I have been trying to understand dB's and -dBm's as they relate to RF signal strength.
I thought I had a pretty good understanding. For example, a -20dBm signal is the same as saying .01 of a mW, or .00001 W.

That made me wonder, if the max measurable RF strength is 0dBm (1 mW), how to account for a very strong radio very close to the RF meter. I reasoned if I have a 50kW AM radio antenna very close, how can I only be getting 1mW 1 foot from it ?

What mistakes am I making ?

 

The big mistake is standing one foot away from a 50kW transmitter aerial. This wouldn't be good for either you or the meter.

 

The big mistake is standing one foot away from a 50kW transmitter aerial. This wouldn't be good for either you or the meter.

not a helpful response.

 

That made me wonder, if the max measurable RF strength is 0dBm (1 mW), how to account for a very strong radio very close to the RF meter. I reasoned if I have a 50kW AM radio antenna very close, how can I only be getting 1mW 1 foot from it ?

How did you get a maximum of .001 watt? Yes, 0 dbm = 1 mW but that is not a maximum. Using that 30 dbm = 1 watt, so 60 dbm = 1,000 Watts and 80 dbm is 100,000 Watts all referenced to 0 dbm. Additionally do not confuse dbm as a measure of power with Field Strength.

Ron

 

How did you get a maximum of .001 watt? Yes, 0 dbm = 1 mW but that is not a maximum. Using that 30 dbm = 1 watt, so 60 dbm = 1,000 Watts and 80 dbm is 100,000 Watts all referenced to 0 dbm. Additionally do not confuse dbm as a measure of power with Field Strength.

Ron

Thank you. That was helpful. I got the 1mW max when I was watching a youtube about the immersionRC RF power meter antenna comparison.

I take it from what you said, that measuring the RF power 1 ft away from a 50 kW antenna would show more than 1 mW. This is why I wrote the post, it just didn't make sense the way I was thinking about it.

 

You may want to run a Google of "field strength meter" to understand what happens to power over distance. Keep in mind that db is merely expressing a ratio, dbm is a measure of power with a reference. We can have dbm (power), dbv (voltage) and a few others with a point of reference. Be careful with things like antennas when a gain is mentioned in db as that gets into another world unto itself.

Ron

 

not a helpful response.

It is a most helpful response motivated by a concern for your physical safety. Don't be so quick to throw stones.

 

I have been trying to understand dB's and -dBm's as they relate to RF signal strength.
I thought I had a pretty good understanding. For example, a -20dBm signal is the same as saying .01 of a mW, or .00001 W.

That made me wonder, if the max measurable RF strength is 0dBm (1 mW), how to account for a very strong radio very close to the RF meter. I reasoned if I have a 50kW AM radio antenna very close, how can I only be getting 1mW 1 foot from it ?

What mistakes am I making ?

Recognize that there is a sign associated with a measure of signal power quoted in dBm. The negative sign means power less than 1 milliwatt; a positive sing means power greater than 1 milliwatt. A typical high performance receiver can allow reception of SSB (voice) signals with an input power of -130 dBm. With a transmitter of 100 watts of output power your signal will propagate from 3,000 to 8,000 miles depending on the time of day, the time of year, and the progress of the approximately 11-year sunspot cycle. After that trip it will probably show up in the receiver as a power level of -110 to -90 dBm.

Check out the following interactive map at:
www.dxmaps.com

 

You may want to run a Google of "field strength meter" to understand what happens to power over distance. Keep in mind that db is merely expressing a ratio, dbm is a measure of power with a reference. We can have dbm (power), dbv (voltage) and a few others with a point of reference. Be careful with things like antennas when a gain is mentioned in db as that gets into another world unto itself.

Ron

I had looked into that, (free space Path Loss) and came away with this : 10 km, 433 mHz, Rx antenna gain 1 dBi, Tx antenna gain 1 dBi, is about= 103 dB loss. A -103dB power loss = 5e-11. So I reason that a radio station broadcasting 50,000W is : (5e^4) x (5e-11) = 25e-7, which is 2.5 uW or 2,500 mW. Now if I measure the field strength 10 km clear distance from a 50 KW antenna, will I read -103dB ?

I may be confusing dB and dBm. But still I am pretty lost.

 

Let's take it one step at a time.
When you use dB, it is a comparison of two things, and you are interested in the ratio of those two things over many orders of magnitude. You can convert any two quantities into a ratio. Let's try it with dollars, or your favorite currency. Let's say you go to work, fresh out of the university, at a starting salary of $62,500/year. The president of the company makes $1.235 Million/year.
Q: Express the ratio of his salary to your salary in dB
A:
\(20\;*\;log\;(\frac{1.235e^6}{63,500})\;=\;25.9\; \text dB\)

Q:Express the ratio of your salary to his salary in dB
A:
\(20\;*\;log\;(\frac{63,500}{1.235e^6})\;=\;-25.9\; \text dB\)

Notice the interesting thing that just happened? Taking the reciprocal of the ratio flips the sign but keeps the same significant figures.

So dB with a negative sign represents the ratio of a "small" thing to a "larger" thing.
And dB with a positive sign represents the ratio of a "large" thing to a "small" thing.

Got it?

 

No. I don't understand why you use 20log instead of 10log.

Also, my question was fairly straightforward. Will I be measuring -103dB ?

 

No. I don't understand why you use 20log instead of 10log.

Also, my question was fairly straightforward. Will I be measuring -103dB ?

We use 20log for quantities like voltage and current, but we use 10log for power because it is derived from the product of voltage and current, or the current squared times impedance, or Voltage squared over impedance. When you take the log of something squared the factor of 2 comes out as a multiplicative constant.

With respect to your question:

2.5 μW ≠ 2,500 mW because a μW is smaller than a mW (not sure why you included this)
Yes, a 103 dB loss is the same as -103 dB is the same as 5e-11

+103 dB ≈ 1.99e10
-103 dB ≈ 5e-11

\(-103\;\text dB = 10\;*\;log\left(\frac{2.5e^{-6}}{50,000}\right)\)

If you wanted to invent a unit for this measurement you might call it:

dB50KW

or dB with respect to 50 kilowatts, so your meter would read -103 dB50KW

Got it?
​

 

You seem to be inconsistent. There was no mention in your salary example of salary squared. So it should be 10log. Got it ?

 

Reading between the lines he means "Thank you for going out of you way and carefully explaining the answer to my question."

 

A Tutorial on the Decibel from the ARRL (American Radio Relay League) is a pretty good read on the subject. The article explains in a nice, easy to understand fashion things like what a decibel actually is and why we see formulas using for example 10 Log or 20 Log. I suggest those who would like a good understanding of how decibels work read the article. Additionally, this forum also has a read on the subject.

Ron

 

The question has been answered.