This page is the second part of my VHF Collinear antenna project with matching-system for 2 dipoles model (Gain = 5.15 DBi).
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Fig. 1 shows the 2 dipoles with details about the coaxial cables. The space between the two dipoles is 1,5 m (0.75 wave length)."Z1"(with BLUE colour) is the "RG58" section which I've described ïn the "Gamma match Dipole" page. On this collinear the length of Z1 is 1.36 m, 1 Lamda "electrical" (Lamda/2 x 2), because the Lamda/2 is not enough from Gamma match up to "Z-TR" (L/2 is too short).
The "Z-TR" (GREEN colour) section its a piece of 75 Ohms coaxial cable (RG59) which acts as "Z" transformer. Further details of "Z-TR" shows the FIG.2
"Z-TR" acts by transforming the 50 Ohms impedance (from each dipole) to 100 and then we have again 50 Ohms on the "T" match connection point (2 impedances of 100 Ohms in parallel = 50 Ohms)
The "Z-TR" section is an "electrical quarter wave" (Lamda/4) x velocity factor of 75 Ohms RG59 (= 0.66). The impedance of Z-TR is given by the formula:
Z-TR = SQUARERoot Z1*Z2
where Z1 is the antenna impedance(50 Ohm) and Z2 the request impedance (100 Ohm).
The "quarter-wave" length on 145.000 MHZ its 0.517 m. So, the "electrical" Lamda/4 is:
0.517 x 0.66 = 0.341 m
That means a length of about 34.1 cm of RG59 coaxial cable.
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FIG.2 above shows the construction details between RG58 and RG59 coaxial cables. The "A" point must be connected with the 2nd "Z-TR" from the other Dipole and both with the RG213 (cathode coaxial to transceiver). This way, we have made a "T-match" for the 2 dipoles.
FIG.3 shows the contruction of "T-Match".
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