[ARC5] dB Power Apples and dB Voltage Oranges, was Re: Selectivity Ratings...

[Christopher Bowne]

When you are measuring the selectivity of a circuit, you typically are looking at a voltage measurement.  dB change relative to a reference voltage follows the equation:

dBx = 20 X log(vx/vref)

so a doubling of a measured voltage relative to a reference voltage to a results in a 6 dB increase.

Power changes follow the equation dBx = 10 X log (px/pref)

so a doubling of power will result in a 3 dB increase.

So if you are running 100 watts output to you antenna, and then put your 1KW output linear amplifier on service, you power output will increase by 10 dB.

The S-meter reading  at a receiving station, however, assuming that it has a 6 dB per S unit calibration that is measuring a voltage, will show an increase of 20 dB, or over 2 S units.  Same result for a field strength meter.

I think I just convinced myself that, as far as the effects observed on a receiving station's S meter are concerned, adding 10 dB of power gain with an amplifieris twice as effective as adding 10 dB of antenna pattern gain, since  unless I am missing something, antenna gain measurements are made comparing differences in received voltages of a field strength meter.

It all goes back to the power equation Power in Watts = Voltage Squared/Resistance.








________________________________
 From: D C _Mac_ Macdonald <k2gkk at hotmail.com>
To: Leslie Smith <vk2bcu at operamail.com> 
Cc: ARC-5 List <arc5 at mailman.qth.net> 
Sent: Tuesday, October 29, 2013 11:56 AM
Subject: Re: [ARC5] Selectivity ratings of ARC-5 receivers.
 

You have to remember that if you double the voltage applied to a fixed resistance/impedance you will also get double the current giving FOUR times the power (6 dB)!


73 - Mac, K2GKK/5 in OKC


Sent from my iPad

> On Oct 29, 2013, at 0:08, "Leslie Smith" <vk2bcu at operamail.com> wrote:
> 
>  Hello Ken,
>  I'll be a goose and make a public fool of myself.
>  Dad always said, "Remember son, there's no bigger fool than a public
>  fool."
>  The units of measurement 2x and 1000x are voltage (measured with a
>  volt-meter or CRO or similar meter)
> 
>  I understand 1 "S" unit equals 6 dB - and in my view this is too
>  small.  
> 
>  "My thinking (your thinking) is that in order to achieve a 3bD
>  increase in signal level at the receiver the transmitter [output
>  power] must be doubled"
>  That's correct, assuming your unit of measurement is power.
>  But at the receiver your unit of measurement is voltage.
>  dB = 20log(E2/E1)  so 20log(1000/1) is 20 * 3 = 60
>  For a 50% increase in voltage level 20 * log (2/1) = 20 * 0.3010 =
>  6dB.
> 
>  Sorry to be long winded
> 
> 
>  73 de Les Smith
>  vk2bcu at operamail.com
> 
> 
>> On Tue, Oct 29, 2013, at 14:31, Kenneth G. Gordon wrote:
>> Reading the ARC-5 maintenance manual, I see that the selectivity for,
>> say, 
>> the R-26 (3 - 6 MHz) receiver is listed at 7.3 Kc at 2X, and 26 Kc at
>> 1000X.
>> 
>> Now 1000X is 60 db, but isn't 2X 3db and not 6db?
>> 
>> Color me confused at this point.
>> 
>> My thinking is that in order to achieve a 3db increase in signal level at
>> the 
>> receiver, the transmitter must be doubled in output power, given
>> identical 
>> conditions.
>> 
>> Or am I all wet here?
>> 
>> Ken W7EKB
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