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

Jay Coward jcoward5452 at aol.com
Tue Oct 29 16:52:23 EDT 2013 

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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