# Relating Transmitter dBm to uV/m and EIRP

1. ## Relating Transmitter dBm to uV/m and EIRP

Hello all,
I would like to calculate the uV/m that would be seen using an EIRP on my transmitter in order to get a rough estimate of the Radiated Power I'll be working with.
I am working on a product using a 433MHz transmitter module (CC1310). The product will fall under FCC 15.231(e) where i'm designing under a specific uV/m maximum.
I need to determine whether the allowable FCC uV/m will be enough for my product.

What I know about my transmitter is Maximum Output Power: 14dBm
This converter tool (https://www.eeweb.com/tools/rf-unit-converter) tells me 14dBm = 1.33E+6uV but that seems rather high.
If anyone here has dealt with FCC or ERP levels I would gladly appreciate any knowledge.

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2. ## Re: Relating Transmitter dBm to uV/m and EIRP

14dBm 25mW, which generates 1.12V @ 50Ω - nearly your stated value.

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3. ## Re: Relating Transmitter dBm to uV/m and EIRP

No you are firstly asking about relation between transmitted power and EIRP, a matter of antenna gain. Secondly between EIRP and field strength. Power density at a specific distance is EIRP/sphere surface area. Convert to field strength by applying field impedance of 377 ohms.

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You can use Rohde&Schwarz field strength and power estimator for the calculation.

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4. ## Re: Relating Transmitter dBm to uV/m and EIRP

I presume you're aware of the limitations for 15.231 E. (max. 1 sec transmission, max. 1/30 duty cycle, min. 10 sec. silence). Also permitted transmitter power is very low, about -22.5 dBm EIRP at 433 MHz.

Most SRD (small radio devices) in the FCC domain are utilizing 915 MHz band (FCC 15.249), -1 dBm EIRP.

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5. ## Re: Relating Transmitter dBm to uV/m and EIRP

Originally Posted by FvM
I presume you're aware of the limitations for 15.231 E. (max. 1 sec transmission, max. 1/30 duty cycle, min. 10 sec. silence). Also permitted transmitter power is very low, about -22.5 dBm EIRP at 433 MHz.
Yes I'm aware and can mold my protocol to those requirements.Thanks.
As far as permitted Transmitter Power I'm going to have to take the hit, I'm stuck at 433MHz or below.
I'm assuming most Hobby 433MHz transmitter products are passed as 15.231(a) at a higher transmit power and then the customer breaks FCC rules by feeding data into it at any interval they wish..
I'm stating this because i'm just curious what commercial products operate at the 15.231(e) power level, what performance can be expected and if that is anywhere near enough for my very specific application.
Most SRD (small radio devices) in the FCC domain are utilizing 915 MHz band (FCC 15.249), -1 dBm EIRP.
My application is transmitting data out of the human stomach, must keep at 433MHz and below unfortunately. Lots of power lost trying to transmit through the saltwater filled body so I will be doing RSSI testing on subjects using 15.231(e) power levels.

Thanks.

6. ## Re: Relating Transmitter dBm to uV/m and EIRP

In this case, transmitter power can be increased according to the expectable body attenuation.

7. ## Re: Relating Transmitter dBm to uV/m and EIRP

Originally Posted by FvM
In this case, transmitter power can be increased according to the expectable body attenuation.
Does the FCC factor in 'expected attenuation' for an application?

I assumed they would measure the device field strength in open-air at 1 meter and that was the limit no matter the applications area of operation.

8. ## Re: Relating Transmitter dBm to uV/m and EIRP

FCC measures actual field strength in 3 m distance. If you can prove that your application environment involves a respective attenuation, it should be considered. It would be good to have an antenna setup that can be used in the lab.

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9. ## Re: Relating Transmitter dBm to uV/m and EIRP

Originally Posted by FvM
It would be good to have an antenna setup that can be used in the lab.
It really would, but I don't know if I could replicate what an FCC certified testing facility would use for Field Strength.
If a testing facility measures the field strength as slightly over the FCC limits then I would need to go back to design and attenuate transmit power, then pay for re-testing and hope I came close. Seems like an extreme waste of money. I know calculating field strength is a normal thing but somehow I highly doubt you can get very close to real-world values. So many variables.

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