Antenna question

P

Pawihte

Guest
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?
 
"Pawihte" wrote in message news:ihcmg2$6k6$1@news.eternal-september.org...

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:
http://radarproblems.com/chapters/ch06.dir/ch06pr.dir/c06p6.dir/c06p6.htm

Tom
 
"Pawihte"
Is antenna gain in dB related to voltage or to power?
Click to expand...

** 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 Allison wrote:
"Pawihte"

Is antenna gain in dB related to voltage or to power?

** 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, this is the kind of useful, level-headed answer that we all like.
Click to expand...

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

OK?

Thanks!
Rich
 
"Rich Grise"
Phil Allison wrote:
"Pawihte"

Is antenna gain in dB related to voltage or to power?

** 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, this is the kind of useful, level-headed answer that we all like.
Click to expand...

** 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 !!


But I, and probably others, would really appreciate it if you could find
it in yourself to drop the frequent "fuckhead" schtuff.
Click to expand...

** Stop posting bollocks on matters you are not competent about.



..... Phil
 
"Pawihte"
Anyway, my question was really not about which ideal antenna is used as a
reference, but which db/dB.
Click to expand...
** Well, now you know there is only one kind of dB.


BTW: Using "db" is incorrect.


..... Phil
 
Tom Biasi wrote:
"Pawihte" wrote in message
news:ihcmg2$6k6$1@news.eternal-september.org...
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:
http://radarproblems.com/chapters/ch06.dir/ch06pr.dir/c06p6.dir/c06p6.htm

Tom
Click to expand...
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.
 
Phil Allison wrote:
"Pawihte"

Is antenna gain in dB related to voltage or to power?


** 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.

Thank you. I understand the math but db or dB is so often also
Click to expand...
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"

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),
Click to expand...
** Sorry - but that ( erroneous) formula gives a POWER ratio.


always with a 'v' added, that I wanted to be sure.
Click to expand...

** 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
 
"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.

http://thatcorp.com/datashts/THAT_2181-Series_Datasheet.pdf

Me, I'm peeved at things like that.


** Wanker.


..... Phil
 
Pawihte wrote:
Phil Allison wrote:

"Pawihte"

Is antenna gain in dB related to voltage or to power?


** 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.


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.
Click to expand...
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
 
On 21/01/2011 22:46, Pawihte wrote:
Phil Allison wrote:
"Pawihte"

Is antenna gain in dB related to voltage or to power?


** 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.

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.
Click to expand...
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)
 
On Jan 21, 2:46 pm, "Pawihte" <pawi...@invalid.invalid> wrote:
Phil Allison wrote:
"Pawihte"

Is antenna gain in dB related to voltage or to power?

** There is only one kind of dB and it always relates to power.

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.
Click to expand...
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== 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" = 1/4 wit
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.

http://thatcorp.com/datashts/THAT_2181-Series_Datasheet.pdf
Click to expand...
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.

** FFS dickwad -

the " Electrical Characteristics " table has a suffix " 2 " after the
words.

Suffix 2 refers to: " Test circuit as shown in Figure 2 ".

Means ALL the data in the table relates to that circuit.
-------------------------------------------------------------

Piss off - you bullshitting, bloody MORON !!!



...... Phil
 
Jamie wrote:
Pawihte wrote:
Phil Allison wrote:

"Pawihte"

Is antenna gain in dB related to voltage or to power?


** 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.


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.



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
Click to expand...

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.
 
On Jan 21, 3:58 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"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.

http://thatcorp.com/datashts/THAT_2181-Series_Datasheet.pdf
Click to expand...
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.
 
Phil Allison wrote:

"Rich Grise"
Phil Allison wrote:
"Pawihte"

Is antenna gain in dB related to voltage or to power?

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

** 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 !!

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

** Stop posting bollocks on matters you are not competent about.

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

Go fuck yourself.

Good Luck!
Rich
 
"Rich Grise"
Phil Allison wrote:

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

** 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 !!

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

** Stop posting bollocks on matters you are not competent about.

Now you're being the FUCKTARD!
Click to expand...

** Jesus fucking Christ !!!!!!!!!!!!!!

You are nothing but a LYING, BULLSHITTING, POSTURING CUNT.

YOU are the one who needs to change and drastically too.

Or better still - fuck the hell off.



...... Phil
 
"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
 
whit3rd" = a fucking 1/4 wit

** 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.

http://thatcorp.com/datashts/THAT_2181-Series_Datasheet.pdf
Click to expand...

** FFS dickwad -

the " Electrical Characteristics " table has a suffix " 2 " after the
words.

Suffix 2 refers to: " Test circuit as shown in Figure 2 ".

Means ALL the data in the table relates to that circuit.
-------------------------------------------------------------

There are NO ambiguities in the data sheet long as you read it ALL.

Fuck off - you bullshitting, fucking MORON !!!




...... Phil
 
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