[BC] dbu to uV

[Mark Humphrey]

Is this the old Jerrold CATV meter with F connector input?  I'm
familiar with those, having used one 25 years ago to make similar
measurements.  However, if 0 dB is referenced to 1000 microvolts (1
millivolt), then it reads in dBmV .  You need to add 60 dB to convert
from millivolts to microvolts.  It also has an input impedance of 75
ohms, but that's probably not a big deal for informal measurements;
consider it a VSWR mismatch of 1.5.

To convert from voltage to field strength, you need to know the
"antenna factor" of your test antenna.  Equations are in this
Wikipedia article:

http://en.wikipedia.org/wiki/Antenna_factor

The "ballpark figure" I've used for a 50 ohm dipole in the FM band is
an antenna factor of 2, in other words, 6 dB.  The Potomac Instruments
dipole supplied with the FIM-71 is very close to this value.

Also, keep in mind that the FCC prediction curves are based on a
standard receive antenna height of 9 meters.  If the elevation of your
test antenna is less, the voltage measured at the receiver will also
decrease.

I ran a 2000 mile mobile drive test of the Philadelphia market about
10 years ago, using a PC-controlled field strength meter, GPS, and 1/4
wave whip on the roof of an SUV, which was 2 meters AGL.  After making
a few supplemental tests with a 30 foot Will-Burt telescoping mast, I
determined that the height correction factor was nearly 14 dB.  So,

Antenna factor =     6 dB
Height correction = 14 dB

Total                  = 20 dB, which is easy to remember.   Add this
to the voltmeter reading to convert to field strength.

Example:

You are using a unity-gain antenna atop your vehicle at 2 meters AGL.

The meter indicates 1 millivolt, which equals 0 dBmV = 60 dBuV
The field strength intercepted by your antenna  =   66 dBuV/m
And, if the antenna were raised to 9 meters AGL, the assumed field
strength at that height (as predicted by FCC curves) would be 80
dBuV/m

Mark



On 4/10/07, Ronald J. Dot'o Sr. <ron.doto at comcast.net> wrote:
>
> Thanks to all who provided links and programs for the above conversion.  My
> situation is that I have an old FIM designed for the cable industry  that
> reads uV and db where 0 db is 1000uV and I needed to know how many uV were
> 70dbu so that I could get a ballpark reading on where the 70 dbu contour (in
> uV) is for a particular station.  Now I know it's 3.16 mV/m and I can do the
> check.
>
> Thanks again,
>
> Ron D
>
>