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

Geoff geoffrey at jeremy.mv.com
Tue Oct 29 18:06:12 EDT 2013 

Increasing power by 2X is 3dB and is readily noticable. Good CW operators 
can hear as little as 1.5dB change and oft times even under 1 dB will bring 
a very marginal signal out of the noise.
DXers, weak signal VHF and EME ops have known that for generations.

Carl

----- Original Message ----- 
From: "K5MYJ" <macklinbob at gmail.com>
To: <n4fs at eozinc.com>; <aj1g at sbcglobal.net>; <k2gkk at hotmail.com>; 
<vk2bcu at operamail.com>; "Jay Coward" <jcoward5452 at aol.com>
Cc: <arc5 at mailman.qth.net>
Sent: Tuesday, October 29, 2013 5:01 PM
Subject: Re: [ARC5] dB Power Apples and dB Voltage Oranges,was Re: 
Selectivity Ratings...


> 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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