monitoring also discussed below. • The program was written in the dedicated RBasic programming language. This language includes commands to access most user-functions of the receiver, access to the PC system clock, and the ability to create, delete and append text files for logging receiver settings and output level.

4. Fig. 1 WinRadio Application control panel Fig. 2 Screenshot of IPS HF Monitor program running

5. Fig. 3 Camden 60m horizontal longwire antenna Fig. 4 Camden 27 MHz whip antenna

6. Fig. 5 Wagga Wagga discone antenna Figures 3 to 5 show typical HF spectra over 24 hours recorded at two locations on a variety of antennas. Camden is located on the urban fringe of Sydney and would be expected to experience relatively dense local radio traffic and man-made noise. The horizontal longwire antenna (Fig. 3) is more sensitive to high-angle, short to medium distance signals arriving by ionospheric propagation. The response of this antenna reflects local ionospheric conditions. There is a reasonably strong diurnal pattern, with increased signal density during local night hours. This is especially true at lower HF frequencies, which are less attenuated by absorption in the lower ionosphere at night. The effect of the dawn terminator can be seen around 18-20UT, with F2 region maximum useable frequency (MUF) reaching a minimum pre-dawn, some atmospheric mixing occurring post-dawn, before absorption increases again on the day-side The vertical whip antenna (Fig. 4) should be more sensitive to low angle long distance signals, but in this case the antenna is designed for a band somewhat higher than 2-20 MHz. Probably much of the received signal is local groundwave, which is relatively constant over the day. A vertically polarised antenna is more sensitive to man-made noise, and a horizontally polarised antenna less sensitive to groundwave. The discone antenna (Fig. 5) is also vertically polarised, and has an essentially flat broadband response above its resonant frequency. In this case it was located in a radio-quiet rural area. Again there is a strong diurnal pattern to the response. The exclusion of low HF frequencies during local night is a function of both antenna response and local noise conditions. Formation and dissipation of the daytime D and E regions can be clearly identified by absorption in the lower HF spectrum.

7. Fig. 6 Single sweep – Camden longwire antenna Fig. 7 Single sweep – Camden 27MHz whip antenna Figures 6 to 8 show single sweeps of the 2-20 MHz band, taken at selected times from the same data as presented in Figures 3 to 5. The longwire antenna at Camden has a significantly better low-frequency performance than the others due to its physical size. Regular peaks in the response at higher frequencies may be resonances on the wire. The 27 MHz whip shows low sensitivity in general at these frequencies. Much of the received signal is probably of local origin. The discone antenna has low sensitivity below its resonant frequency. This sweep was taken near local dawn, when fOF2 would be at a minimum. Hence acquisition of distant signals would be limited. The figure emphasizes the very low radio noise environment. Fig. 8 Single sweep – Wagga Wagga discone antenna

8. Shortwave fadeout monitoring • In the event of a large (M-class and above) solar X-ray flare, high energy photon deposition in the D-layer results in increased absorption of HF radio waves. This effect only occurs in the sunlit sector, is greatest where the sun is overhead, and progressively affects higher HF frequencies with increasing X-ray intensity. • The IPS HF monitor aims to observe such events when they occur locally, in terms of ionospheric response. At the completion of each spectrum scan, the WinRadio HF Monitor is programmed to scan a number of discreet frequencies at the lower end of the HF spectrum. The frequencies are chosen locally and ideally should have a constant signal strength from a distant source. If there is a large flare during local daylight hours, the response of the monitored frequencies should track the progression of a shortwave fadeout event. • The following sequence of figures show one such event captured by the Camden HF monitor during an X-ray flare originating from a recent particularly active solar region (AR808). This flare peaked briefly at the X1.1 level at 0300UT on September 9 2005, when the sun was close to maximum zenith over Camden.

9. Fig. 9 GOES12 satellite plot of solar X-ray flux. Only one flare in this sequence occurred with the sun overhead E. Australia Fig. 10 WinRadio SWF Monitor plot. X-ray intensity peaked briefly at 0300UT. SWF evident about this time. Fig. 11a IPS Camden 5d ionogram for 09 Sep, 0239UT Fig. 11b IPS Camden 5d ionogram 0254UT

10. Fig. 11c IPS Camden 5d ionogram 0259UT Fig. 11d IPS Camden 5d ionogram 0304UT Fig. 11 shows a sequence of ionograms from the IPS 5d ionosonde co-located at Camden NSW with the WinRadio HF Monitor, around the time of the SWF event of 09 Sep 2005. The flare at 03UT on September 9 (Fig. 9) was impulsive in nature. The ionograms show, over a relatively short interval, the disappearance of ionospheric reflection from lower HF frequencies, extending briefly to all HF frequencies near peak X-ray flux, and gradual return to more normal conditions as the flux decayed. The event was also tracked by the HF monitor (Fig. 10). The fact that HF Monitor signals were not greatly reduced by SWF suggests significant contribution to the received signal from local and groundwave sources. The antenna in use at the time was the 27 MHz whip. Fig. 11e IPS Camden 5d ionogram 0334UT

11. Future directions and conclusion • The WinRadio HF Monitor offers a simple and reliable near-real-time indicator of actual HF conditions, at relatively low-cost. It requires occasional user-intervention due to the nature of the application and the Windows operating system. Most IPS sites are networked so that remote desktop access to the HF Monitor PC is possible, allowing control of remote monitors from IPS Head Office. • The main difficulty to this point has been to find a suitable antenna. Ideally a vertical, broadband, omni directional antenna would be used. The discone appears to be a good choice but, for a reasonable physical size, is limited at the lower end of the spectrum. IPS hopes to install vertical monopoles at two sites in the near future which, while primarily intended for oblique path monitoring, may be available at times for omni directional HF monitoring. • Selection of suitable monitoring sites is also important. The instrument clearly performs better at radio-quiet locations. The IPS sites at Culgoora, in Central NSW, and Learmonth, WA would appear to be good choices. However, the eventual deployment of the vertical monopoles may dictate site selections. • WinRadio receivers may be adapted to oblique path monitoring if found to be compatible with transmitting equipment available to IPS for this purpose. The newer HF-only versions of the receivers may be used to collect quantitative data on digital HF broadcast transmissions. • Further Information • WinRadio product range and technical information: www.winradio.com • RBasic Programming language: www.rbasic.com • IPS products and services: www.ips.gov.au • Acknowledgement • The contribution towards this project of Stacey Osbrough, LaTrobe University – IPS vacation student during the summer 2003/04 is gratefully acknowledged. • Contact • Mike Hyde, IPS Radio and Space Services, PO Box 1386, Haymarket NSW 1240 • mike@ips.gov.au