FSM case study - Wideband interference on 40m

Signal appears to arrive from bearing  340º / 160º True from Canberra at 28/06/2005 21:30 measured with small un-tuned square loop and IC-R20 hand held receiver.

Field strength measurements made with FSM, R5000 receiver, HP355D attenuator and a half wave dipole.

Fig 1 shows on the LHS, RMS, QP and Pk field strength measurements at around 17:00 when the interference was not present, but background level was elevated  due to long distance paths operating, and the RHS, field strength measurements at around 21:30 when the interference was present.

For S-meter comparison, S-meters tend to respond similarly to the QP detector, and QP field strength of 29dBμV/m would result in S9 (50μV) ± 3dB with a low half wave dipole antenna. It can be seen that this interference was around S9.

A summary of the measurements and chart is available for download in the Excel spreadsheet summary of measurements.

A ten second mp3 sound clip from 27/06/2005 (25KB) is available to assist in identification of the interference.

Files saved from FSM and DPLOT for one of the measurements are available for download:

Fig 1: Chart from Excel summarising ambient conditions and interference

Fig 2 is a clip of the FSM screen at the completion of one of the measurements.

Fig 2: FSM screen

Fig 2 is a clip of the wavelet measured by FSM and exported to DPLOT for one of the measurements. The graphic display of the wavelet helps in identifying and understanding the character of the interference. The pulse modulation can clearly be seen, the PRF can be determined and in the case of this interference, it varies from day to day.

Fig 3: Wave plot from FSM / DPLOT

More information

If you have something to add or for more information, have a look at article Recent wideband interference on 40m on Google Groups or Recent wideband interference on 40m on Usenet.

You can file reports with the WIA Intruder watch by email.


© Copyright: Owen Duffy 1995, 2016. All rights reserved. Disclaimer.