Antenna question

I'll be grateful if you could explain this:

Is antenna gain in dB related to voltage or to power? For example, suppose there are two receiving antennas with different gains, but identical in frequency, impedance and terminating load, and antenna A has a 6dB gain over antenna B. If they are both pointed at the same transmitting source from the same distance, what will be the ratio of the voltages they deliver at the load - 4:1 or 2:1?

Conversely, if the two antennas are used as radiators and pointed at the same receiving antenna, what will be the ratio of the voltages they induce in the receiving antenna?

Or are they really the same db number for voltage and power? If voltage, 6db will be in dbv or x2, resulting in x4 power for the same impedance, and x4 power is 6db. Does that make any sense?

Antenna gain is usually measured in dBi. The "i" is for isotropic. Same power in all directions. For some examples see here:

Tom

"Pawihte"

** There is only one kind of dB and it always relates to power. 3 dB is a doubling of power, 6 dB is a quadrupling of power.

In a circuit where impedance remains the same, 3dB is a 41% increase in voltage and 6 dB is a doubling of voltage.

.... Phil

Phil, this is the kind of useful, level-headed answer that we all like.

But I, and probably others, would really appreciate it if you could find it in yourself to drop the frequent "f*****ad" schtuff.

OK?

Thanks! Rich

** No point whatever in me or anyone posting GOOD information when others like yourself regularly undo any benefit to be obtained by posting utter BOLLOCKS !! ** Stop posting bollocks on matters you are not competent about.

.... Phil

Thank you. I have read about dbi and understand the concept of a theoretical isotropic antenna. But I believe dbd, relative to an ideal dipole, is also used. Some specs simply say db or dB. In the same way, I often see dbv (not necessarily in connection with antennas) for voltage ratios where dbv = 20*log(V1/V2), but not always with the v inserted. Anyway, my question was really not about which ideal antenna is used as a reference, but which db/dB.

Thank you. I understand the math but db or dB is so often also used to express a voltage ratio as dBv = 20*log(V1/V2), and not always with a 'v' added, that I wanted to be sure. Thanks again.

"Pawihte"

** Well, now you know there is only one kind of dB.

BTW: Using "db" is incorrect.

.... Phil

"Pawihte"

** Sorry - but that ( erroneous) formula gives a POWER ratio. ** When another letter is added to "dB" it changes the meaning.

Normally, dBs are pure ratios - but if a "m" or "v" or "V" or "W" suffix is added - it becomes an actual level and no longer a ratio.

You need to find out what each suffix stands for and then use the number to compute the actual quantity.

Eg. "m" = milliwatt - so 6dBm equals 4 milliwatts.

.... Phil

When a function is plotted on log/log scales, the 'unit' for time is a two-fold frequency ratio, which borrows from music the term 'octave'. The 'unit' for power is a tenfold power ratio, the 'Bel'. So far, so good.

But, lots of logarithmic functions are discussed in loose language, and we've all done damage to these concepts. I had a transducer/ amplifier combination, whose gain I recorded in dB related to 0 dB=3D=3D 1V output per 1A input...

The THAT 2181 data sheet (it's a current-in/current-out amplifier) refers to gain always in dB, but never explains that this is a current ratio, not power. Me, I'm peeved at things like that.

The two-items-that-are-proportional graph has "3 dB per octave" character but what that conventionally means is log(2)/log(10) Bel per octave, which would be 3.01029 dB per octave...

"whit3rd"

The THAT 2181 data sheet (it's a current-in/current-out amplifier) refers to gain always in dB, but never explains that this is a current ratio, not power.

** The data sheet makes it clear the gain ( and other figures) figures refer to a given test circuit ( figure 2) that is clearly a voltage amplifier.

Me, I'm peeved at things like that.

** Wanker.

.... Phil

db normally indicates a ratio and usually is mention in the material that you are reading, as a gain, etc.

How ever....

When not specifying a gain/ratio it's either stamped with an additional symbol or you assume the reference to be in a particular subject... For example, a couple of references that would be know only by those in the field..

TV 0 db = 1mV into a 75 ohm load

Audio type equipment

0 db = 1mW into a 600 ohm load and gives you ~ 0.7746V rms. * 1.414 = 1.08 pk.

Some other 0 db level references..

dbj = 1mV, dbk = 1 KW, dBm = 1 mW into 600 ohms dBv = 1V..

a voltage gain is dBvg

And I've seen the use of dBpg "Power gain" also...

But mostly, only the dB is used and noted near by as to what it really is pointing to.

etc..

Jamie

In electronics dB almost always refers to a power ratio

dB = 10*log(P1/P2) [1]

but since, for a constant impedance, Power is proportional to Voltage squared you can go straight from the voltage ratio to dB with

dB = 20*log(V1/V2)

This is a short cut which works as explained below

P = V^2/R [2]

So substituting P for V^2/R equation 1 becomes

dB = 10*log( (V1^2/R) / (V2^2/R) )

Now cancel the Rs (this only works for constant impedance otherwise the Rs won't cancel)

dB = 10*log( (V1^2) / (V2^2) )

re-arrange

dB = 10*log( (V1/V2)^2 )

Then using the general mathematical equation:

log(x^y) = y*log(x) where x and y are any numbers

dB = 10*log( (V1/V2)^2 ) = 2*10*log(V1/V2)

fer

Define 'clear'. The figure 2 circuit is 'typical application circuit' on my copy of the datasheet, NOT 'test circuit'. There's a test circuit mention in one footnote, on the 'electrical characteristics' data, but nothing similar in the discussion or on subsequent data and graphs.

Because that circuit's input and output/feedback resistors are the same, the ratio for its voltage gain and for IC current gain is identical in any case. Neither voltage ratio nor current ratio is explicitly referenced by the datasheet, however, just the ambiguous 'dB gain'. The best hint that this isn't a power gain, is that the test doesn't specify any load.

0 dBM = 1mV into a 75 ohm load.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

Now you're being the FUCKTARD! I was trying to encourage you to post useful information without the cussing, but apparently even when I try to say something positive, you don't like it.

Go f*ck yourself.

Good Luck! Rich

ix is

to

But isn't an antenna gain said to be so many dBs, and assumed to be a voltage ratio? If there is no load on the antenna, high impedance input, why would power gain mean anything?

I hear AM radios need about 120 dB gain, and assume the meaning is with a 1 microvolt input, the audio output is about 1 volt, or 120dB higher.

-Bill

"Bill Bowden"

But isn't an antenna gain said to be so many dBs, and assumed to be a voltage ratio?

** dBs are always power ratios.

For a fixed impedance, the corresponding voltage ratio can be easily calculated.

If there is no load on the antenna, high impedance input, why would power gain mean anything?

** The "gain" of an antenna is specified with the stated load impedance applied.

.... Phil

Which brings us back to my original question since I'm not "in the field" of radio communication. When the gain of an antenna design is specced as 6dB or 10dB or whatever, is that a voltage gain or a power gain?

"Pawihte" <

** This is like teaching a pig to sing ...

Oink, oink ....

.... Phil