Radio Information - Just the facts

Teken

Teken

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Aug 11, 2020
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Can those who are well versed in radio theory and practical applications break down in plain English what this means?!?

Pretend I am ten years old and you need to explain the same:

The output power of the unit should be set lower than the effective power if the antenna being used results in more than the legally permitted output power while running at 30dBm.
 
Different antennas have different gain (directional) in relation to a spherical antenna standard which radiates equal power from a sphere measured in dBi. dBi = gain from isotropic antenna. 0dBi (sphere). When you design an antenna to radiate all the power in a particular direction, you will have more TX power available in that direction than radiating from all directions (isotropic). You will see directional antennas classified in gain dBi. Meaning that much more gain than isotropic antenna. Evidently they want you turn down the output power of your unit to conform with max FCC regulations, meaning if you have a 3dBi antenna, turn down output power 3dB. Inreality, who will ever know you are transmitting more power than regulations, unless you are slamming someone else nearby and they report you. You must be talking about the max output power of a wireless Router, so most likely no one would know any difference.
 
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Oneup said:
Different antennas have different gain (directional) in relation to a spherical antenna standard which radiates equal power from a sphere measured in dBi. dBi = gain from isotropic antenna. 0dBi (sphere). When you design an antenna to radiate all the power in a particular direction, you will have more TX power available in that direction than radiating from all directions (isotropic). You will see directional antennas classified in gain dBi. Meaning that much more gain than isotropic antenna. Evidently they want you turn down the output power of your unit to conform with max FCC regulations, meaning if you have a 3dBi antenna, turn down output power 3dB. Inreality, who will ever know you are transmitting more power than regulations, unless you are slamming someone else nearby and they report you. You must be talking about the max output power of a wireless Router, so most likely no one would know any difference.
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Appreciate the high level response and insight. Let me ask you an obvious question if radio is set to transmit at say 19 dBm.

A value of 20 dBm or any increased value (21~30) equates to a higher power output correct?
 
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Correct. dBm is an absolute power measurement (Watts) and dBi is just a relative power measurement compared to an isotropic antenna. So if you had a +3dBi antenna, and an output of 30dBm (1 Watt) from your router, you would have an effective radiated power of 33dBm at that antenna. Therefore you would need to turn your router power down to 27dBm to meet the 30dBm effective power at the antenna to meet the max power requirement of FCC. The narrower the beam of the antenna, generally equates to a higher gain antenna, since it is concentrating the power in smaller area. dB's are just a logarithmic scale to measure power. 0 dBm = 1mW, 30dBm = 1W, 40dBm = 100W, etc.
 
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Oneup said:
Correct. dBm is an absolute power measurement (Watts) and dBi is just a relative power measurement compared to an isotropic antenna. So if you had a +3dBi antenna, and an output of 30dBm (1 Watt) from your router, you would have an effective radiated power of 33dBm at that antenna. Therefore you would need to turn your router power down to 27dBm to meet the 30dBm effective power at the antenna to meet the max power requirement of FCC. The narrower the beam of the antenna, generally equates to a higher gain antenna, since it is concentrating the power in smaller area. dB's are just a logarithmic scale to measure power. 0 dBm = 1mW, 30dBm = 1W, 40dBm = 100W, etc.
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Really appreciate the feedback and comprehensive overview. What is the dBm equivalent to 3-4 watts?!?
 
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A good rule of thumb to use: A 3dB increase or decrease will approx double the power or approx decrease power by half. So 30 dBm is 1W, so 33 dBm is 2W, and 36 dBm is approx 4W.
Or use formula W=10(dBm−30)/10) Use online conversions is easiest.
 
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Teken said:
The output power of the unit should be set lower than the effective power if the antenna being used results in more than the legally permitted output power while running at 30dBm.
Click to expand...
Wouldn't this be more clear if instead of "effective power", it would say "maximum power"? As it stands, it's gibberish to me.
 
tigerwillow1 said:
Wouldn't this be more clear if instead of "effective power", it would say "maximum power"? As it stands, it's gibberish to me.
Click to expand...

Effective power implies the output power of the radio + gain of antenna. I agree it is worded somewhat confusing. Just keep in mind the gain of the antenna could be negative also.
 
tigerwillow1 said:
Wouldn't this be more clear if instead of "effective power", it would say "maximum power"? As it stands, it's gibberish to me.
Click to expand...

I am so happy someone else is as confused as me. One of the problems I freely admit is not knowing all the (Radio Lingo).

As such this is also made harder to understand when replies are structured in a way that adds even more confusion.

One of the problems I am / was having is why people are stating values such as 19 dBm and saying this is the same as 1 watt?!?

If there are other factors that could translate 19 dBm into 1 watt I’d love to hear about it. I’m not sure why so many so called HAMS go out of their way to make things so complicated?!? :banghead:

This has probably been one of the major reasons I refused to take any so called radio license.

As it hasn’t stopped me from deploying hardware that just works and is also respectful to other people / frequencies etc.

At some point I’m sure getting a license is on the road map. But, for now having a call sign or transmitting on higher power only during an emergency hasn’t been a problem for anyone in our group.
 
Oneup said:
A good rule of thumb to use: A 3dB increase or decrease will approx double the power or approx decrease power by half. So 30 dBm is 1W, so 33 dBm is 2W, and 36 dBm is approx 4W.
Or use formula W=10(dBm−30)/10) Use online conversions is easiest.
Click to expand...

:thumb:
 
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