Wireless Monitoring Duty Cycle

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Zee

Senior Member
Location
CA
I have a customer who is a little concerned about EMF radiation from her SMA SB7000US inverter.

It has a radio piggyback card and antenna installed which communicates with a Sunny Beam (tabletop display unit) wirelessly.
(Perhaps some of you remember this nifty little device. It has a built in mini solar panel!)
The inverter is on the exterior of her (stucco) bedroom wall, while the Sunny Beam is in the house on the other side of her bedroom.

I dug up the manual for the Sunny Beam.
I found this:
868 MHz acc. to ETSI EN 300 220 Duty Cycle < 1 %

What does "Duty Cycle < 1 %" mean?
How would this compare to a standard household Wi Fi router?
How would the net energy emitted compare?
Is it safe to assume it emits nothing at night (no solar energy)?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I dug up the manual for the Sunny Beam.
I found this:
868 MHz acc. to ETSI EN 300 220 Duty Cycle < 1 %

What does "Duty Cycle < 1 %" mean?
How would this compare to a standard household Wi Fi router?
How would the net energy emitted compare?
Is it safe to assume it emits nothing at night (no solar energy)?

Duty cycle <1% means it is transmitting less than 1% of the time.

The ETSI EN 300 220 standard specifies a maximum of 25 milliwatts (25mW) effective radiated power (ERP) at 868 MHz:

https://www.google.com/url?sa=t&rct=...rdww1m#page=22
The reason for this relatively low maximum power level is to reduce interference with other communications systems such as cellular and two-way land mobile.

Handheld cell phones are typically limited to a maximum transmit output of 600 mW, and the ERP is similar because the antenna gain is close to unity (0 dB). However, power control typically reduces the Tx output unless you are at the fringes of the cell coverage area.
The original AMPS analog cell phones had a duty cycle of 100% during a call because they used FDMA and duplexer filters to allow simultaneous transmission and reception. 2G, 3G, 4G, etc. phones typically have a duty cycle less than 50% but considerably more than 1% during a phone conversation.

2.4GHz WiFi routers and access points typically have output powers of 50 to 100mW, but can be as high as the 1 Watt range although that is unusual. There are of course exceptions for longer range outdoor links.

Another thing to consider is that as you get further away from an antenna (beyond the "near field") the received power goes down as the square of the distance (because the area of a sphere is proportional to the radius squared).

I don't know if the Sunny Beam device transmits at night or not.

Anyway, I think you should be comfortable in reassuring your customer that the Sunny Beam device does not emit any higher levels of electromagnetic fields than widely used wireless devices, and if anything it is less.
 

pv_n00b

Senior Member
Location
CA, USA
Syncro covered the standard, all good information. I'm not sure why the Sunny Beam would be transmitting at all, it should just be receiving information from the inverter transmitter. Maybe they are referring to the Radio Piggy-Back.

If your customer has been googling "EMF danger" then they have been reading a lot of crackpot websites that have told them that even low levels of EMF will cause all kinds of health issues. There is really no science you can lay on someone who has bought into this stuff to allay their fears. If they are not too far gone maybe they will be open to being told it's comparable something else in their home like a wifi router, bluetooth device, or smart meter that they accept. Otherwise, it's probably just easier to tell them to chuck the tabletop monitor if they don't feel safe and pull the radio out of the inverter.
 

Zee

Senior Member
Location
CA
Thank you very much. I like the hard data and numbers.:cool:

I do want to get this right once and for all:
how does signal strength (EMF strength) decrease with distance?
I thought it was related to the CUBE of the distance.
( I figured it was cubic since it radiates in 3-D. Like you said, an expanding sphere.)
You mention it is the square.
I have also heard it changes "logarithmically".


Yes, if someone is sure they are being poisoned, at some point there is only so much you can say.:dunce:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Thank you very much. I like the hard data and numbers.:cool:

I do want to get this right once and for all:
how does signal strength (EMF strength) decrease with distance?
I thought it was related to the CUBE of the distance.
( I figured it was cubic since it radiates in 3-D. Like you said, an expanding sphere.)
You mention it is the square.
I have also heard it changes "logarithmically".


Yes, if someone is sure they are being poisoned, at some point there is only so much you can say.:dunce:

It decreases as the square of the distance, i.e., by the Inverse Square Law. Double the distance, the signal strength decreases by a factor of four. The term you are looking for is "exponentially", and yes, that's what the Inverse Square Law describes.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Thank you very much. I like the hard data and numbers.:cool:

I do want to get this right once and for all:
how does signal strength (EMF strength) decrease with distance?
I thought it was related to the CUBE of the distance.
( I figured it was cubic since it radiates in 3-D. Like you said, an expanding sphere.)
You mention it is the square.
I have also heard it changes "logarithmically".


