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4.3 Load Couples into Power Line
Occasionally, an antenna will strongly couple into nearby power lines (e.g. a long wire
antenna in the attic). When this occurs, the plot lines may be “thicker” and the readings will
shift around a lot. There are two methods to eliminate this effect
1. Power the Bravo MRI II from batteries. The wall power pack must be unplugged from
the Bravo MRI II power jack to do this.
2. Connect the ground sleeve of the coaxial connector to an earth ground.
If the above methods do not clear up the plots, there is most likely an interfering signal
causing the problem.
4.4 Tune an antenna to resonance
To tune an antenna to resonance, set the Bravo MRI II center frequency to the desired
resonant frequency. Connect the Bravo MRI II to the antenna. Using the adjustment
provided by the antenna manufacturer, tune the antenna for a zero phase crossing at the
Bravo MRI II center frequency. The tuning adjustment could be one of several methods, and
should be mentioned in the instructions for the antenna.
4.5 Measure the length of a coax
To find the length in degrees (at some frequency) set one of the plot types to Reflection
coefficient angle (see paragraph 3.9.1.1.6). Set the Bravo MRI II center frequency to the
frequency of interest. Note the RCA reading. By adjusting the sweep width to the widest,
and stepping through a number of center frequency values, determine how many positive to
negative zero crossings there are below the frequency of interest. The length in degrees is:
L = (180)(number of zero crossings) + modified RCA
Calculate the modified RCA using this algorithm: if the RCA is positive then subtract off
360 from RCA (if RCA is negative, don’t subtract anything). Now multiply the new RCA by
negative ½. The result will be a number between 0 and +180.
To convert the length to wavelengths, divide L by 360.
EXAMPLE:
I have chosen 14.200 MHz as this is where I want to tune one of my antennas. We want to
measure a coax that disappears into a wall panel and comes out at the roof egress box, so we do
not know it’s actual length. We have reason to believe that the velocity factor of this coax is the
standard .66, but this in not really important since this calculation is based, already, on
measurements with velocity incorporated. To give the measurement in feet would require Vf .
By stepping through several sweep widths and center frequencies I have determined that there
are only 2 positive-to-negative zero crossings and the RCA value is – 46 degrees. Using the
formula from above we start with multiplying 180 times 2 (crossings)= 360. Since the RCA is
already negative, we do not need to subtract out 360 from our number of –43. The Mod-RCA
number is = -46 times –1/2, or +23. To finish out the calculation we add the first half of 360 to the
Mod-RCA of 23 to get 383 degrees.
To get wavelengths we divide by 360 to get 1.0639.
To convert to feet we do the following:
Divide 982.08 by 14.3 (MHz) = 68.677Ft. (wavelength in air), times Vf (.66) = 45. 327 ft (in coax).
Multiply our L of 1.0639 times 45.327 = 48.223 Ft or just over 48 Feet 2 1/2 inches. The plans
called for 1 ½ wavelengths, so I think we have a problem to look for.