Front to Back Ratio: [Definition, Examples + Role] in Directional Antenna by Nourishing Tech Appetite

This is a related post of “How to choose the best antenna for airtel jio etc”. You can read it if you wish and then you can come to this current post which is the second part but it is specially focused on directional antenna. 

 

The main objective of the post is to provide you more ideas on directional antenna like how greater front to back ratio(F/B) impacts on the performance in antenna, its definition and the simple ways to check this ratio to evaluate the performance.

 

Here we have used Question and Answer session to grasp more ideas on the topics. So, let’s begin.

Q-What is front to back ratio of Directional Antenna?

Ans-Directional antenna is designed to radiate most of its power from main/front lobe to forward direction rather than rear direction to get most of the signal from front direction. It is designed in such a way to get signal from a specific direction, it means you can use it to optimize signal from specific directions according to your choice.  The ratio in between front signal power and back signal power is called the F/B ratio. It is a measure through we can check the quality of antenna and greater F/B ratio is always an indication of good directional antenna.

 

Q- How high F/back ratio contributes in getting good signal?

Ans- If forward radiation power is better and greater than its opposite direction then, it is said as a good or greater front to back ratio and, the performance of antenna is also increased. This happens because of its ability or features to minimize or terminate the signal that comes from opposite or other direction, so as to work great in only forward direction.

Let’s get into it to get clear the views on the topic. The antenna’s performance is determined by its F/B ratio and can be calculated through a formula:

Front to back ratio = (Pfront/Pback) 

Front to back ratio = Pront- Pback

The radiation power strength in the forward direction is indicated by Pfront and the power strength of opposite direction is called Pback.

Q-Determining a good directional antenna or how to know whether the antenna is good F/B ratio or low F/B ratio

Ans-Suppose a performer giving a speech on a stage and you are sitting and facing towards the performer. You want to listen but noises are coming from back like talking and murmuring of others audience. To listen carefully you have to focus on performers’ voices and at the same time you also have to ignore the noises coming from opposite direction or surroundings.

Here in this example the talk of the performer is like a signal that you want to listen so, it can be assumed as a desired signal which is coming from the forward direction

Again, you want to focus on performer’s voice but sound of others audience are distracting you. In this case, the sound which are coming from behind or surrounding will be assumed as an unwanted signal that you should ignore to listen clear.

In this circumstance your ability decides that how good and clear you could listen while rejecting others noises.

Let’s see other connected facts which could affect the situation. If the performers’ talk is loud enough than the other distracting sound and your hearing capability is good enough then, you can easily listen to the performer. So, it is like greater F/B ratio where your capabilities show that you can listen well and focus on the performers’ voice and other noise does not affect you.

On the other hand the low front to back ratio shows the circumstance where the performer’s voice is too low and distracting sounds of audience is too high. In this case, listening to the performer will be difficult as well as challenging for you.

The same rule is applied to directional antenna. The higher F/B ratio indicates the ability of antenna, where, it could manage the incoming and outgoing signal in the desired direction very well and reduce the unwanted signal from opposite direction to give a good performance.

Q- How to know whether your antenna has high front to back ratio or not? 

Ans- To know this, professional tries several tests to get the exact value which can be hard for normal people because it needs some equipments and software like signal meter testing, field testing and antenna patterns testing etc.

Q- So, is there any simple testing guide that can be done by simple users?

Ans- Yes, we can do it. The process is very simple and only we need a signal indicator that is already available on device like router or modem with external antenna port. Therefore, I am giving a simple guide through which we can test the performance and monitor its signal strength. Let’s see the how.

Note- This test alone can’t give detailed or accurate measurement. If you want fast network then so many indicators, application and tools are needed to get the quality signal strength.
Just attach the external antenna to the modem and connect with a computer. 

 

Go to the setting of your modem. The setting may vary according to the device and manufacturers.  We have to open the setting where we get device information. See the example.

 

Now you have to do some field testing. You just need to put the antenna facing to the location of your desired cell tower. Adjust its angle till getting the best and strongest signal.(greater rsrp) After placing the antenna just monitor the indicator (you can see RSRP, Reference signal receive power) value. (it is indicated in dbm). The value you get is the Pfront(signal Power received from the main lobe). Note it down for further calculation.

Field testing 2- Now you have to rotate your antenna to 180 degree and by doing this the antenna‘s back portion will be to the direction of tower. Check the indicator to get dBm value. The value you get is the Pback(Power of signal of back lobe of the antenna). Jot it down.
 

Compare those received value. You will see the huge difference and the big difference is considered as a good antenna having higher front to back ratio.

Let’s see some examples

Suppose you point your antenna to the desired cell tower and get RSRP value -50dbm and when you face the back portion to the tower then you get RSRP -70dbm. Here dbm is a unit to measure the power level of signal strength

Here we have got two received signal strength which are-50dBm and -70dBm. Now let’s convert the both value in to miliwatts (mW). For this we will use the formula

Power (in mW) =10^ (dBm/10)
We will put -50dBm in formula
Power (in mW) =10^ (-50/10)
Power (in mW) = 10^ (-5)
Power (in m W) = 0.00001 mW
Therefore the received power of -50dBm is = 0.00001 mW

The same conversion we will do for -70dBm
Power (in mW) = 10^ (-70/10)
Power (in mW) = 10^ (-7)
Power (in mW) = 0.0000001 mW

Now when we compare these two power which are -50dBm = 0.00001 mW and -70dBm = 0.0000001 mW then we find that -50dBm received signal power is stronger and greater than -70dBm-

Example 2

Suppose Pfront is -30dBm and Pback is -80dBm then we will convert the value to miliwatts by the formula

Power (mW) =10^ (dBm/10)
Placing the value -30dBm in formula
Power (mW) =10^ (-30/10) =10^ (-3) = 0.001 mW

Conversion for -80 dBm
Power (mW) =10^ (dBm/10)
Power (mW) =10^ (-80/10) = 10^ (-8) = 0.00000001 mW

From the above example we have found that the received power of -30dbm is greater than -80 dBm

Now let’s calculate the front to back ratio
We would use the formula 

          
Front to back ratio (db) = pfront- Pback
Putting the value in formula we get
Front to back ratio (dB) = -30 - (- 80) = -30 + 80 = 50dB

So, the answer is 50 dB

Now let’s find out that how stronger is Pfront from Pback in power level. For this we can use formula – Power Ratio = 10^ ((Pfront – Pback)/10)

Putting the value in the formula we get
Power Ratio = 10^((-30 – (-80)/10) = 10^ ((-30 + 80)/10) = 10^ ((50)/10)
= 10^ 5 = 100000

It means the Pfront is 100000 times stronger than Pback in power level. 

Though high front to back ratio in antenna plays an important role, there are many factors which can affect its performance in real world. We can disscuss it in another post till then if you know some you are welcome to give in comments.