2m pager QRM SE Qld

[VK4ADC]

I have been looking into some pager-sourced QRM which is causing triggering of a number of 2M repeaters in SE Qld, notably those which don't require an access tone. VK4RBN on 147.000 (Tx, Rx 146.400), VK4RAX on 147.075 (Tx, Rx 147.675) being the most obvious ones plus I suspect that the Dstar repeater VK4RBX on 146.7875 (Tx, Rx 146.1875) is also being hit as well. I have been using my SDR plus two receivers to isolate the most likely source to being 148.6375 but there are so many paging transmitters on it that pinpointing which individual Tx is faulty is hopeless.

The SDR coupled with Spektran shows me at least one spurious travelling up in frequency on paging transmitter activation but before any paging data is sent, and I suspect that there is more than one spur doing the triggering. It does not appear to be a transmitter intermod product as the mostly co-sited 148.5625 transmitters do not have to be up for the repeater triggering to occur. It is not a receiver intermod product either as it is occurring on multiple receivers of different brands and on multiple antennas. The SDR here is fed from a discone antenna via a variable attenuator to ensure that it is not overloaded and all are producing the same results.

I really only became aware of it yesterday but I am guessing that it has probably been happening for a while. My thought is that if others with spectrum analysers or SDRs can confirm that there is a travelling spur, and its probable transmitter location determined, we might be able to have an approach made to Vodaphone/Hutchison to see about getting it fixed. It's worth a try.

If you have the right gear, do some exploring between 146 and 149 with a decent antenna connected and report what you find.

73 Doug VK4ADC

[VK4ADC]

I tried SDsharp as the spec analyser today after noting paging tones come over the top (partial FM capture) of some users on VK4RAX (Rx 147.675), still with the attenuator in-line and adjusted to prevent overload. Vertical graticule is 5dB steps (approx).

   

   

This latter image shows some transmitted intermod as the extra spurs are still there with more attenuation in line to the SDR.
The 148.5625 frequency up alone does not show spurs..

[VK4ADC]

I have been doing a bit more research today and want to rescind my thought that it is primarily a Vodaphone pager on 148.6375 that is causing the triggering of VK4RBN and VK4RAX repeaters. Yes, at least one of the transmitters on 148.6375 is producing spurious but it is not the cause of the triggering....

The screen shot below, with explanatory labels, reveals that the problem transmitter is actually on 148.8125 MHz, denoted Pager 3 at the RHS in the image.

   
Click on the image to see in greater detail.

Hard to see but there are wandering spurious only when the Pager 3 transmitter is active.

The ACMA database search reveals only two probable sources :

Register of Radiocommunications Licences
Assignments found
Results 1 - 13 of 13 assignments.
(UNLIKELY ONES OMITTED...)

ID Frequency Emission Designator Authorisation Date T/R Client Licence Site

1341805 148.8125 MHz 16K0F2D 15/Dec/2016 T Mater Misercordiae Public Hospital (72693) 1918219/2 Mater Hospital Raymond Tce SOUTH BRISBANE QLD 4101

1341806 148.8125 MHz 16K0F3E 15/Dec/2016 T Mater Misercordiae Public Hospital (72693) 1918219/2 Mater Hill Hospital Main St SPRINGFIELD LAKES QLD 4300

Given the problem has only recently started, it could be either site BUT the likelihood that I am hearing South Brisbane at some 29.5 Km away is unlikely, that makes Springfield Lakes at 8 Km away as the probable source. I have only heard paging tones on 148.8125 yet Springfield is licenced as 16K0F3E - voice - so that makes me think that they are using it purely tone paging.

Being able to determine it as the one source will be easy if I travel to the Springfield area with a suitable receiver and look at it in close physical proximity. Alternatively, someone near South Brisbane may be able to check that one...

At least it may be easier to get it fixed rather than as with a wide area network like Vodaphone's..

More to come in due course
Doug VK4ADC

Added in 16 minutes 37 seconds:
Better view of the wandering spurs without a lot of activity on 148.5625 or 148.6375. Makes it easier to define the "culprit".
VK4RBN and VK4RAX are the signals at LHS, 148.8125 the signal at LHS..
Spurs from 148.6375 are easier to spot too.

   

[VK4ADC]

The fun never stops here. The results this morning have been a little more definitive given the connection of the 10 element 2M yagi in lieu of the omnidirectional discone. The bearings from my QTH to Mater at Springfield are about 320 degrees, the Mater at South Brisbane around 12 degrees.

