09-03-2017, 08:09 AM
I have had a little flack over the time it took for the QRM to be resolved in its entirety but the following info may help summarise and settle the timeline. I must also point out the ACMA was advised as soon as I defined the source but I was allowed (?) to work though this privately with the hospital.
1. The paging system at the hospital is used for urgent care paging and the service cannot be simply turned off.
2. The triggering of the repeaters was spasmodic rather than continuously on the frequency and inhibiting total use.
3. The interference started after a previous transmitter failure and the replacement transmitter generated the spurious.
4. The faulty transmitter was supplied from the USA, as was the replacement, with no Australian agents involved, and it takes time to communicate, configure and ship a further replacement unit from that country.
5. When it was discovered that the hospital had a second/standby installation and it did not cause QRM, they were encouraged to swap across to it even though the coverage within the hospital grounds was not confirmed as 100%.
6. The last week before the actual transmitter changeover occurred was interference free because they were using the second transmitter facility.
7. In essence, there was about 3 working weeks of QRM, the end of the first week was when I defined the frequency and facility and I am unsure as to whether anyone else even bothered to search for it.
The second week was involved in working with the hospital and making sure they understood the issues involved, and with them starting the arrangements to obtain a replacement for the faulty transmitter - without a lot of cooperation for the USA initially.
I was absent from home for most of the third week but I was made aware the QRM continued. I contacted the hospital on the Friday after my return, queried progress on the replacement and as a result of some tests, the operation on the second transmitter commenced on the Saturday morning.
The fourth week was QRM-free and the faulty transmitter was finally replaced on the Friday PM and was confirmed as being stable.
8. Be aware that the person I worked with has multiple roles within their organisation and is not just dedicated to things like paging facilities. If it is electronic and in any way involved with wired or wireless communications, that is within their area of responsibility.
I was emailed a statement on 28/2/17: "I have made a request to a Brisbane ACMA field officer about two weeks back to investigate why they didn't have an "efficient cavity filter fitted between the transmitter and antenna" as per the special conditions of their license at South Brisbane and Springfield sites."
I responded with "A single in-line bandpass cavity resonator would not provide significant filtering of spurious transmitter outputs closer than several MHz away. The ruling used to be that they were only required on “communal sites” anyway and I doubt that either Hospital would really be classed as a communal site, even if there are other VHF or UHF base station facilities nearby. If the original pager installation date at South Brisbane preceded the licence requirement for a filter, the staff there may never have been aware of that requirement.
A resonator slightly detuned due to mechanical drift over time or maladjustment is more likely to create a reactive load and cause transmitter instability if the tendency is already there."
A further response was
"Their license conditions state that one must be fitted.
A suitable filter would provide >12dB at 1mhz and >18db at 2mhz. This would at least have gone some way in minimising the impact (geographical range) of their interference.
By not having one fitted, they caused more interference than if they had one and they have not complied with their license conditions.
Their slowness to rectify the issue also caused concerns within ACMA.
Irrespective of being a communal site or not, a filter must be fitted if the conditions of the license stipulate one.
Too many products are entering the market with non existent filtering (baofeng radios for example) and potentially causing harmful interference and contributing to raising the noise floor for all spectrum users. Commercial spectrum users not adhering to their license conditions only makes the situation worse."
Finally, I do not want to get involved in a verbal war since the matter has been resolved as far as interference to amateur service facilities is concerned. Whether the resonators/filters get fitted or not does not come within our control either - that is between the ACMA and the body concerned.
I will admit that I was very tempted to respond by doing a path loss evaluation between the repeaters and the pager to see what signal levels were actually received based on knowing the level of the spurious observed with the SDR versus the main emission level, and then adding attenuation figures based on a typical resonator response curve. Given the spurs were only about 15-20dB down at most, I suspect that the repeaters would still have been triggered but the source would have been a lot harder to discover. I refrained.
FINIS - THE END
1. The paging system at the hospital is used for urgent care paging and the service cannot be simply turned off.
2. The triggering of the repeaters was spasmodic rather than continuously on the frequency and inhibiting total use.
3. The interference started after a previous transmitter failure and the replacement transmitter generated the spurious.
4. The faulty transmitter was supplied from the USA, as was the replacement, with no Australian agents involved, and it takes time to communicate, configure and ship a further replacement unit from that country.
5. When it was discovered that the hospital had a second/standby installation and it did not cause QRM, they were encouraged to swap across to it even though the coverage within the hospital grounds was not confirmed as 100%.
6. The last week before the actual transmitter changeover occurred was interference free because they were using the second transmitter facility.
7. In essence, there was about 3 working weeks of QRM, the end of the first week was when I defined the frequency and facility and I am unsure as to whether anyone else even bothered to search for it.
The second week was involved in working with the hospital and making sure they understood the issues involved, and with them starting the arrangements to obtain a replacement for the faulty transmitter - without a lot of cooperation for the USA initially.
I was absent from home for most of the third week but I was made aware the QRM continued. I contacted the hospital on the Friday after my return, queried progress on the replacement and as a result of some tests, the operation on the second transmitter commenced on the Saturday morning.
The fourth week was QRM-free and the faulty transmitter was finally replaced on the Friday PM and was confirmed as being stable.
8. Be aware that the person I worked with has multiple roles within their organisation and is not just dedicated to things like paging facilities. If it is electronic and in any way involved with wired or wireless communications, that is within their area of responsibility.
I was emailed a statement on 28/2/17: "I have made a request to a Brisbane ACMA field officer about two weeks back to investigate why they didn't have an "efficient cavity filter fitted between the transmitter and antenna" as per the special conditions of their license at South Brisbane and Springfield sites."
I responded with "A single in-line bandpass cavity resonator would not provide significant filtering of spurious transmitter outputs closer than several MHz away. The ruling used to be that they were only required on “communal sites” anyway and I doubt that either Hospital would really be classed as a communal site, even if there are other VHF or UHF base station facilities nearby. If the original pager installation date at South Brisbane preceded the licence requirement for a filter, the staff there may never have been aware of that requirement.
A resonator slightly detuned due to mechanical drift over time or maladjustment is more likely to create a reactive load and cause transmitter instability if the tendency is already there."
A further response was
"Their license conditions state that one must be fitted.
A suitable filter would provide >12dB at 1mhz and >18db at 2mhz. This would at least have gone some way in minimising the impact (geographical range) of their interference.
By not having one fitted, they caused more interference than if they had one and they have not complied with their license conditions.
Their slowness to rectify the issue also caused concerns within ACMA.
Irrespective of being a communal site or not, a filter must be fitted if the conditions of the license stipulate one.
Too many products are entering the market with non existent filtering (baofeng radios for example) and potentially causing harmful interference and contributing to raising the noise floor for all spectrum users. Commercial spectrum users not adhering to their license conditions only makes the situation worse."
Finally, I do not want to get involved in a verbal war since the matter has been resolved as far as interference to amateur service facilities is concerned. Whether the resonators/filters get fitted or not does not come within our control either - that is between the ACMA and the body concerned.
I will admit that I was very tempted to respond by doing a path loss evaluation between the repeaters and the pager to see what signal levels were actually received based on knowing the level of the spurious observed with the SDR versus the main emission level, and then adding attenuation figures based on a typical resonator response curve. Given the spurs were only about 15-20dB down at most, I suspect that the repeaters would still have been triggered but the source would have been a lot harder to discover. I refrained.
FINIS - THE END
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.