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ANTENTOP- 01- 2004, # 005

Something about ATU




The "cold" end of L1 should be grounded or isolated carefully from the metal case of ATU. Otherwise you probably will have strong arc between the ungrounded end of L1 and a wall of the ATU, as it is shown in Fig. 8. C1 and C1 should be very qualitative with aerial or vacuum dielectric. The clearance between plates of capacitors should be not less than 2-mm/ to 200-W bypass power. Stray capacitance of C1 and C2 to the metal case of ATU should be no more than 25-pF, otherwise the efficiency at 24-28-MHz drop.


Figure 8 Arcing inductor


If you want to connect symmetrical antennas feeding through symmetrical ladder lines to the T- Matching Unit, use symmetrical transformer 1:4 or 1:6. BTW, many of symmetrical antennas, feeding through ladder lines have large reactive component, which bad to transformation by simple transformers 1:4 or 1:6. The T - Matching Unit suppress harmonics up to 10-15 dB.


T - Matching Unit with a “digital” inductor


L1 has to have slide contact to operate the MU in proper way. Sometimes, even extra half of a turn influences to the matching. It restricts usage of an inductor with taps, or demands personal selection taps for real antenna, that certainly, restricts possibilities "tap" MU. Simple decision on this problem by W3TS made, he offered a "digital" inductor that Fig. 9 shows. Really, it is possible with the help of several switches very fine to tune needed inductance. Electronic relays with special chips allow realize automatic ATU. Military also use the method at their automatic ATU.


T - Matching Unit with mirror parts


For practical design of any ATU it is not conveniently to have two capacitors insulated from the ground. AEA corporation (USA) does the MU as Fig. 10 shows. You can see that they changed C to L. Really, schemes of MU figured in Fig. 7 and Fig. 10 are equivalent.


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