query on RFI interference on Tower Masts

[bcoleman1]

Hi All, Hope all is well.
We have 2 Repeater units in Ireland at a Tower mast site. ( Co. Louth)
A digital broadcast signal antennae was installed on frequency 175mhz on the same Tower mast early April last.
Unfortunately, the RFI knocked /de-sensitised, out our radio mesh repeater which was on its own freq and we restored it by installing a cavity filter.

However, we have a legacy unidirectional alarm repeater named Ateargo ( Italian vendor). It is on 167 TX/transmit freq. and 172 RX/Receive freq. that got knocked offline as well and 20 odd RMA units are missing daily timer test since early April.

I found out the name of the Co. whose client installed the digital radio broadcast signal and he kindly turned it off one night for me , however it made no difference, still same 25-26 missing RMA units.

Alarm rma units meant to receive into the Tower site in Louth first then transmit to Dublin Ateargo Receiver before transmit over ip link to NOC/ARC Monitoring centre in Belfast.

I have ran a bird watt meter model 43 on original Ateargo repeater at the high site/Louth and SWR readings are good.
I have swapped out the power supply and battery on it too. Kenwood unit.

​

by the way we tried installing even a dual cavity filter on the Ateargo repeater but it made no difference either.

I also tried setting up a test environment of spare Ateargo repeater unit and ateargo rma test radio however could not get any alarms in from a house attic
in Monasterboice which is at 1400m height - the highest point between Mourne mountains and Dublin mountains.

Any ideas what good be causing the problem,/advice greatly appreciated please/

Thank you.

 

[bcoleman1]

Hi All
We had a issue with Ateargo legacy unidirectional alarm repeater unit. A digital broadcast antennae on the same tower desensitised alarm signals going into ateargo. We got it solved end of last july by installing a yagi antennae towards direction of the main ateargo repeater/receiver in Dublin. However in the last 2-3 months - 4-to 5 of the same rma units within 10km-36km range are missing daily signal timer test. Appreciate any advice how to try fix the issue please/Thank you. ( some days only one unit might miss daily timer test example a unit 10km frrom the Tower mast, more frequently 4 to 5 units are missing daily timer test.

 

[prcguy]

For problems like this you need to look at the receiver input of the units experiencing interference with a spectrum analyzer to understand what the receiver sees. If the problem comes and goes you may have to watch the spectrum analyzer for some time or use a peak hold function if it has that to record signals over time. The interference could be on your frequency or it could be a very strong signal nearby that the receiver can't deal with.

A 175MHz digital broadcast makes me think very high power TV transmitter and having that at 175MHz is only 3MHz away from one of your repeater receiver inputs. In the US a typical digital TV broadcast signal is 6MHz wide and covers 174 to 180MHz putting it only 2MHz from your repeater input if you share the same frequency plan.

A spectrum analyzer will tell you the precise signal level your repeater receiver is seeing plus you will see if the digital TV transmitter sidebands are suppressed within specification and if they are encroaching onto your repeater input frequency. A digital TV transmitter usually has a very elaborate filter to keep things legal but the high power they produce is a problem at most repeater sites. It may be possible to solve the problem with an elaborate filter at the front end of your repeater receiver using at least a 10" dia 1/4 wave band pass cavity filter or several smaller versions in series to make a very narrow band pass filter with minimal loss. You would also need at least a single if not dual isolator on the repeater transmitter and a similar band pass filter on the transmitter to keep the digital TV transmitter signal from entering your transmitter and creating IMD which may contribute to the problem.

I have seen a lot of repeaters in both VHF and UHF installed at high power TV transmitter sites where some are working fine with extra filtering and some that cannot be fixed. In that case you can either move the repeater frequency far from the TV transmitter frequency or move the repeater to another site.

 

[bcoleman1]

Many thanks for your terrific reply, appreciated. Hope all is well. rgds, barry

 

[Ubbe]

I had a case where a guy that manages towers and rented out space had an issue that his VHF antenna at one site that where used for several VHF receivers had lost coverage. He assumed it had to be an antenna problem so he let a company replace the antenna and the coax but it didn't help. As I knew him from setting up equipment in many of his towers he asked me to take a look at it.

The system measured fine from the antenna connector but as soon as the antenna where connected it lost some 20-30dB of sensitivity. I could see that the noise floor of a spectrum analyzer where rising up in the VHF band when the antenna where connected and as there where several antennas at the site I could see that the ones pointing more or less to a broadcast tower some 1km away had the most noise and I asked our FCC to take a look at it.

It tuned out that there had been a new DAB 220MHz transmitter installed but the bandpass filter that where supposed to be used where so heavy that they left it at the ground floor for a week trying to figure out how to get it to the site room at the top.

If there's a constant RF issue then it can be a continuous transmitter like broadcast or a control channel that could be the problem but if it is periodic then it, at least being one part of it, has to be a transmitter that gets keyed on and off in pattern that correspond to the times when there's a problem.

As it needs to be a strong transmitter to cause issues then you can even use a scanner with its own antenna and with a spectrum display feature or a SDR dongle to see if any transmitters are keyed up when the RF problem starts.

Our FCC doesn't even log a RF interference issue if a repeater doesn't have a circulator isolator on its TX, as all filters will be passing the TX frequency to the antenna and all nearby TX transmitters in the same frequency band will go straight from the antenna and into the TX circuits and starts to work like a diode mixer.

/Ubbe