RFID Antenna & Tech questions

[Altruistic69]

Hi,

I'm a project manager for one RFID company, I have been reading alot about the technical part for my own understanding but I still couldnt understand some / most part of it. Appreciate if you guys can help me with some questions, I'm a beginner.. so please use simple english when explaining to me.

1) Currently I'm doing UHF RFID project, in my country there's a regulation by the government which stated the maximum EIRP is 2 Watt.
a) How do i calculate it? I mean how do I know whether my team is following this?
b) Appreciate if you can explain to me what is EIRP & ERP (the difference too) and the sub elements that I should understand in measuring it.

2) I read about antenna gain and some other stuff that important during configuring or purchasing antenna (7dBi / 9dBi / 12dBi). How should I go about this? How do I choose?
a) What is (dB)? How do I calculate it?
b) What is (dBm)? How do I calculate it?
c) What is (dBi)? How do I calculate it?
d) What us (dBd)? How do I calculate it?

 

[roxxon]

1 b) EIRP = Equivalent Isotropic Radiated Power, ERP = Effective Radiated Power (without the "isotropic")
That is a power, which would have to be connected to / radiated by an isotropic radiator (EIRP) or a dipole (ERP) to get the same field strengths. What the government (or whoever) actually wants to restrict are the field strengths - but they vary and depend on the distance to the radiator. So, what actually is regulated is the total radiated power, implicitely. I suggest you consult wikipedia or your books on further information and examples.

a) There are various ways. If you know the directivity of your antenna you can simply measure the received power (with an antenna with known parameters) and calculate the radiated power using the free space loss (approximatively).

2a) dB is a logarithmic measuring unit always applied to a (power) ratio. In the general case: dB = 10 * log10(Pin/Pout) gives the attenuation in dB instead of the linear % which you get by the ratio directly.
2b) dBm is the dB ratio with respect to 1mW (therefore, the small 'm'): dBm = 10 * log10(P/1mW)
2c) dBm is the dB ratio of an antenna gain with respect to the isotropic radiator (therefore the 'i'), which has a gain of 1 (linear) = 0 dBi.
2d) dBd is the dB ratio of an antenna gain with respect to the dipole antenna (therefore the 'd'). Since the dipole has a gain of about 2.15 dBi this means: dBd = dBi + 2.15dB

...hope that helps. Some of those basics you should definitively find in any good RF book, wikipedia or whatever.

 

[Altruistic69]

Okay pls correct me..

EIRP = Transmitter power output (dBm) + Antenna Gain (dBi) - Line Loss (dB)

EIRP
Measured in dBm (decibles in miliwatts) / Watts

Antnnea Gain
Measured in dBi (decibels referenced to an isotropic antenna)
How to know / calculate this?

Cable Loss
Measured in dB (decibels)
How to know / calculate this?

Transmitter Power Output
Measured in dBm (decibles in miliwatts) / Watts
How to know / calculate his?

Since the country's regulation for EIRP is 2 Watts, therefore the calculation will change to:
2 Watts / 33 dBm = Transmitter power output (dBm) + Antenna Gain (dBi) - Line Loss (dB)

Correct?

 

[tony_lth]

Okay pls correct me..

Antnnea Gain
Measured in dBi (decibels referenced to an isotropic antenna)
How to know / calculate this?

You can measure it. Or check the antenna specifications.

Cable Loss
Measured in dB (decibels)
How to know / calculate this?

You can use network analysis or power meter or spectrum analysis to measure it.

Transmitter Power Output
Measured in dBm (decibles in miliwatts) / Watts
How to know / calculate his?

You can use power meter to measure it.

Since the country's regulation for EIRP is 2 Watts, therefore the calculation will change to:
2 Watts / 33 dBm = Transmitter power output (dBm) + Antenna Gain (dBi) - Line Loss (dB)

Correct?

yes., Correct.

 

[Altruistic69]

Transmitter Power Output : This one we normally set/configure at the reader correct? If there are 4 anttenas connected to the reader, and let say I set to "A" power output, does it mean "A" power will go to each reader or it will be distributed equally? And the EIRP is the calculation for all antennas or single antenna?

Antnnea Gain : This one normally given & shown in the market, such as "UHF RFID Linear Polarization Antenna 7dBi/9dBi/12dBi". And for circular it will be "UHF RFID Circular Polarization Antenna 7dBd/9dBd/12dBd". Is this correct? So it doesn't mean I always have to go for higher dBi gain during purchasing (like megapixel), I will only go for higher gain antenna such as 12dBi / 12dBd if there should be alot of line loss (perhaps due to low cost cable) and even if I choose to go for the highest dBi gain antenna 12dBi, I'll need to make sure it will still be in 2 Watts range of the EIRP?

Cable Loss : Do you mind pasting me some websites for this kind of device? Thanks.

EIRP and ERP actually measured in dB or dBm? I'm getting different answer from the internet.

EIRP is power radiated by Isotropic antenna & ERP is power radiated by Dipole antenna. Which one is isotropic and which one is dipole? I have Motorolla fixed reader, CSL fixed reader, SiRIT fixed reader, how do i know which is which and whch calculation to use (EIRP / ERP).

