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Lower frequency comms transmitters need less power?

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cupoftea

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Hi,

At 1:07 of this video ..


…they discuss that they use the Harvi device to send a signal 30 metres to another device. The Harvi has no power supply, but just harvests energy from the mains via a CT (clipped to the house incoming line). Is this the reason that they use 868MHz comms?...ie, this relatively low frequency has lower power transmitters than say 2.4GHz?

Harvi
 

Frequency has little effect on transmitter power consumption. The frequency used will depend on local licensing regulations but regardless of that, a short range transmitter will only consume one or two mA and even then it is probably pulsed so it makes little overall difference which is used.

Brian.
 
Also, in an application like this...just transmitting small amounts of data over 30 metres or so......would there be a need to do antenna tuning?...say if you were using 868MHz.....dont the transmitter datasheets tell what geometry and shape of antenna is needed?....why would anyone need to tune an antenna for such an applciation? Do you think every item would even need production tuning?
 

I contrived a current detector to log cycling times of my refrigerator, I jumpered a single wire into its power cord. (It carried several Amperes.) I wrapped that one wire around a transformer a few times. I found it was able to 'harvest' sufficient power to light a red led dimly.

transformer winding lights led.GIF
 
Sub-GHz radio is typically achieving larger range in buildings than 2.4 GHz, better transmission through walls and around corners. Also less concurrent services on air.

For this reason, sub-GHz has been often used for home automation and similar purposes. Biggest disadvantage: you need different frequencies for different countries.

Reliable transmission over 30 m can be achieved with less transmitter power in case of doubt. But worst case, 30 m can't be guaranteed inside buildings, neither at sub-GHz or 2.4 GHz.
 
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