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

Tue Oct 29 19:17:55 EDT 2013

Of course you are right Mike,,,I forgot about the effect of the V squared in the power equation....

________________________________
  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: Tuesday, October 29, 2013 1:41 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
this as your conclusion is in error. 73 - Mike 

Mike B.
 Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Christopher Bowne
Sent: Tuesday, October 29, 2013 1:19 PM
To: D C _Mac_ Macdonald; Leslie Smith
Cc: ARC-5 List
Subject: [ARC5] dB Power Apples and dB Voltage Oranges, was Re: Selectivity
Ratings...

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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