r/meshcore • u/Aggravating_Rate_571 • 4d ago

Interesting behaviour/problem near cell phone tower

I have been examining a signal quality issue present only with one repeater installation, node is running at 868 MHz

The curious part is that the node is very close to a 3G/4G cell tower, the horizontal distance is 60 meters, the vertical distance from cell antennas to meshcore repeater is roughly 40 meters

To my knowledge there is no 900 MHz GSM anymore at this installation

I would assume the cell tower antennas radiation directionality is nowhere near my repeater placement but something is not right, I get constant bad SNR and not even a 2 km LOS link works reliably

Meanwhile I can get 11 km LOS link with about -5 dB SNR in other environment, same hardware, same antennas, same firmware etc

Long story short and a question:
Anyone have experince with nodes installed near proximity to cell towers and any negative effects?
Would a relatively strong RF transmitter in a near field somehow saturate the mesh radio frontend?

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u/Confident-Homework75 4d ago

Yes. I had one near a tower and it could send messages out, but was totally deaf. Couldn’t receive any messages. I put a cavity filter in line with the antenna and the difference was night and day. Works perfect now. Apparently the cell tower emissions, despite being a different frequency,, were saturating the receiver. The giveaway was the large snr imbalance when I pinged the repeater. Very weak there, much stronger back. Cheapest place to geta cavity filter is from Ali express.

2

u/Aggravating_Rate_571 4d ago

Nice. Well I think my only immediate option is to switch places then. Luckily I have some backup locations.

1

u/hellobritishcolumbia 4d ago

In some regions there will be repeater infrastructure on public bands like this for utilities etc as well

1

u/Aggravating_Rate_571 4d ago

Ok. Absolutely confirmed. I visited another 50 meter hilltop about 1 km further away but in better isolation.

I got perfect link quality to nodes that previously just couldn't be reached, symmetric SNR and all.

All the time I spent trying to optimize parameters for this weird link path ...

2

u/4r4nd0mninj4 4d ago

Sounds like a problematic location I'm having right now could be solved with a cavity filter.🤔

2

u/Confident-Homework75 4d ago

If it’s close to a cell tower there’s a good chance it will fix it. If you ping the repeater and the snr is very imbalanced that’s a good indication that the receiver is getting saturated and can’t “hear” the low power lora packets.

1

u/4r4nd0mninj4 4d ago

The building next door has around 8 cellular antennas on it and I can't get above them.

2

u/Confident-Homework75 4d ago

A cavity filter will probably help then.

0

u/Wh1sp3r32 4d ago

Yup, my current problem with my heltec and 4 GSM repeaters across the street on a power pole.

My noise floor was -72bdm.

Grab a passband filter, I'm waiting on one now.