Calculating power transmission of a wireless transmitter

[sonaiko]

I am trying to design a wireless transmitter.

I reached a stage where I need to calculate how much power I should transmit. According to FCC rules, I am only allowed to transmit with electric field intensity of 500 micro V/m without a license.

I referred to Power & Field intensity equation, I found out that P = 0.3 E^2.
That means radiated power should equal to 75 nW.

I am not sure if this is a reasonable result! I feel this is so low as I was expecting to transmit something in the mW range.

Please inform me if the result I got is reasonable, and if that is a value which average transmitters use to radiate the wireless signal.

Thank you.

 

[davenn]

Re: Transmitted Power

ok FCC rules means you must be in the USA

>>electric field intensity of 500 micro V/m<< interesting definition

you need to see if somewhere they state a mW power level, a common level is 10mW. That would be a easier way to work your situation out

Dave

 

[waqashussain86]

Re: Transmitted Power

I am also working on wireless power transmission, in this project power amplifier is used to boost up the signal before transmitter. I want to know that you also using it or not ?

 

[keith1200rs]

Transmitted Power

75nW sounds about right for spurious emissions. You need to find a specific frequency band for your purpose where you will be allowed to transmit higher power.

Keith

 

[disisku_22]

Re: Transmitted Power

Hi,

I am only allowed to transmit with electric field intensity of 500 micro V/m without a license.

---- this value is for harmonics/spurious not for main peak power level, as per FCC for fundamental power at 2.4G is 50mV/m at 3m = 0.75 mW (-1.25 dBm EIRP).

refer attachment for further details.

regards
Roshni.

 

[FvM]

Re: Transmitted Power

Yes, 500 uV/m (at 3 m distance) is a standard FCC spurious emission limit value. You won't be able to receive it above a few meters distance under typical conditions.

A popular FCC band for SRD (license free small radio devices) is 902 - 928 MHz. Without frequency hopping, 50 mV/m (about -1 dBm effective isotropic radiated power) is allowed. See FCC 15.247ff for details.

 

[sonaiko]

Re: Transmitted Power

Ok thanx for the helpful information.

I am actually in japan not in US.
I found **broken link removed** figure for the maximum allowed electric field measured in V/m for any frequency in Japan.
I am still not sure how to know my maximum allowed transmitted power using this figure.
nW is too small and it does not make sense.

 

[keith1200rs]

Transmitted Power

Yes, that is a spurious emissions graph. You are allowed to transmit a lot more power in specific frequency bands. What are you actually trying to do?

Keith.

 

[sonaiko]

Re: Transmitted Power

Yes, that is a spurious emissions graph. You are allowed to transmit a lot more power in specific frequency bands. What are you actually trying to do?

Keith.

There is no specific frequency, but rather a range. I am ok to work in a 800MHz~2GHz range. Any frequency in this range is acceptable for me.

So how should I know how much power I can transmit in Japan if for example I work on 1GHz band?

Btw, what is the spurious emissions and how is that different that my actual transmitted power?

Thanx

 

[keith1200rs]

Transmitted Power

There is a band 950-956MHz in Japan which is probably the most suitable. It is not one I have used, but I suspect it is 10mW like the European 868MHz and US 915MHz.

The spurious emissions are what is transmitted at frequencies other than in the selected band. It also applies to devices which aren't actually supposed to be transmitters. Anything with a clock in it will radiate - e.g. a microprocessor.

Keith.

 

[sonaiko]

Re: Transmitted Power

There is a band 950-956MHz in Japan which is probably the most suitable. It is not one I have used, but I suspect it is 10mW like the European 868MHz and US 915MHz.

The spurious emissions are what is transmitted at frequencies other than in the selected band. It also applies to devices which aren't actually supposed to be transmitters. Anything with a clock in it will radiate - e.g. a microprocessor.

Keith.

That is a great answer.
Yet, do you have any documents or links where I could find this information stated?
I need something official to base my work on or else I will be building my Tx on air!

By the way, what do u call what we are talking about now?
maybe "power allowed for frequency spectrum"?
or "unlicensed allowed transmitted power"? .. etc
or what? So I can make my online search easier..

Thank you again.

Added after 18 minutes:

One more question

Look **broken link removed**, and under Radio Law Enforcement Regulation Article 44 .

