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4 Applications and Measurement Examples
4.1 Make a ½ wavelength coaxial line
To make a coaxial line that is tuned to one half wavelength:
1. Start with a piece of coax that is slightly longer (about 5 to 10%) than calculated. The
formula for a ½ wave cable is:
L = 491(vf) / fr
where
L = length in feet
vf = velocity factor (usually between .6 and .9)
fr = frequency.
2. Connect one end of coax to Bravo MRI II (preferably using a coaxial connector), the
other end is left open. Set the Bravo MRI II center frequency to the desired ½ wave
frequency. There should be an impedance peak (and a zero phase crossing) slightly to
the left of the center frequency. The zero phase crossing should go from upper left to
lower right. Use wire cutters to snip off small pieces of the coax from the unconnected
end. Every time a piece is removed, the zero phase crossing should move to the right.
When the zero phase crossing reaches the center frequency, the coax is at ½ wave
length.
3.
Alternate method: Far end of coax is shorted. Look for zero phase crossing from lower
left to upper right. Impedance will be very low. This method is less preferred, as the
coax must be re-shorted after every snip.
4.2 Make a ¼ wavelength coaxial line
To make a coaxial line that is tuned to one half wavelength:
1. Start with a piece of coax that is slightly longer (about 5 to 10%) than calculated. The
formula for a ½ wave cable is:
L = 245.5(vf) / fr
where
L = length in feet
vf = velocity factor (usually between .6 and .9)
fr = frequency.
2. Connect one end of coax to Bravo MRI II (preferably using a coaxial connector), the other
end is left open. Set the Bravo MRI II center frequency to the desired 1/4 wave
frequency. There should be a low impedance and a zero phase crossing slightly to the
left of the center frequency. The zero phase crossing should go from lower left to upper
right. Use wire cutters to snip off small pieces of the coax from the unconnected end.
Every time a piece is removed, the zero phase crossing should move to the right. When
the zero phase crossing reaches the center frequency, the coax is at 1/4 wave length.
3. Alternate method: Tune the Bravo MRI II to twice the ¼ wave frequency, and look for
the zero phase crossing (upper left to lower right).