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Ambient noise survey, Bowral NSW on 40m, 13/08/2010

Overview

This article documents a small survey of ambient noise at 7.060MHz in the eastern part of rural town Bowral in a large block residential environment.

Field strength measurements made with FSM, an 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

Fig 1: Measurement locations

Measurements were made on a clear 2kHz wide channel as close as practicable to 7.060MHz at six locations from a vehicle parked at the roadside, shown in Fig 1. Five measurements were made at each location, so there are 30 measurements in the Residential area spread over six locations.

Antenna details and calibration assumptions are at http://www.vk1od.net/antenna/m40/index.htm .

Analysis

The FSM data records of the observations is available for download in the Excel spreadsheet of observations.

Fig 2: Statistical summary of FSM measurements of ambient noise by location

Fig 2 shows the distribution of measured ambient noise at each location.

Fig 3: Distribution of receive noise power expected in a lossless isotropic antenna based on ambient noise measurements

Fig 3 shows the distribution of receive noise power in 2kHz bandwidth expected in a lossless isotropic antenna based on the ambient noise measurements. The median value is between S3 and S4 using the convention that S9=50µV in 50Ω.

Conclusions

This is a very limited survey of locations chosen at random. The residential locations are in East Bowral, large block residential with underground LV AC distribution. Measurements were not made in extreme situations, eg under HV lines, adjacent to transformers, in the middle of any parks, around shops or schools. A larger survey could afford a representative sampling of prospectively higher and lower noise locations 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.

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