t-match construction
the t-match with its balanced feed point is one method of feeding a balanced dipole. a suitable 4:1 balun is used in conjunction with the t-match to perform this function. the t-match, with a 4:1 balun and balanced dipole can be substituted for a folded dipole with its 4:1 coax balun.
schematic:
spacing between arms & dipole can be anything convenient for construction
adjustment of the t-match with a 4:1 balun is simple - just adjust the position of the shorts on the arms until you get an acceptable swr. for the lowest swr it may be necessary to change the length of the driven element. after changing the driven element readjust for the lowest swr.
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6m
***** 6m antenna t - match *****
frequency of the antenna (mhz) 50
lambda 5995.85 mm or 236.057 ins
dipole 2836.04 mm or 111.655 ins
dipole diameter 12.7 mm or 0.5 ins
arm length 709 mm or 27 ins
arm diameter 6.3 mm or 0.25 ins
element spacing: make it convenient for construction
***** 4:1 coaxial cable balun *****
the frequency selected is: 50 mhz
the coax you selected is: rg-213/u
the velocity factor of the coax selected is: .66
the physical length of a half-wave coaxial (4:1)
balun is: 1978 mm or 77 ins
2m
***** t - match *****
frequency of the antenna (mhz) 144.300
lambda 2077.56 mm or 81.794 ins
dipole 982.69 mm or 38.688 ins
dipole diameter 4.765 mm or 0.187 ins
arm length 245 mm or 9 ins
arm diameter 4.765 mm or 0.187 ins
element spacing: make it convenient for construction
***** 4:1 coaxial cable balun *****
the frequency selected is: 144.3 mhz
the coax you selected is: rg-213/u
the velocity factor of the coax selected is: .66
the physical length of a half-wave coaxial (4:1)
balun is: 685 mm or 26 ins
(note: the physical length is for the shield. keep the center conductor as short as possible. keep the insulation long enough for high power. example: for 432 mhz use a 5 mm or .197 ins length of insulation.)
***** balun checking procedure *****
to make or check a 4:1 balun when the velocity may be unknown use a .7 factor and cut the coax longer than the formula indicates for the shield. cut the leads as short as possible but to a useable length, short them together on one end. on the other end connect the leads together to form a small loop. use a grid-dip oscillator (gdo) with minimum coupling to the loop and tune for a sharp dip. adjust the gdo while listening for the gdo on your receiver. reduce the length of the coax in small increments until the frequency on the receiver approaches the desired frequency. with each adjustment keep the leads just long enough to make the connections. you may also use the above method to check any balun which was made using this type of baluns. the velocity factor in some coaxial cable has been found to vary from manufacturers specifications so it is a good idea to check your balun to make sure it can do the job.
keep the leads short and use minimum coupling
70cm
***** t - match *****
frequency of the antenna (mhz) 431
lambda 695.57 mm or 27.385 ins
dipole 329.01 mm or 12.953 ins
dipole diameter 4.765 mm or 0.187 ins
arm length 82 mm or 3 ins
arm diameter 3.175 mm or 0.187 ins
element spacing: make it convenient for construction
***** 4:1 coaxial cable balun *****
the frequency selected is: 431 mhz
the coax you selected is: rg-213/u
the velocity factor of the coax selected is: .66
the physical length of a half-wave coaxial (4:1)
balun is: 229 mm or 9 ins