Yes, if someone is sure they are being poisoned, at some point there is only so much you can say.:dunce:
As ggun says the signal strength goes down as the inverse square of the distance: 1/(distance)2
This is because as the signal radiates the power is distributed (spread out) over a larger spherical area, and the area of a sphere is 4πR2 where R is the radius (the distance from center to outside of a sphere). So the signal strength at a radius R2 is a factor of R12/R22 smaller at than at R1, where R2 > R1.
Light, which is just a higher frequency form of electromagnetic radiation follows the same law. So the intensity of light from a lightbulb also goes down with distance by the inverse square.
The signal is propagating away from the source and dispersing, not "filling up" a volume of space and so the cube of distance does not apply.
You mentioned logarithms, so in decibels the signal strength is reduced for every doubling in distance 10 log10[1/22] = -6.02 dB.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
It decreases as the square of the distance, i.e., by the Inverse Square Law. The term you are looking for is "exponentially", and yes, that's what the Inverse Square Law describes.
For the record, an inverse square law is not exponential decrease. An inverse square law is an example of a power law, which for a decreasing function of d is of the form C*d-n for some constants C and n, with n > 0. While an exponential decrease would be of the form C*e-A*d for some constants C and A, with A > 0. In the exponential law, the variable is in the exponent.

BTW, an exponential decrease has the property that it decreases faster than any power law.

Cheers, Wayne
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
BTW, an exponential decrease has the property that it decreases faster than any power law.
Cheers, Wayne
Right, and that's why any coax, fiber optic cable, or other guided wave structure will eventually have more loss than free space propagation if you go over a far enough distance. This is true no matter how low loss the cable is. Free space has the inverse second power or 6 dB loss per doubling of distance and so it initially drops the signal much more than a cable does. But a cable has an exponential loss versus distance just as Wayne described, and therefore it has a fixed dB loss per every unit of distance (eg., meter or foot) and so eventually accumulates a higher loss than free space propagation..

By the way, in radar the level of the returning reflected signal goes down with the inverse 4th power of distance because it has two paths to follow: first the transmitted signal propagates to the target and then the reflected signal travels back to the radar receiver. Later in WWII the Germans could receive Allied radar transmissions and therefore estimate how far away allied planes were by the received signal strength. A US physicist Luis Alvarez came up with a technique whereby the Allies reduced the transmitted power by approximately the inverse 3rd power with distance to target as they flew in. Therefore the Germans thought the planes were flying away because they were receiving diminishing signal levels (because this inverse 3rd power reduction in transmitted power exceeded the second power increase in signal level from the continued reduction in path loss as the planes got closer to the target). Even after applying the inverse 3rd power reduction in transmitted power with distance, the received radar signal was increasing linearly with distance because the path loss of the radar return was decreasing as inverse 4th power. I thought this was all quite clever.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
For the record, an inverse square law is not exponential decrease. An inverse square law is an example of a power law, which for a decreasing function of d is of the form C*d-n for some constants C and n, with n > 0. While an exponential decrease would be of the form C*e-A*d for some constants C and A, with A > 0. In the exponential law, the variable is in the exponent.

BTW, an exponential decrease has the property that it decreases faster than any power law.

Cheers, Wayne

I stand corrected, although the Inverse Square Law shows that the signal strength approaches zero asymptotically as distance from the source increases, which is similar to an exponential decrease as it approaches zero forever and never gets there. Firesign Theatre aficionados will recognize it as the Antelope Freeway Effect. :D
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Right, they both approach 0 in the limit.

The exponential decay approaches zero faster in the sense that for any inverse power law (make the coefficients C and n as big as you like) and any exponential decay (make the coefficients C and A as small as you like), there will be some value of the variable beyond which the exponential decay function will be smaller than the inverse power law function.

Cheers, Wayne
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Right, they both approach 0 in the limit.

The exponential decay approaches zero faster in the sense that for any inverse power law (make the coefficients C and n as big as you like) and any exponential decay (make the coefficients C and A as small as you like), there will be some value of the variable beyond which the exponential decay function will be smaller than the inverse power law function.

Cheers, Wayne

Antelope Freeway 1/4 mile... :D
 
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