Wednesday, February 16, 2011

VHF or UHF Yagi BALUN Calculator.


For the VHF/UHF frequencies, a 4:1 impedance ratio coaxial balun is normally used. Two sections of identical coaxial cable are needed. One section (A) has a convenient length to reach between the antenna and the transmitter.  Its characteristic impedance is Z.

The other section (B) is a half-wavelength long at the center of the frequency of interest.
The "physical" length is found from: 5904/F = L.  The complete formula is:
        L = 5904 * V/ F  in MHz
L is the cable length, in inches F is the operating frequency, in megahertz
V is the velocity factor of the coaxial cable. 
The result is found by multiplying L by V.
To find the "Electrical" length, divide this result by F.
The velocity factors of common coaxial cables are shown in the following table.
Coaxial cable velocity factors
Regular polyethylene 0.66
Polyethylene foam 0.80
Teflon 0.72

Both radio signals and light travel almost 300,000,000 meters (186,363 miles) per second.

When designing a matching or phasing BALUN for a VHF or UHF Yagi, the quarter wave transformer is where these calculations will come to life.   Most coax cables we use in HAM radio have varying velocity factors (VF).  That is; RF signals travel at different speeds through these coaxial cables, depending on the cable type we use.   For example, the coax cable you are using has a velocity factor of .80%.  This indicates that the electrical length is actually 80 percent of of its "free space length".  When making a VHF or UHF BALUN or phasing transformer we must be sure we have included the velocity factor in our computations.

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