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

Tue Oct 29 17:01:58 EDT 2013

Does an S-Meter reflect POWER or VOLTAGE?

Receiver sensitivity is measured in microvolts! NOT POWER!

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Jay Coward" <jcoward5452 at aol.com>
To: <n4fs at eozinc.com>; <aj1g at sbcglobal.net>; <k2gkk at hotmail.com>; 
<vk2bcu at operamail.com>
Cc: <arc5 at mailman.qth.net>
Sent: Tuesday, October 29, 2013 1:52 PM
Subject: Re: [ARC5] dB Power Apples and dB Voltage Oranges, was Re: 
Selectivity Ratings...

>
> In tech school we learned it like this: For a decernable increase in 
> received signal, you had to increase transmitter power by four times. I 
> think there were problems on the FCC 2nd class test like this.
> Jay
>
>
>
> -----Original Message-----
> From: Mike Feher <n4fs at eozinc.com>
> To: 'Christopher Bowne' <aj1g at sbcglobal.net>; 'D C _Mac_ Macdonald' 
> <k2gkk at hotmail.com>; 'Leslie Smith' <vk2bcu at operamail.com>
> Cc: 'ARC-5 List' <arc5 at mailman.qth.net>
> Sent: Tue, Oct 29, 2013 1:11 pm
> Subject: Re: [ARC5] dB Power Apples and dB Voltage Oranges, was Re: 
> Selectivity Ratings...
>
>
> Chris -
> Without going into the details, I'll let you do it, but, you should 
> re-think
> his as your conclusion is in error. 73 - Mike
> Mike B. Feher, N4FS
> 9 Arnold Blvd.
> owell, NJ, 07731
> 32-886-5960
>
> ----Original Message-----
> rom: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
> ehalf Of Christopher Bowne
> ent: Tuesday, October 29, 2013 1:19 PM
> o: D C _Mac_ Macdonald; Leslie Smith
> c: ARC-5 List
> ubject: [ARC5] dB Power Apples and dB Voltage Oranges, was Re: Selectivity
> atings...
> When you are measuring the selectivity of a circuit, you typically are
> ooking at a voltage measurement.  dB change relative to a reference 
> voltage
> ollows the equation:
> dBx = 20 X log(vx/vref)
> so a doubling of a measured voltage relative to a reference voltage to a
> esults 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
> utput 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
> dB per S unit calibration that is measuring a voltage, will show an
> ncrease of 20 dB, or over 2 S units.  Same result for a field strength
> eter.
> I think I just convinced myself that, as far as the effects observed on a
> eceiving station's S meter are concerned, adding 10 dB of power gain with
> n amplifieris twice as effective as adding 10 dB of antenna pattern gain,
> ince  unless I am missing something, antenna gain measurements are made
> omparing differences in received voltages of a field strength meter.
> It all goes back to the power equation Power in Watts = Voltage
> quared/Resistance.
>
>
>
>
> _______________________________
> From: D C _Mac_ Macdonald <k2gkk at hotmail.com>
> o: Leslie Smith <vk2bcu at operamail.com>
> c: ARC-5 List <arc5 at mailman.qth.net>
> ent: Tuesday, October 29, 2013 11:56 AM
> ubject: Re: [ARC5] Selectivity ratings of ARC-5 receivers.
>
> You have to remember that if you double the voltage applied to a fixed
> esistance/impedance you will also get double the current giving FOUR times
> he power (6 dB)!
>
> 3 - Mac, K2GKK/5 in OKC
>
> ent 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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