Background:
I have a unique need to have two commercial WiFi routers wirelessly bridge with one another over RF cabling (coaxial with SMA connectors) rather than using a standard antenna. The idea is to remove Router A's antenna and connect a cable to the antenna port. On Router B, disconnect its antenna and connect the other end of the cable to Router B's antenna port. Repeat as necessary for all antenna ports (4 in my case; a 4x4 MIMO router).
When attempting to use female/female RF cabling with SMA connectors to the antenna port directly, I seem to be saturating the transceiver. As a result, I was looking into purchasing an attenuator (4 for each of the 4x4 MIMO antenna connectors).
Question:
How do I determine the required dB of attenuation for a 4x4 MIMO 5 GHz antenna port so I can hard-wire the two radios (via SMA RF cabling) to simulate a wireless link between their antenna ports?
Reference:
A user on superuser.com had a similar inquiry, but the response is quite dated and there isn't sufficient explanation on why 60 dB of attenuation was recommended.
See post here: https://superuser.com/questions/300715/connecting-a-wifi-router-to-receivers-with-a-cable-instead-of-antenna
******EDIT:******
For those interested, I have performed the test using 60 dB fixed attenuators supporting up to 18 GHz. When using the 5 GHz band, we were able to successfully record data transfer rates of exactly what we expected in a wireless bridge configuration. The only issue we saw with our configuration was the fact that there was leakage from the SMA connector leads, so clients that were not connected via the wired SMA/coaxel cable were also able to connect to the server.
I cannot give you a precise answer, but I can help you ballpark an answer that will work.
Lets do some simple free space path loss calculations, wikipedia has a lot of information on the theory, but I usually just use an online java script calculator. Using that calculator I put in the following information
Calculate path loss, and you get: -62.9dB. So that is the attenuation that you would naturally get with the above configuration. Thus I would think a 60 dB attenuator sounds just right. :)
Also to back up the numbers, IEEE 802.11 (WiFi) usually allows transmission of about 20dBm, and the sensitivity of a receiver is usually in the range of -55dBm to -80dBm (this is from experience, I don't have a canonical source, but look up any IEEE 802.11 module product information sheet).
This means that:
transmit_power_in_dBm - path_loss_in_dB > receiver_sensitivity_in_dBm
or the path loss will be to large for the receiver to receive anything. Conversely the path loss must not be too low, or the amplifier in the receiver will be saturated (and my actually be harmed). So I would suggest you aim for somewhere around:
attenuator_in_dB ≃ transmit_power_in_dBm - receiver_sensitivity_in_dBm - received_signal_strength_in_dB
If we put received_signal_strength to 40dB to get a signal that is 40dB above what is at minimum needed to receive anything, we get:
20dBm - (-70dBm) - 40dB ≃ 50dBm
I.e. attenuation of 50dBm, somewhere between 50dBm and 60dBm looks very appropriate. :)
Note: By connecting the wifi units directly, without attenuation, you might already have broken the receivers in both units. (Specifically the low noise amplifiers.) So before you proceed using the units, test them again with their antennas connected. You might have to buy new ones.