Hi,

I have question about the definition of antenna gain. I'm looking antenna for RFID application and according to FCC 15.247 maximum conducted power is 30dBm and EIRP is maximum 36dBm. If you want to have maximum output power, you need to have antenna gain 6dBi. I think this is clear so far.

You can use circular polarized 9dBic antenna, which would meet 36dBm EIRP power requirement.

So my question is what is the definition of antenna gain, if I design for example dipole antenna with 2dBi gain and then I tell to customer that you should place it quarter wave distance from metal plane (which will increase the gain)?

 

Anytime you measure something in dB, you are making a comparison. In the case of an antenna, the thing you are comparing an antenna to is an "isotropic" radiator. The literal meaning of the word is "same in all directions".

https://en.wikipedia.org/wiki/Isotropic_radiator

To the extent that an antenna has directional characteristics in azimuth or elevation, it will exhibit "gain" with respect to an isotropic radiator. The statement that an antenna has "gain" does not imply there is some kind of secret amplification happening, it is just a function of the radiation pattern created by the antenna itself and the surrounding features.

 

Welcome to AAC.

EIRP is the operative metric, not antenna gain. If anyone operates a system beyond the specified EIRP they are not in compliance. But don’t forget Type Acceptance. Before an unlicensed intentional radiator can be legally marketed, it must be certified as compliant in a given configuration. This includes all of the parts as a system, including antenna.

Once certified, there are three classes of permissive changes that can be made without requiring recertification.

1. Class I is a change that does not degrade the performance of the system compared to the certified version.
   
   
2. Class II is a change that does degrade the performance, but not beyond the regulatory limits—additional paperwork must be filed. but recertification is not required.
   
   
3. Class III is a change to software in an SDR radio that changes its operating frequency—additional paperwork must be filed as for Class II

So, technically, for a device like and RFID transceiver you can’t legally operate it with an antenna different from the one provided by the manufacturer during certification, or that doesn’t change the performance of the system.

In some radio services, antenna gain has a specified limit. In those cases “accidentally” placing a reflector behind the antenna increasing the gain beyond regulatory limits would be operation outside compliance and not permitted. If you were to advise a customer to do it, that would be intentional circumvention of the regulations under which your device was certified and put you in jeopardy.

So, no, there are no clever “tricks” to circumvent EIRP or other limits. The FCC is very good at trying to protected shared bands, particularly in the case of Part 15 devices. The numbers aren’t arbitrary, they add up to the EIRP—even in those cases where a maximum antenna gain is specified.

 

Welcome to AAC.

EIRP is the operative metric, not antenna gain. If anyone operates a system beyond the specified EIRP they are not in compliance. But don’t forget Type Acceptance. Before an unlicensed intentional radiator can be legally marketed, it must be certified as compliant in a given configuration. This includes all of the parts as a system, including antenna.

Once certified, there are three classes of permissive changes that can be made without requiring recertification.

1. Class I is a change that does not degrade the performance of the system compared to the certified version.
   
   
2. Class II is a change that does degrade the performance, but not beyond the regulatory limits—additional paperwork must be filed. but recertification is not required.
   
   
3. Class III is a change to software in an SDR radio that changes its operating frequency—additional paperwork must be filed as for Class II

So, technically, for a device like and RFID transceiver you can’t legally operate it with an antenna different from the one provided by the manufacturer during certification, or that doesn’t change the performance of the system.

In some radio services, antenna gain has a specified limit. In those cases “accidentally” placing a reflector behind the antenna increasing the gain beyond regulatory limits would be operation outside compliance and not permitted. If you were to advise a customer to do it, that would be intentional circumvention of the regulations under which your device was certified and put you in jeopardy.

So, no, there are no clever “tricks” to circumvent EIRP or other limits. The FCC is very good at trying to protected shared bands, particularly in the case of Part 15 devices. The numbers aren’t arbitrary, they add up to the EIRP—even in those cases where a maximum antenna gain is specified.

Thank you for comments. I agree that there is no tricks. Rules are clear, but I think this accidental reflector is a bit gray area. Basically, I have been look for product, which is kind of dipole array and it has more or less omni-directional radiation pattern with 3dBi gain. That I agree. I'm practice this product is always used with ground plane in RFID portal. I expect that gain will increase 6dB, but it is not mentioned anywhere. Radiation has to go somewhere if you block another half with metal plate. I this case end user can put extra 6dB of power (well 3dB in order to meet 30dBm conducted power requirement), since they are not aware that the antenna gain is actually 9dBi instead of 3dBi. It would make big difference.

Is there some mechanism in FCC to check the concern about violation is valid?

 

Thank you for comments. I agree that there is no tricks. Rules are clear, but I think this accidental reflector is a bit gray area. Basically, I have been look for product, which is kind of dipole array and it has more or less omni-directional radiation pattern with 3dBi gain. That I agree. I'm practice this product is always used with ground plane in RFID portal. I expect that gain will increase 6dB, but it is not mentioned anywhere. Radiation has to go somewhere if you block another half with metal plate. I this case end user can put extra 6dB of power (well 3dB in order to meet 30dBm conducted power requirement), since they are not aware that the antenna gain is actually 9dBi instead of 3dBi. It would make big difference.

Is there some mechanism in FCC to check the concern about violation is valid?

Since the certification includes the antenna system, the conducted limit doesn’t apply if the antenna gain would cause an EIRP in excess of the regulatory limits. If there is more antenna gain, the maximum permissible conducted power would decrease until it matched the maximum EIRP—the real controlling number.

I don’t know of any resource that can handle questions like that free of charge. Large companies usually have lawyers and engineers for it, or hire consultants.

 

Since the certification includes the antenna system, the conducted limit doesn’t apply if the antenna gain would cause an EIRP in excess of the regulatory limits. If there is more antenna gain, the maximum permissible conducted power would decrease until it matched the maximum EIRP—the real controlling number.

I don’t know of any resource that can handle questions like that free of charge. Large companies usually have lawyers and engineers for it, or hire consultants.

According to my understanding certification of UHF RFID readers are certified with selected antennas. Fairly often end user is using some other antenna than used in certification. Usually maximum conducted power from the reader is up to 32.5dBm, so you can compensate cable loss. Anyway, you could pass certification with the specific antenna without "accidental" reflector, but in real life you would use this reflector every time.

I guess it would be possible to say that your antenna gain much lower than it actual is in real life unless someone sue you.

 

Actually, the certification has to be conducted with the equipment in the configuration in which is will be used, and there is this:

Limit Clause FCC Sections: 15.203, 15,204
An intentional radiator shall be designed to ensure that no antenna other than that furnished by the
responsible party shall be used with the device

Excerpted from a compliance report for an RFID reader where the testing firm was certifying that this rule was followed.

 

Actually, the certification has to be conducted with the equipment in the configuration in which is will be used, and there is this:

Limit Clause FCC Sections: 15.203, 15,204
An intentional radiator shall be designed to ensure that no antenna other than that furnished by the
responsible party shall be used with the device

Excerpted from a compliance report for an RFID reader where the testing firm was certifying that this rule was followed.

Certification can be done without accidental reflector and then someone ignorant just add reflector. Manufacturer of the reader device cannot prevent someone using whatever antenna they have. Usually you check the gain from nameplate and then you calculated maximum power.