VK2WI AM broadcast on 7.146MHz - quality issues

VK2WI runs a broadcast of WIA news and ARNSW news on Sunday mornings from the ARNSW member funded Dural transmitter site.

Amongst the many frequencies they broadcast on, is 7.146MHz which carries a high power AM signal under special permit.

There is a long history of technical problems with transmissions from VK2WI, and in recent times, the 7.146MHz high power AM transmission is the headliner.

The waterfall displays in this article are from an independent receiver, VK1HW in Canberra.

Fig 1:

Fig 1 shows a waterfall plot of VK2WI's AM signal on 04/02/2012. The display shows the occupied bandwidth to be in excess of 30kHz. The current Amateur LCD limits necessary bandwidth to 8kHz, this transmission is clearly in breach of regulations and the interference created is exacerbated by the high power in use. The permitted channel for the 7.146MHz transmission is 7.142-7.150MHz.

Fig 2:

Fig 2 is a waterfall of VK2WI on the right on 15/04/12 after they took action responding to a report by myself of bandwidth exceeding 20kHz (it was interfering with the AM net channel 7.121-7.129MHz). Even after corrective action, it is around 16kHz wide. For comparison, that is my AM signal on the left, stronger and narrower, dismissing any notion that the observed excessive bandwidth of VK2WI is due to the receiver. Having reduced the bandwidth to 16kHz, the transmitter was kept on air for the duration of the broadcast.

After a weekend off air for corrective action, the transmitter returned on 29/04/12.

Fig 3:

Fig 3 shows the signal on 20/04/12. In this case, occupied bandwidth is again excessive at more than 20kHz. To their credit, the operator took the transmitter down on receiving a report of the problem, but one must wonder why the transmitter was keyed up with its history and clearly uncorrected problems. 

Conclusions

Amateur Radio NSW operates the transmitter under a high power permit, and funded by member funds. One might expect that given the high power operation, that the signal would be exemplary... and it clearly is not, there is a long running problem with the 7.146MHz AM transmission.

It would appear that the station operators do not effectively monitor the signal quality, and first and foremost, it should be the duty of the responsible operator to ensure that transmissions comply with regulations.

It would seem that listeners who queue up to give reports in the broadcast callbacks do not identify the problem, or reports are ignored... probably the former. With the wider use of receivers that show the information presented in the waterfalls above, whether on the radio's own display or an attached computer, many listeners must be aware of the problem and disinclined to report it.

With the active dumbing down of ham radio, it is understandable that reports of excessive bandwidth may be well intentioned but ill informed, which puts more emphasis on the need for the station operator to reliably inform themselves of transmission quality independently of callbacks etc. It is also understandable that, for the same reason, many stations observing the extreme bandwidth lack the confidence to report it.

Changes

VK1OD on the 'net. I appreciate feedback, click on the ... in webm...@vk1od.net for my email address.

© Copyright: Owen Duffy 1995, 2012. All rights reserved. Disclaimer.