25-01-2017, 01:10 PM
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
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
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
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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.