FSM case study - Ambient noise survey on 40m
This article documents a small survey of ambient noise at 7.1MHz in a number of sites in Canberra in both residential and business (CBD) environments.
Field strength measurements made with FSM, IC7000 Transceiver, HP355D attenuator and a MobileOne M40 helical whip. Measurements were normalised to 1Hz bandwidth within FSM, and the chart below extrapolates the field strength to 9kHz
Measurements were made on a clear 2kHz wide channel as close as practicable to 7.1MHz at ten sites from the vehicle parked at the roadside, shown in Fig 1. Three sites were in Civic business area, and seven were in residential areas of North Canberra. Ten measurements were made at each Business site and five at each Residential site, so there are 30 measurements in the Business area spread over three locations, and 35 measurements in the Residential area spread over seven locations. The sites are available for download in KML format.
Antenna details and calibration assumptions are at http://www.vk1od.net/antenna/m40/index.htm .
The FSM data records of the observations is available for download in the Excel spreadsheet of observations.
The median for the Business observations at 17.8dBμV/m falls a little above the ITU-R P.372-8 predicted median of 14.4dBμV/m, perhaps unduly influenced by one site which was more consistent and higher level than the others.
Site B1 at 1 Akuna St was notable in that the median field strength was 10dB higher than expected, and the individual observations were more consistent than expected with an inter-quartile range of just 0.5dB.
The site was revisited on Sunday 03/12/2006 at about 06:30 to further explore the emissions. There was no significant vehicular traffic, most businesses were closed, just the conditions for lowest noise observations.
The B1 measurement site was opposite the Waldorf serviced apartments on the corner of Akuna Street and London Circuit. A brief survey of the site was conducted using an Icom ICR20 portable receiver and a small loop antenna. Distinctive emissions were observed continuously from about 3.5MHz to 8.5MHz, and consistently from the direction of the core of the Waldorf building which has pedestrian access around most of its perimeter). Bearings taken form the car park on London Circuit opposite the Waldorf building also were conclusive. The nature of the emissions suggests a possible in-house BPL system.
Site B1 appears atypical, and possibly due to a BPL system, but without certain knowledge of that, the data points have not been deleted from this survey. (The median field strength for the 20 observations from the remaining Business sites is 13.6dBμV/m, so site B1 inclusion raises that by about 4dB).
The median for the Residential observations at 10.1dBμV/m falls very close to the ITU-R P.372-8 predicted median of 9.7dBμV/m.
This is a very limited survey of sites chosen at random. The residential sites are old North Canberra, rear spine LV AC distribution mainly in flat 4 wire three phase configuration with some limited ABC, HV is on the street. Measurements were not made in extreme situations, eg under HV lines, adjacent to transformers, in the middle of any parks. A larger survey could afford a representative sampling of prospectively higher and lower noise sites and may show a slightly larger range but probably with a similar median.
The greatest contribution to uncertainty is the Antenna Factor of the helical whip on the vehicle. Directivity is less an issue than the assumptions of current distribution, therefore the effective height, and the voltage drop due to loading by the receiver as a termination.
The summary of the observations of Business environment site appears skewed by an atypical site (one of just three sites), and should not be relied upon.
These observations do not support the proposition that ITU-R P.372-8 underestimates the ambient noise experience in residential environments.
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