The following images were obtained from SDRsharp using the SDR dongle with the variable attenuator in line set to 10dB to prevent total pager overload.

The first one is with the yagi at 320 degrees (TN (True North)) and the VK4RBN and VK4RAX repeaters (LHS) have clearly been triggered by the pager signal at RHS. No travelling spurii are noticeable.
   
Click on the image to see more detail.

The second image is with the yagi at 12 degrees (TN) and, again, the VK4RBN and VK4RAX repeaters (LHS) have clearly been triggered by the pager signal at RHS. The more obvious travelling spurious emissions are shown sweeping across the 2M band in a couple of places. Given the higher signal strength in this direction, that places the South Brisbane transmitter as the probable culprit.

  pager8 12deg.jpg (Size: 198.53 KB / Downloads: 1,633)
Click on the image to see more detail.

It still needs to be confirmed by someone close to South Brisbane but at least the details are becoming clearer.

Added in 31 minutes 34 seconds:
By looking at the 2M FM band rather than just the top end near the pagers, it appears that VK4RCC at Ocean View on 146.625 TX, 146.025 Rx, is also being triggered.

   

[VK6LD]

Hi Doug,

Just thought I'd leave a reply to let you know your detective work makes very interesting reading. SDR Sharp looks to be an interesting piece of software. I must give it a try.

Cheers,

Rob...
VK6LD

[VK4ADC]

Rob

I wasn't sure how valuable a low cost ( < $20) SDR dongle would be before I bought it but, with the right software and figuring out how to use that, it is beginning to look better all the time. It will never replace a proper spectrum analyser but doesn't do a bad job for what it cost. Just as an aside, I fed my sig gen into it yesterday and it was seeing well better than -120dBm at 70CM.

The QRM saga continuing:
I contacted Doug VK4OE by phone this morning and asked him to connect his spectrum analyser to an antenna and check for the travelling spurious. He got back to me this afternoon to advise that the paging carrier was about -30dBm and the spurs were about 30dB below that (ie -30dBc).

I tried phoning the Mater Hospital after that and received no information but did ask for my contact details to be passed on to the people who looked after their pocket pager system, the same ones who were apparently there trying to fix pager issues (but probably not the RF ones). No phone calls back yet.

Will update further as things become closer to resolution, meanwhile the pager continues to hammer the three voice repeaters.....

[VK6LD]

Hi Doug,

Thanks for the details - I might have to get an RTL dongle and give it a try.

In the past when I've had to notify ACMA regarding QRM issues to repeaters, once you get past the disinterest to resolving interference to amateur repeaters during initial contact and get ACMA appreciating that the problem actually is other commercial users operating outside of their license conditions, then ACMA are usually quite good working with the commercial users to rectify the problem.

Cheers,

Rob...

[VK4ADC]

Hi Doug,

Thanks for the details - I might have to get an RTL dongle and give it a try.
....
Cheers,

Rob...

Do an eBay search for "USB 2.0 Digital DVB-T SDR+DAB+FM HDTV TV Tuner Receiver Stick RTL2832U+R820T2 ZX". Mine came from Darwin for about $13.60 including free postage, took about a week to get to me. There are a few others that seem cheaper but are outside AUS and take about 3-4 weeks to arrive. I ended up cutting the mini-coax and putting an SMA male crimp on the end - my lead is about 100mm long.

Don't load the Blaze TV software from the supplied mini-CD as that driver may cause issues later when trying to get SDR# to run. ( see http://www.rtl-sdr.com/tag/sdrsharp/ )There are a few driver support files to be installed but once up, it's great. Spektrum software runs well on a Win 64bit platform and gives a wider frequency span that SDR#, no 32 bit format available. (see http://www.rtl-sdr.com/spektrum-new-rtl-...-software/ )

   

Still waiting for a phone call about the pager....

[VK4ADC]

Now for some good news.

I had a phone discussion this afternoon with a person in charge of that pager facility at the hospital who confirmed that a new transmitter was placed in service there Monday last week. I advised that that was the day the current problem was first noticed here.

I have emailed screen shots from SDR# to them to clarify what the issue is and was advised that the details will be passed on to the transmitter manufacturer/agent.

There is no timeline for it getting resolved but at least I have direct contact details so that I can keep in touch for updates.

In the meantime, the false triggering will continue to annoy many amateurs in SE Qld - but - at least there is a light at the end of the proverbial tunnel.

I will post here with any news.....

73 Doug VK4ADC

[VK3HZ]

Well sleuthed Doug.

