Ask HN: Question about radio waves

#1

Post by ratsoman2 » Thu, Oct 18, 2018, 5:40 AM UTC

What frequency would be best for within 1 mile in a city or suburban area?

Follow up, Why are restaurant notification buzzers so limited in distance and how could they improve this.

thanks

Re: Ask HN: Question about radio waves

#2

Post by Gibbon1 » Thu, Oct 18, 2018, 6:14 AM UTC

There are four components that effect range. Transmit power, attenuation, receive sensitivity, and interference.

Transmit power is easy, more power more range.

Attenuation depends on frequency in a cluttered environment. Everything being equal lower frequencies work a lot better than high ones. Contrary to what you'll hear or read radio signals in a cluttered environment doesn't follow an inverse square law in practice. It's a lot worse than that.

attenuation = const * distance ^ -k

Where k depends on frequency. For FM/AM radio (important where the tower is on a hill) k = 2.5

900MHZ k is ~3 and for 2.4GHz is ~3.5

So 900MHZ is a lot better than 2.4GHZ. 433 is likely better than either.

Receive sensitivity depends on the antenna and front end design. And modulation type and strongly on the signal bandwidth. And very importantly the presence of interfering signals. Low data rate spread spectrum will give you the highest receive sensitivity.

Re: Ask HN: Question about radio waves

#3

Post by wahern » Thu, Oct 18, 2018, 6:43 AM UTC

Gibbon1 wrote: post #2

There are four components that effect range. Transmit power, attenuation, receive sensitivity, and interference. Transmit power is easy, more power more range. Attenuation depends on frequency in a cluttered environment. Everything being equal lower frequencies work a lot better than high ones. Contrary to what you'll hear or read radio signals in a cluttered environment doesn't follow an inverse square law in practi…

Isn't interference just another way to describe sensitivity? From the classic Salon article, The Myth of Interference: "'Interference is a metaphor that paints an old limitation of technology as a fact of nature.' So says David P. Reed, electrical engineer, computer scientist, and one of the architects of the Internet." https://www.salon.com/2003/03/12/spectrum/

Re: Ask HN: Question about radio waves

#4

Post by Gibbon1 » Thu, Oct 18, 2018, 7:34 AM UTC

wahern wrote: post #3

Gibbon1 wrote: post #2

There are four components that effect range. Transmit power, attenuation, receive sensitivity, and interference. Transmit power is easy, more power more range. Attenuation depends on frequency in a cluttered environment. Everything being equal lower frequencies work a lot better than high ones. Contrary to what you'll hear or read radio signals in a cluttered environment doesn't follow an inverse square law in practi…

Isn't interference just another way to describe sensitivity? From the classic Salon article, The Myth of Interference: "'Interference is a metaphor that paints an old limitation of technology as a fact of nature.' So says David P. Reed, electrical engineer, computer scientist, and one of the architects of the Internet." https://www.salon.com/2003/03/12/spectrum/

Any receiver design, bandwidth and modulation technology will have an ultimate sensitivity. Far as I understand this is due to both noise created by the circuit itself and the thermal noise over the band.

You can't get better than that.

Then you add interference which raises your noise floor both statically and dynamically. Unlicensed bands tend to be polluted with short coherent pulses of RF.

One issue with interference. While attenuation is symmetrical between two transceivers, interference is utterly not symmetric.