Wireless signal strength explanation needed

Question

Could you kindly explain me or advise following please...

I have a TP-link WiFi router. When I measure signal strength just in the vicinity of the router- literally I touch my smartphone (with WiFi signal strength measurement application) to a WiFi router antenna - I get signal strength -15dBm. The manufacturer datasheet says - "WiFi transmission power <30dBm".

So why do i have -15dBm then just at immediate vicinity of the router ? Where does 45 dBm go? (from 30 dBm down to -15)

Thank you so much.

Answer 1

I can think of a few reasons why your WiFi signal strength is 45 dB down from what your router's documentation suggests.

1. Your phone and antenna aren't calibrated. Your app makes signal strength estimates using a guess about what kind of phone and antenna you're using, but that's just a guess: mobile phone handsets' WiFi antennas vary widely, and the orientation of the phone, how you're holding it, and where it is relative to other conductive objects can make a difference of many decibels. The signal levels given by your app are surely meant to be relative, so you can compare reception at one location to reception at another location, rather than absolute. An accurate absolute measurement of signal strength requires expensive calibrated test equipment with exact placement and orientation in a calibrated test range. Typically the test range includes a Faraday-cage enclosure that seals other signals out.

2. Your router may not be set for maximum power. Maximum power is completely unnecessary for most people, and more power contributes to interference in the common situation where there are many routers with overlapping WiFi coverage, so manufacturers often set the default power level to medium, rather than high. If you log into your router's command interface you can adjust the band, channel, and power level settings.

3. Then of course there is the free-space path loss; your phone isn't directly connected by cable to the router, so a lot of the power transmitted by the router is broadcast into space and bypasses your phone's antenna.

Answer 2

A Wi-Fi transmitter, like a lightbulb, throws out photons in a lot of directions. Only some of them will be captured by the receiver. Most will go elsewhere, eventually being absorbed by walls, atmospheric water vapor, and so on to be converted to heat.

The Friis transmission equation explains this in more rigorous terms.

Even if the two antennas are touching, it's unlikely 100% of the transmitted signal will go into the receiver. This is easy to verify if you have a third device, like a laptop. When you touch your phone to the access point, does your laptop entirely lose the WiFi signal? If all the power was going to the phone there would be none left to reach the laptop.

Perhaps if you connected the transmitter and receiver directly with a transmission line, you would receive nearly the full power. However this would almost certainly not work, since the receiver is not designed to handle that much power. The receiver may even be damaged. -15 dBm is an extremely good WiFi signal.

Answer 3

First, the router power will be +20 or +22 dBm. Subtract a few dB on the way to the antenna, more if it's an internal antenna.

Second, the free space path loss will be at least 20 dB, from phone to router, a few cm apart. This means that even held close by, the phone antenna only captures 1% of the transmitted signal. This will be made worse by the polarisation not being well matched, you might need to rotate the phone to get maximum coupling.

Also, the phone antenna probably has a gain of -2 dBi, being so compact and squeezed in to the phone with several other antennas.

But most importantly, your phone probably can't measure powers above -15 dBm anyway. That's off the scale for a wifi chip, which usually operates between -30 and -90 dBm.

Try about 1 m away, rotating the phone to find the maximum, then you might get more realistic numbers. I get about -25 dBm which is as expected.