Interference on 2 metres

[VK7HH]

Further to my previous posts here - http://www.vklogger.com/forum/viewtopic ... 23&t=13824

Initially I thought, as per the posts above that my receiver was deaf. However, yesterday whilst operating the field day, I found it increasingly annoying that I had so much noise on 2 metres. This stayed around almost the entire time, and made it very difficult to copy weaker stations on 2m.

Last night, the noise had disappeared. I checked around 4:45pm local here today and it also was not present. Then 10 mins later it had come back. The issue presents itself as just general noise with the S meter rising to S5-8. It also varies with frequency. I can tune up and down the band and it can disappear, although yesterday it seemed to be affecting the whole band. Beaming in any direction does not cause a reduction in noise.

As this is located at a remote site, naturally I looked up the ACMA database to see what transmitters are located on site. The only transmitters that seem to run for long duration on site are trunking at 850 MHz. The control channel transmits 24/7 into a high gain vertical just a few metres away. It is possible this is the cause, however since it's running 24/7 and the interference does disappear, it's unlikely.

Possible powerline noise perhaps? The powerlines are located about 30-40m away. As it's a remote site, it's hard to tell if the noise disappears when it's raining.

Actually.. just as I type this, the noise has completely disappeared. Just trying to identify the cause so I know whether putting a bandpass filter on the radio will actually help the situation. 432/1296 are unaffected. I've also been over nearly every plugpack, switchmode transformer, ethernet switch and I'm confident after turning off every device except the radio that the noise is not generated externally on our site.

It is worthy to note how the system is built.

LFA 8 element yagi > RG 213 tail to LDF-450 feedline > Tokyo Hi Power HL-160V amplifier > 2m/70cm MFJ diplexer > FT857 radio.

Here is a recording -
https://www.youtube.com/watch?v=iUqe0XsHxQw

[VK3ALB]

Hi Hayden.

A couple of years ago I started to noticed the S meter on 2m and 70cm on the rigs in my study bouncing to full scale. Around the same time we started getting TV interference. Short story, a pole some 70m from our house was causing interference. Usually on dusk for a few hours but never if it was raining. I ended up making a real nuisance of myself to the local power authority who finally replaced the pole hardware and the noise disappeared. You might have to go to site and DF the interference. You video is similar to what I heard.

[VK7HH]

Thanks Lou,
I suppose compounding the issue is the fact the remote site is located on a mountain top, with poles that are not so easy to access. That said - it might be a good leverage (i.e. interference to a comms site).

What DF techniques did you use? Next trip I’m going to stock up on equipment. Already been around the site without much success with a handheld on AM. We also have a remote scanner on the air band too - that’s also getting significant interference.

[VK3ALB]

Hi Hayden,

The inspector that came out to investigate used a scanner with AM facility with one of those small el cheapo log periodic TV antennas. He also had a big rubber mallet to hit the base of the pole and had some TV test gear to measure the BER in our house. I did the same things using my FT817 and a 3 el 2m yagi with a stepped attenuator. Amazing how easy it was to pinpoint the faulty area on the pole.

[VK7HH]

Thanks Lou!

I'll add it to the kit to go for a search. We have a tape measure yagi used for direction finding, so that will work well.

Regards
Hayden

[VK5PJ]

Hayden,
does it sound like switch mode noise? I have it from the solar charger, my Victron Blue wanders across the band

[VK7HH]

Hi Peter,
Not so much, it's not really got any sort of defined noise to it.

We have checked our switchmode power supplies in our hut. The only thing not tested was a Victron changeover charger/inverter. This however is 99% of the time on float and located inside the metal building.

I've been told that the Toyko amps can oscillate, perhaps that is a probable cause? In any case, I think a trip to the mountain is required. Noise wasn't there this morning, but it's back now. We haven't had any rain for a while so difficult to test from here if it's the powerlines.

[ZL4TAE]

Hi all
This sounds to me like front end overloading from some other service in VHF band
Amateur radio equipment like many scanners often have virtually no front end selectivity.
And it may be it could be miles away
In my experience, incoming signals stronger than -40dBm are likely to cause front end overload
If this is the case then a cavity filter will soon help you out
Otherwise you may need a spec an to see whats out there
Good luck, Terry ZL4TAE

[VK7HH]

Thanks for your input Terry.

I have a cavity ready to go. I can't leave it up there as the return loss is poor at one end of the band 144.1, but good up the top end, 144.5 etc.

I'd have to build or purchase a proper bandpass filter for the 2 metre band, but before doing so I wanted to first find out the source of the noise.
Interesting thing is that when it appears, it is fairly constant, doesn't disappear for a long time. It also sort of peters out over a duration, maybe a few minutes from what I've heard. So, I do have my doubts on if it is front end overload as if it was a voice channel on a certain frequency then I'd expect it to pop up, then disappear, then pop up again which it doesn't do. If it was a data channel, then it would be there all the time, theoretically.

If it were a mix of two or more transmitters, then I'd expect the same behaviour as a voice channel as described above. Moving my yagi around doesn't help, indicating it's probably in the near field of the antenna. More investigation required.

Hayden

[VK7HH]

So I finally made it to the site today to undertake some testing.

First I did a sensitivity test of the radio, -120dBm for 12dB SINAD on FM. As per specs.
Then I inserted an iso-tee or RF sampler. The isolated port is about 30-35dB down and I placed a good 50 ohm load on the antenna port. Turning up my RF generator I came to a figure of -87dBm. So the actual number was 33dB.

I then removed the load and replaced it with the antenna feedline. Immediately the receiver went quiet. Turned up the generator a further 15dB before I arrived at my 12dB SINAD measurement. This indicates the noise floor degredates the receiver by 15dB over what the bench sensitivity is, so effective sensitivity is 105dB. This explains why my receiver is deaf.

Now the next challenge - find why!

I hooked up my spectrum anazlyer and there was no noticeable spikes in the VHF band. Only low signals, presumably from other sites. I could barely see the FM transmitters on a mountain about 20km away. The strongest signal was a 860MHz trunking transmitter that is about 10 metres away. The signal measured was -40dBm.

I inserted a cavity filter I had on hand and this made no discernible difference. So I guess I can rule out front end overload.

It’s looking more likely to be a bad switchmode supply or perhaps Ethernet noise or a clock from nearby IP links. These are difficult to troubleshoot as they can’t be switched off.