I'll bet it's a switch-mode supply (600kHz) powering the pager, probably supplied by the local installers.

Are there any spurs at +600 kHz (and 1.2 MHz ...)?

Regards,
Dave
VK3HZ

[VK4KD]

Great detective work, Doug!

I don't have all the fancy gear here, but I can confirm that, from a simple J-pole at Eatons Hill, I can see a moderately strong pager burst on 148.8125 coinciding every time with a triggering of VK4RBN. Seems to also be affecting VK4RAX maybe 4 times out of 5; I can't yet correlate it with the triggering of VK4RCC, though.

It only seems to hit VK4RBN on initial TX; listening to a couple of particularly long bursts on 148.8125, the repeater is dropping within a couple of seconds and not reopening until the next burst commences.

My FT-8800R isn't seeing anything on VK4RBN input from here, for the record (but I am blocked by hillside in the direction of the Mater.)

Now, off to go hunting for one of them thar SDR dongles :^)

73,

Phil
VK4KD

[VK4ADC]

We have conducted tests today (2/2/17) and confirmed that the issue is related only to the primary paging transmitter and that the standby unit does not cause any problems. They will try to leave it on the backup as much as they can but it will not be possible to leave it solely there overnight.

This issue comes with it's own things to mention...

(a) Comment : "100 pagers are used for all the nursecall , critical codes and general communication so we need to ensure a replacement is sent before taking further action." and, in my personal opinion, health and safety takes precedence over false triggering of a few amateur repeaters...

(b) The backup transmitter cannot be left in service alone. There must be a backup in case of primary failure and it is not possible to change the order of the transmitter devices.

© The transmitter was sourced in the USA and unless there are local service agents capable of resolving the instability, there will be a delay while another transmitter is readied and shipped to Aus.

We are lucky that they are cooperating in this case. It will be resolved but the timeline is a question still to be answered.

Please do not phone the hospital to query or complain : leave the contact arrangements with me as a sole agent representing all of the local amateurs. I was made aware that there have been at least a couple of phone calls already today about the interference and I can assure you that you won't get any answers if you call.


Doug VK4ADC



"

[VK4KD]

Absolutely, critical medical care trumps our annoyance. Glad they're cooperating with you, Doug -- sounds like you found some good folks over there. Good luck in your ongoing dealings with them.

-Phil (for the record, *not* one of the ones who's called the hospital.)

[VK4ADC]

The word today is that a replacement transmitter is on its way and should be installed early next week.

In the meantime, please be patient about the QRM.

73 Doug

[VK4KD]

Hi Doug,

Have you heard anything from the hospital lately?

The pager was still triggering VK4RBN late last week, but as of tonight it seems to be fixed -- I'm seeing plenty of bursts on 148.8125 but no corresponding opening of the repeater.

Would be interesting to get confirmation, and I'm also sure that the hospital would appreciate feedback that the problem seems to be resolved.

73,

-P.

[VK4ADC]

The pager transmitter was due to be replaced Monday morning this week but the person responsible has been down sick. The hospital has been utilising a backup transmitter since the end of last week (late Friday) and it has not been causing any interference to the repeaters and no sign of spurious on the SDR waterfall etc.

I was just looking at it again a few minutes ago (Wed AM) and still no sign of repeater triggering is occurring & no spurious are visible here.

I have been waiting for the new transmitter to be installed and evaluated before essentially closing off my comments on this topic but your query has prompted a more immediate response. Maybe more later today or tomorrow once I have heard from them..

Doug

[VK4ADC]

The replacement hospital paging transmitter was placed in service at 3PM today (Friday 24th Feb 2017) following nearly a week of operation on their standby transmitter. The original (faulty) transmitter is being sent back to the USA.

Neither transmitter is causing false triggering of any repeaters that I can note on the SDR waterfall so I consider the QRM resolved permanently.

Thanks for their cooperation has been sent on behalf of the SEQ amateurs.

Thanks also to those who have read and responded to my forum posts. It does go to show that these issues have a chance of being resolved by amateurs if the "big stick approach" is avoided and common sense prevails, not to mention having a little patience.

73 Doug VK4ADC

[VK3ALB]

A good demonstration of trouble shooting and an excellent outcome.

Well done.

[VK4KD]

Thanks for your work with the hospital on this one, Doug. A good outcome, and seemingly on pleasant terms from beginning to end. Well done!

(And thanks to everyone else for their patience, too!)

73,

-P.

[VK4ADC]

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