It sames that equipments working in 950MHz or 2.45GHz need no licensing under the condition of operating at field intensity E = 35 micro V/m or less.

When I transfer this value to watts, it results in nW!! Doesnt make sense, does it?

 

[keith1200rs]

Transmitted Power

In the UK they are often called "License Free" bands because you don't need to apply for a license to use the transmitter provided it meets the requirements. If you want to transmit in the 88-108MHz band, by contrast, you need to apply for a license.

This may help:

www.arib.or.jp/english/html/overview/doc/5-STD-T96v1_0-E1.pdf


Keith.

Added after 5 minutes:

Just saw your additional post. I think the link you have is specific to RFID. 950-956MHz is not universal - it is specific to Japan (and possibly China). So, if you made an RFID system which worked on 950-856MHz it would have to work at very low levels outside Japan i.e. the spurious emissions level. If you want a "universal" frequency, then I think you would have to use the 2.4GHz band.

Keith.

 

[sonaiko]

Re: Transmitted Power

In the UK they are often called "License Free" bands because you don't need to apply for a license to use the transmitter provided it meets the requirements. If you want to transmit in the 88-108MHz band, by contrast, you need to apply for a license.

This may help:

www.arib.or.jp/english/html/overview/doc/5-STD-T96v1_0-E1.pdf


Keith.

Added after 5 minutes:

Just saw your additional post. I think the link you have is specific to RFID. 950-956MHz is not universal - it is specific to Japan (and possibly China). So, if you made an RFID system which worked on 950-856MHz it would have to work at very low levels outside Japan i.e. the spurious emissions level. If you want a "universal" frequency, then I think you would have to use the 2.4GHz band.

Keith.

Thank you.

Can you tell me how to know P in watts from E in V/m? Maybe I am doing something wrong.

And assuming I work in the universal 2.4GHz band, how much power am I allowed to transmit? I cannot find anything about that in the link I sent you.
Plus, I do not think the link was only talking about RFID. Have you check the very last part of it (very weak equipments)?

 

[FvM]

Re: Transmitted Power

Thank you for posting the japanese 950 MHz SRD regulation. In my opinion, some specifications, particularly channel mask and 2nd order harmonics are rather critical. Equipment complying to the 860 MHz ETSI or 915 MHz FCC rules would need serious redesign to keep it. I also fear, that some popular transmitter chips won't be able to keep the channel mask because of their spurious emissions during turn-on and turn-off.

It may be a more simple way to use standard 2.45 GHz equipment, that's also allowed for license free operation in Japan. But I didn't yet review the respective regulations.

P.S.: Regarding "universal" equipment. A transmitter that can be tuned between 860 and 955 MHz can be used in many countries with an EIRP of at least 1 mW to maximum 10 mW.

P.P.S.: Effective radiated power and field strength are simply linked by the sphere area related to the respective receiption distance and the free space radiation resistance (377 ohm).

 

[sonaiko]

Re: Transmitted Power

P.S.: Regarding "universal" equipment. A transmitter that can be tuned between 860 and 955 MHz can be used in many countries with an EIRP of at least 1 mW to maximum 10 mW.

P.P.S.: Effective radiated power and field strength are simply linked by the sphere area related to the respective receiption distance and the free space radiation resistance (377 ohm).

So I can comfortably use the 860MHz~955MHz at 10mW without license? Can you link me to an official document that say so? Not that I do not trust you, but I need it to show some documentation at work.

Can you post the equation that relates power with E? I remember E=V/d then we get V if we know the distance then power through V^2/R. But I do not remember for the sphere!

 

[keith1200rs]

Re: Transmitted Power

For Europe I think it is ERC70-03

www.erodocdb.dk/Docs/doc98/official/pdf/REC7003E.PDF

Keith.

 

[FvM]

Re: Transmitted Power

So I can comfortably use the 860MHz~955MHz at 10mW without license?

No. A transmitter with these parameters can be configured to comply with the specific regulations. For Japan, it could use 10 mW under certain conditions given in the above posted document. For Europe (ETSI), the transmitter would be limited to 5 mW, for U.S. to 1 mW, if not using frequency hopping. Also the frequency has to adjusted accordingly.

The specific spurious and harmonics limits have to kept, also limitations for transmit duration and duty cycle. Also compliance measurements by an officially comissioned institution may be necessary, possibly the deviice gets a registration number. In so far, the procedure is not actually comfortable.