[BC] dbu to microvolt program

[Peter Burk]

Richard,

Ron D said:
>Several months ago someone on the list provided a link to a program that
may have been on some manufacture's site that converted dbu to uVolts and
visa versa.

I took the original poster's reference to dbu to be the traditional fcc
definition of dBu, not shorthand for microvolts as in your dB?v.

Richard said:
>Where we disagree is whether or not  the frequency is required to convert 
the value of a radiated field from units of V/m to units of dB?V/m --  

We do not disagree that you can convert volts/meter to dB referenced to 1
microvolt/meter.  Both terms are in the same units. Ron D is asking to
convert dBu to ?V.

Perhaps it will be easier to understand the point if you consider a real
world example.  A signal is known or predicted to have a 40dBu signal at
your location. You have a receiver with 20?V sensitivity. Can you receive
the signal? Answer:  It depends on the frequency. 

Note also that the other things you refer to are all taken into account in
coming up with the measured 40dBu signal level.  We do assume 50 ohms and a
dipole antenna, but none of the other factors you mentioned earlier enter
into this calculation.

Richard also said:
>For all the conversions built in to that Excel spreadsheet, strangely it 
does not include the simple ones needed to convert a field strength between 
units of dB?V/m and volts/m.

These are the formulas you gave and they are right. I don't have the
spreadsheet in front of me, but ISTR dB to volts. Again, doing dBu to volts
using the spreadsheet requires the two-step process outlined earlier.

Peter Burk
Burk Technology



-----Original Message-----
From: Richard Fry [mailto:rfry at adams.net] 
Sent: Tuesday, April 10, 2007 8:41 AM
To: Broadcast List
Subject: [BC] dbu to microvolt program

Peter Burk wrote:
>dBu, as used in RF, is referenced to 1uV/meter. Only at about
>40 MHz is dBu equal to dBuv, which presumably references to
> 1 1 microvolt.   ...  You simply can't get dBu from microvolts
> without the frequency.
_________________

Hi, Peter -

As I posted yesterday, we don't disagree on the point that the frequency 
must be known (with other elements) in order to convert an arriving 
(radiated) field intensity into the voltage or current or power that exists 
in some impedance connected to the feedpoint of a receiving antenna 
immersed in that field.

Where we disagree is whether or not  the frequency is required to convert 
the value of a radiated field from units of V/m to units of dB?V/m --  
which, I believe, was the need of the original poster in this thread.  Note 
we haven't changed the impedance in space across which that field exists, 
only the units used to quantify the field.  No receiving antenna is 
involved; that field exists whether an antenna is there or not..

Volts are related to decibels with respect to 1 ?V only by the expression 
20*log(E in microvolts).   There is no frequency term there.  So for 
example to express a field strength of 1 mV/m as a field strength in 
dB?V/m, we have 20*log(1000) = 60 dB?V/m (or "60 dBu" to many).

Note that the FCC uses this relation when defining the field strengths in 
"dBu" for various allocation, protection, and minimum/maximum field 
strength purposes.  The 60 "dBu" contour is always the 1 mV/m contour, 
whether the r-f frequency is in the MW or the UHF spectrum.

For all the conversions built in to that Excel spreadsheet, strangely it 
does not include the simple ones needed to convert a field strength between 
units of dB?V/m and volts/m.  Maybe they were too simple.  And I am puzzled 
about #7 in the spreadsheet.  None of the units is given in terms of 
radiated/received field, so why is the frequency needed?  I'll try to get 
the answer from the author, and report back to the list.

Rich 

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