Applications And Measurement Examples; Make A ½ Wavelength Coaxial Line; Make A ¼ Wavelength Coaxial Line - AEA VIA Bravo MRI Operating Manual

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 in MHz.
2. Connect one end of coax to VIA Bravo (preferably using a coaxial connector), the other
end is left open. Set the VIA Bravo 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 quarter 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 VIA Bravo (preferably using a coaxial connector), the other
end is left open. Set the VIA Bravo 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 VIA Bravo to twice the ¼ wave frequency, and look for the
zero phase crossing (upper left to lower right).
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