That's a nice calculator

[LeoG]

https://absolute-emc.com/calculator/convert/dBm

[WRYZ926]

It would be nice if antenna manufacturers and retailers would list the antenna gain in dBd instead of dBi.  The dBd value is more realistic and also what most coax loss calculators use. The conversion from dBi to dBd is pretty simple to remember, subtract 2.14 from the dBi value to get dBd.

[LeoG]

I was playing around with radio mobile online and notice that their calculations are off because they are using dBi numbers in dB and dBm equations.  That's what prompted me to look for a calculator for µV to dBm.

 

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They have it as

(46.23-1.3+9.8+9.8-1.6)-132.0=-69.07dBm

I believe it should be

(46.23-1.3+7.65+7.65-1.6)-132.0=-73.37dBm

 

[LeoG]

1 hour ago, WRYZ926 said:

It would be nice if antenna manufacturers and retailers would list the antenna gain in dBd instead of dBi.  The dBd value is more realistic and also what most coax loss calculators use. The conversion from dBi to dBd is pretty simple to remember, subtract 2.14 from the dBi value to get dBd.

dBi numbers are bigger therefore you can sell more antennas.

[WRYZ926]

10 minutes ago, LeoG said:

dBi numbers are bigger therefore you can sell more antennas.

Yes I know and that is why they advertise gain in dBi. But it would still be nice if they would list the gain in dBd.

[Groundwire]

On 4/10/2025 at 2:47 PM, WRYZ926 said:

It would be nice if antenna manufacturers and retailers would list the antenna gain in dBd instead of dBi.  The dBd value is more realistic and also what most coax loss calculators use. The conversion from dBi to dBd is pretty simple to remember, subtract 2.14 from the dBi value to get dBd.

Its 2.15 not 2.14

[WRYZ926]

Just now, Groundwire said:

Its 2.15 not 2.14

I'm definitely not going to argue over 0.01. Plus 2.14 is what comes up most often when searching for the conversion for dBi to dBd.

[Groundwire]

On 4/10/2025 at 4:24 PM, WRYZ926 said:

Yes I know and that is why they advertise gain in dBi. But it would still be nice if they would list the gain in dBd.

It doesn't matter, just do the math, dbi is a real gain figure.  if you know anything about antenna theory you will find that they lie about dbd gain figures all the time as well. 3dbd gain from a 1/4 vertical? Haha, please. The real antenna companies dont lie to sell, usually.

[Groundwire]

3 minutes ago, WRYZ926 said:

I'm definitely not going to argue over 0.01. Plus 2.14 is what comes up most often when searching for the conversion for dBi to dBd.

Nobody's arguing. One is right one is wrong. You made a mistake, no big deal

[WRYZ926]

4 minutes ago, Groundwire said:

Nobody's arguing. One is right one is wrong. You made a mistake, no big deal

Well it seems that we are both correct and also both are wrong. I just did a web search and half of the links say 2.14 while the other half say 2.15. Either way 0.01 is not going to make a difference.

And yes most manufacturers are optimistic when it comes to claims on antenna gain. This is especially true with hand held and mobile antennas.

[LeoG]

14 hours ago, WRYZ926 said:

Well it seems that we are both correct and also both are wrong. I just did a web search and half of the links say 2.14 while the other half say 2.15. Either way 0.01 is not going to make a difference.

And yes most manufacturers are optimistic when it comes to claims on antenna gain. This is especially true with hand held and mobile antennas.

So we can call it 2.145 to make it more complicated.

[WRYZ926]

7 minutes ago, LeoG said:

So we can call it 2.145 to make it more complicated.

Sounds good for me.

In reality 0.01 is not going to make any difference in the calculations. That's like arguing over the differences between 20 watts output and 25 watts output.

[LeoG]

So no one has any opinion on the calculation from Online Radio Mobile?

Quote

Quote

They have it as

(46.23-1.3+9.8+9.8-1.6)-132.0=-69.07dBm

I believe it should be

(46.23-1.3+7.65+7.65-1.6)-132.0=-73.37dBm

 

 

[SteveShannon]

6 minutes ago, LeoG said:

So no one has any opinion on the calculation from Online Radio Mobile?

 

Their calculation is correct. Total gain calculations should be relative to an isotopic antenna. 

[LeoG]

So if the parameters were in dBd with their respected reduction in strength (dBi-2.15db) then the resultant output would still be correct only because the calculations were consistent and done with all dBd gains instead of dBi?

The answer they give is in dBm, what would it be if you used dBd gain measurements?

[SteveShannon]

42 minutes ago, LeoG said:

So if the parameters were in dBd with their respected reduction in strength (dBi-2.15db) then the resultant output would still be correct only because the calculations were consistent and done with all dBd gains instead of dBi?

The answer they give is in dBm, what would it be if you used dBd gain measurements?

In that case your calculations would be 4.30 db too low. 

An isotropic antenna is theoretical, a single infinitely small source with a gain of 0, meaning multiply the input times 1 to get the output when working with linear values.

Having a dBd gain value for an antenna is convenient but for a calculator like this you always want to take the antenna gain back to dBi so you’re not inadvertently throwing the output value off. 
Maybe a better way to explain it is that the dBd value is only useful for comparing an antenna to a dipole.  For purposes of calculating total gain you always need to start at 0 which is the dBi number.