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Joined: Sun Jun 11, 2006 7:34 pm
Location: Hilleroed, Denmark

Measuring and calculating signal strength from interfering neighbor - Regulatory measurement methods debatable?

Wed Jul 01, 2015 2:46 am

Hi, We have a problem here in Denmark.

Set up a large camping site with 20 transmitters (mainly 2,4GHz Groove), interconnected with fiber and some with 5GHz Sextants MIMO links. All was working fantastic until a new neighbor arrived on the scene, selling long distance access using 2,4 and 5GHz Ubiquity equipment.

Now we suffer major problems with especially upload. When they turn off the transmitters all is fine again.

We agreed with them on the 5GHz - we don't really need 1W transmitting power so we simply put the links down in the 5,2GHz area - links are fine and test upto 200/200Mbps trafr.

Regulatory in Denmark require 0,1W max in 2,4GHz and 5,2GHz areas and up to 1W in the 5,7GHz.

On the 2,4GHz accessnet however all is rubbish now. They put the links in "in between channels non 802 mode" so the only way to spot them is by spectral analysis. Routerboard to our rescue ;-) We see them fine!

There is about 500 meters from our site to their mast of interference. In my understanding of how far 100mW in the 2,4GHz will allow connection, we should hardly be able to measure them in that distance?
We see them with at least -50 to -60dBm all over the place - if measured by SXT we see a gain of -40dBm.

Again, I think that would interfer with anything so we called the local "noice police" - the government assigned specialists - to do a on site measurement of the interference.

Here is the problem:

I am not a "Spectral expert" but simply rely on my "common sense and experience" when it comes to signal strength and how to measure them.

The "expert" brought along a parabolic antenna and measured the signal 462m away from the mast - he concluded that with a signal strength of 44,6dBuV (-62dBm) - the signal would be 158mW on the transmitter.

His calculation:

Spectrum analyzer Tektronix SA2600 (ResBW = 3MHz and VideoBW = 10mHz)
34cm parabolic antenna 2,3 - 2,5GHz gain 15dBi
Cable: RG213 4 meter loss at 2,45GHz = -0,5dB
Connectors: 2 pcs. loss -0,5dB
Adjustment of 5,2dB (10log(10/3)) as the ResBW is 3MHz and not 10MHz

His formula to calculate ERP:
P(rx) = measured value in dBm (dBuV = -107dB)
P(gain) = Antenna gain + other loss
P(Rout) = P(rx) - P(gain)
lambda = 0,122m (3*10^8 / 2,45*10^9 = 0,122)
Distance damping: D(L) = 21,5 + 20 log(distance in meter / lambda)
P(total) = P(Rout) + D(L) + 5,2

Measurement 1 (control site):
Distance = 100meter
Measured value = 54dBuV (-53dBm)
Freq: 2,434GHz

Ptotal = -53 - 14 +21,5 + 20log(100/0,122) + 5,2 = -53 - 14 + 21,5 + 58,2 + 5,2 = 17,9 dBm
Conclusion: P(total) = 62mW

Measurement 2:
Distance = 470meter
Measured value = 44,6dBuV (-62,4dBm)
Freq: 2,434GHz

Ptotal = -62,4 - 14 +21,5 + 20log(470/0,122) + 5,2 = -62,4 - 14 + 21,5 + 71,7 + 5,2 = 22,0 dBm
Conclusion: P(total) = 158mW

So his conclusion is that all is fine.

I boosted up a Groove (1W) to the max with a 6dB antenna attached. He measured it to be legal - just around 100mW he concluden.

Boosted up 2 more (1W, no reduction for antenna and Current TX rate show -20dBm) - he still saw absolutely nothing illegal in this setup.

Please help me out here - either I am 100% misunderstanding all I have learned about WiFi the last 20 years or something is simply not right in those measurements.

I send him home with the clear understanding that I would advise him strongly to recalculate his measurements as I really believe his calculator to be broken.
I also made it clear to him that if he were to "bluestamp" these measurements, I would no longer have any problems making 4-6km access legally in the 2,4GHz area - This is entirely possible with the gain he finds "within legal boundaries" - He will simply revolutionize building rural long haul networks here.

I also wondered how he could at all assume to me measuring effect that way. As I understand it, e.i.r.p should be measured in a very short distance form the radiating antenna - e.g. 1 meter.

He agreed that there might be errors and that "common sense" talks against being able to do long haul 2,4GHz with as little as 100mW - My experience say "no more than max 100m" - but then again - I may be wrong...

Am I simply stupid or does the authorities not know how to measure here in Denmark?

Any and all help is highly appreciated. Our problem persist - The signal is still blocking our network and if I fail to prove him wrong, we are facing MAJOR problems...

Thanks in advance!

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Re: Measuring and calculating signal strength from interfering neighbor - Regulatory measurement methods debatable?

Fri Sep 25, 2015 2:55 pm

Hi there

What I believe is happening is an incomplete understanding of measure setup

The link below is a video that describes resolution and video bandwidth

It is relevant since what the instrument measure depends on the "setup" and the signal behavior

Let me give an example: Assume the Tx "jumps" between frequencies every 1ms
If the analyzer "stays" on a frequency for 2ms it will "see" only half of the power, since the other half the transmitter is on another frequency.

Ideally, what you wish is to capture the actual power out of every single instant, but to do this you must be "sync" with the actual transmitter behavior.

This opens the door to some "juggling" by a transmitter that uses non standard "protocol", kind like what happened for the diesel gate for cars....

Hope the above helps

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