Questions about dBs, dBis and dBms in WiFi

[Mitrixsen]

Hello, everyone.

I am studiyng for ENCOR and I have questions regarding watts and decibels. Fortunately, it’s only theory since I understand the calculations.

My book says the following:

I get what’s happening here but not the explanation behind it? If we use the logaritmic comparison function for dB, we will find out what the loss is in dB, which is -65.

So what are they trying to tell us in the later paragraphs? Like they’ve just converted the watts into dBm which makes it easier to work with but what is this page trying to say? I am not quite wrapping my head around it.

The second pages say the following:

I don't get this part:

By itself, an antenna does not generate any amount of absolute power. In other words, when an antenna is disconnected, no milliwatts of power are being pushed out of it. That makes it impossible to measure the antenna’s gain in dBm. Instead, an antenna’s gain is measured by comparing its performance with that of a reference antenna, then computing a value in dB.

Nothing generates power when it’s disconnected.. Are they trying to say that the antenna doesn’t produce any power on its own but it rather radiates the power the transmitter sends to it? So we use a different unit here, dBi to represent the gain/loss?

Thank you
David

[Scott Fella]

It always gets confusing, but, dB as you know is a general unit for just measuring relative power, while dBi is specifically used to describe the gain of an antenna relative to an isotropic radiator.  So when you are trying to calculate EIRP on a link, you would use dBi in your calculation to subtract any cable loss, from the transmit power your unit is configured for and the manufacture gain.  An isotropic radiator is a theoretical antenna that radiates uniformly in all directions without any focus or gain, which is just a starting point for you.  You will never see a perfect uniform pattern in the real world but this gives you a base to help understand your RF.

You just have to search around and also look at outdoor wireless in your search to get a better grasp of dB vs dBi as that should eventually help.

https://www.data-alliance.net/blog/dbi-db-dbm-dbmw-defined-explained-and-differentiated/

https://lte.callmc.com/understanding-antenna-db-dbm-dbi-and-dbd/