Thank you very much guys, sorry if my understanding is low. I'm not for engineering / technical background, I'm more to business that wish to learn.

 

[lambdaanttech]

1. It depends how the antennas are connected . Are they all 4 combined ? If you have 1 transmitter with 4 combined antennas then the ERP = XdBm (tx power) + YdB (total gain of the antennas) . Roughly you get 3dB by combining 2 antennas , combining 4 antennas means aprox. +6dB more gain over the gain of a single antenna .

2. It easier to calculate with dBd , because when it actually comes to real measurements you will be using a "real" antenna such as a dipole for example .
2,15dBi = 0 dBd , dBi = dBd + 2,15 and dBd = dBi - 2.15 . Often circular antennas are expressed in dBc (reference compared to orthogonal circular antenna) .
Higher gain antennas will be used it your path loss increase , you need 6dB more gain for 2x distance .

3.Cable loss can easly be obtained by the specs of the manufacturer or measuring it on a network analyzer .

4.EIRP or ERP is always expressed in dBm , you are refering a ratio (dB) to 1 miliwatt (m)

5.An isotropic antenna is a fictive antenna that doesnt exist .An isotropic antenna is a fictive point that radiates equally in each direction . It doenst matters if you calculate with EIRP or ERP they are both the same since EIRP is calculated with the RF power in dBm + antenna gain in dBi and ERP is calculated the same power in dBm and antenna gain in dBd . The difference is always 2,15 .
I suggest to calculate with dBd values , its easier when it comes to on the field measuring .


Hope it helped .

Best regards,
Kenny

 

[Altruistic69]

I have a fixed reader, normally a reader can be connected to 4 antennas right? so the power will be distributed equally & should i calculate erp for each?

 

[Altruistic69]

I have difficulties understanding this.

Assuming the Power = 1 Watt (30dBm)
Assuming the Antenna Gain shown by the hardware specs in dBi = 5dBi
Assuming signal loss = 0

EIRP = 30dBm + 5dBi - 0 = 35dB (3.162 Watts)
ERP = 30dBm + (5dBi - 2.15) - 0 = 32.85dB (1.928 Watts)

Assuming the Power = 1 Watt (30dBm)
Assuming the Antenna Gain shown by the hardware specs in dBd = 5dBd
Assuming signal loss = 0

EIRP = 30dBm + (5dBd + 2.15) - 0 = 37.15dB (5.188 Watts)
ERP = 30dBm + 5dBd - 0 = 35dB (3.162 Watts)

It looks like EIRP (shown by the hardware specs in dBi) is the same with ERP (shown by the hardware specs in dBd).

A) But why does the ERP (shown by the hardware specs in dBi) isn't the same with EIRP (shown by the hardware specs in dBd)?
B) Now I'm confused when which one should I use to calculate and when will I be using it.
C) Some say it doesnt matter if I use ERP or EIRP because it will give the same answer. But based on the above calculation, it seems like that's not the case.

 

[lambdaanttech]

I'm not familiar with RFID readers , it depends how the antennas are connected .
Probably those 4 antennas are being used for 4 different directions ? If they are pointed in 4 different direction then you can calculate the ERP for a single antenna if you know the rf power out .
If you have +33dBm rf power , does it means +33dBm on each antenna port or is it total +33dBm divided over the 4 antenna ports ?

You can have maximum 2 watt (+33dBm) EIRP so to make things easy you can calculate with dBi values of the antenna .
Dont confuse EIRP with ERP , they have a differente meaning .
+30dBm +5dBi = +35dBm EIRP = +32,85 dBm ERP
+30dbm + 5dbd = +35dBm ERP = +37,15dBm EIRP
In both cases your are exceeding the regulated EIRP power of +33dBm .

Its better to install a higher gain antenna and reducing the power , since an antenna works reciprocal its also gives you extra gain on reception .

 

[Altruistic69]

1) What I know is, the transmitting power output at the RFID reader can be adjusted to 1 Watt / 30dBm. The reader will be connected to 4 external antennas (different direction). Therefore should I use the 30dBm to calculate each antenna or should I divide 30dBm by 4 antennas which is 7.5dBM in my calculation for ERP?


Technical Requirements by ETSI EN 302 208-1 v1.3.1

As you can see this technical requirements & method of measurements that I get from ETSI.
2) This is the calculation for circularly polarized antenna, how about linear polarized antenna and omnidirectional antenna? Can I use the same calculation?

3) TPO (dBm) = ERP – Antennen Gain (dBi) + 5.15 + Line Loss (dB)
= 30dBm – 9dBi + 5.15 + 0.6dB
= 26.75dBm / 0.473 Watt

a) Does it mean I have to set to 0.473 Watt at my reader to comply to the government regulation which is ERP 2Watt?
b) How do they get 5.15?
c) It's just getting confusing. Apreciate if someone can show me some example with elaboration on how to calculate ERP & EIRP for linear antenna & circular antenna in an event when the specification of the antenna gain is expressed in dBi & dBd.



And the above calculation i mant for what?