Subscriber Discussion

RF Signal Not Near Zero

KJ
Kenny Johnson
Feb 15, 2017

So I have two test radios set up in this room for testing.

They are both set to WDS Bridge and are connected via MAC address.

They are EnGenius enh500's.

The signal on both of them is around .-40

I wonder why I cant get the signal close to 0 even with them in the same room?

 

U
Undisclosed #1
Feb 15, 2017
IPVMU Certified

The signal on both of them is around .-40

-.40 is pretty close to zero though.

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Avatar
Jon Dillabaugh
Feb 16, 2017
Pro Focus LLC

-40 is plentystrong enough. In fact, much lower attenuation than that could lead to damaging the radios. 

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BS
Bob Schenck
Feb 16, 2017
IPVMU Certified

If you are talking about RSSI readings in decibel milliwatts (dbm) then you can't reach 0 on Wi-Fi reception strength.  As Jon points out that would likely overload the receiver.  -30 dbm is pretty much the max I've ever seen on a Wi-Fi signal strength meter.  

The best explanation of dbm range for Wi-Fi is this explanation from Microsoft that explains how Windows converts raw measurements into a 0-100 signal strength rating.

A percentage value that represents the signal quality of the network. WLAN_SIGNAL_QUALITY is of type ULONG. This member contains a value between 0 and 100. A value of 0 implies an actual RSSI signal strength of -100 dbm. A value of 100 implies an actual RSSI signal strength of -50 dbm. You can calculate the RSSI signal strength value for wlanSignalQuality values between 1 and 99 using linear interpolation.

So any reading of -50 dbm or higher is considered a full strength connection by Windows.  Any reading under -100 dbm is considered no signal.  

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SP
Sean Patton
Feb 16, 2017

Getting a reading of 0 would possibly be catastrophic; long term exposure to high level input signals can actually burn out the input of the radio circuit. Its not real likely without high gain antennas, but still not a good practice. (I think Jon was alluding to this).

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UI
Undisclosed Integrator #2
Feb 16, 2017

 

I would be very careful setting up radios with directional antennas at a distance of less than 75'.  Oversaturation of the receiver and permanent damage becomes likely after even just a few minutes of transmission.  Also, once any PtP link is installed, you really should decrease the Tx power on each leg if the RSSI is above (lower) than -58 dBm.  Achieving a -42 signal will not provide any additional throughput or stability and will lead to premature failures.

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KJ
Kenny Johnson
Feb 16, 2017

Thanks for all the responses!

So what should the signal be ideally?

I read that closer to 0 is better...

But these posts seem to say that the signal should never really be close to 0

BS
Bob Schenck
Feb 16, 2017
IPVMU Certified

"Closer to zero" is referring to the fact that WiFi received signal readings are going to be below 1 milliwatt, and therefore are negative numbers on a dBm scale.  You can see how dbm relates to actual milliwatts on the table below.  So a negative 40 is a much better signal than a negative 90 because it is actually 100,000 times the received power level in wattage.  Because the working power range for Wi-Fi signals is so large, the dBm scale makes signal readings easier. 

So -40 dBm is closer to zero than -90 dBm and therefore a stronger signal, however it is possible for a signal to be too strong.  The decibel scale takes a little getting used to because even though we always talking about positive wattage values, the dBm scale goes negative for readings under 1 milliwatt.

 

 

By the way, I think this is a great topic that doesn't get discussed enough in Wi-Fi networking.  Thanks for bringing up your example.

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CB
Cullen Brannan
Feb 16, 2017

Thanks for the explanation/table Bob that actually makes a lot of sense.  I knew the target for most radios was between 40-50 dBm, but I didn't understand the relation to power. 

Avatar
Jeffrey Hinckley
Feb 16, 2017

-40 is a great signal.  Most radios will self adjust to signals -40 to -50 (these are the optimum signal levels).  Signal levels in the -20 range or stronger could actually damage the radios.  The limits (threshold) are normally about -65 to -70 before the radios will downshift modulation schemes.  Worst signal is normally around -80 to -90 (and will have major problems if noise is present).

RSL = Antenna1 + Antenna2 + Power output - FSL (free space loss) - Other (connector, line loss, environment)

FSL = 20logd (KM) + 20logf (MHz) + 32.44

So for example, lets say you have a radio, 5.8 GHz, 16 dbi antenna, 10 dbm output, 0.5 km link. 

FSL = 20log(0.5) + 20log (5800) + 32.44 = 101

RSL = 16 + 16 + 10 - 101 = -59 (if the threshold is -70, you have 11 db of "fade margin")

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SP
Sean Patton
Feb 16, 2017

And I just noticed Jeff's message, after I typed this up:

50-60 Great

60-70 Good

70-80 Okay

80-90 Maybe

90+ Nope. 

These also depend on how noisy of an environment you're, and how good of a radio/antenna structure you have. 

Wireless is something that's easy to attempt and easy to get working in a limited environment, but very difficult to get perfect. Outdoor wireless amplifies (pun intended) this difficulty 10 fold. Especially after climbing to the top of a water tower, when its cold enough that there's snow on the ground.

 

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