TYansmatcfi circuit 335
The split capacitor network shown in Fig. 19-4 is used to transform a source impedance that is less than the load impedance. In addition to matching antennas, t his circuit is also used for interstate impedance matching inside communications equipment. The equations for design are:
One version of the transmatch is shown in Fig, 19-5. This circuit is basically a combination of the split-capacitor network and an output tuning capacitor (C3). For the HF bands, the capacitors are on the order of 150 pF per section for Cj and 250 pF for C2. The roller inductor should be 28 jaH. The transmatch is essentially a coax-to-coax impedance matcher and is used to trim the mismatch from a line before it affects the transmitter.
Perhaps the most common form of transmatch circuit is the tee-network shown in Fig. 19-6. This network is lower in cost than some of the others, but suffers a problem- Although it does, in feet, match impedance (and thereby, in a naive sense, "tunes out* VSWR on coaxial lines), it also suffers a high-pass characteristic. This network, therefore, does not reduce the harmonic output of the transmitter. The simple tee-network does not serve one of the main purposes of the antenna tuner;
Improved SPC transmateh circuit.
harmonic reduction. An alternative network, called the SPC transmateh, is shown in Fig, 19-7. This version of the circuit offers harmonic attenuation as well as matching impedance.
Figure 19-8 shows commercially available antenna tuners based on the trans-match design, The unit shown in Fig. 19-8A is manufactured by MFJ, It contains the usual three tuning controls, here labeled Transmitter, Antenna, and Inductor. Included in this instrument is an antenna selector switch that allows the operator to select a coax cable antenna through the tuner to connect input to output (coax) without regard to the tuneT and select a balanced antenna or an internal dummy load. The instrument also contains a multifunction meter that can measure 200 or 2000 W (full-scale) in either forward or reverse directions. In addition, the meter operates as a VSWR meter.
Figures 19-8 shows a tuner from the United Kingdom, This instrument, the Nevada model, is low-cost but contains the three basic controls. For proper operation, an external RF power meter or VSWR meter is required. This tuner and a Heathkit transmatch antenna tuner are shown in Fig. 19-8B. There are SO-239 coax-
7Varisfnatch circuit 337
Commercial HF antenna-matching networks.
ial connectors for input and unbalanced output along with a pair of posts for the parallel line output. A three-post panel is used to select which antenna the RF goes to: unbalanced (coax) or parallel The roller inductor is in the center and it allows the user to set the tuner to a wide range of impedances over the entire 3- to 30-MHz HF band.
Coaxial cable balun transformers
A balun is a transformer that matches an unbalanced resistive source impedance (such as a coaxial cable) and a iwtianced load (such as a dipole antenna). With the circuit of Fig. 19-9, you can make a balun that will transform impedance at a 4:1 ratio, with =■ 4 X flj. The length of the balun section coaxial cable is:
Lu, = the length in feet V = the velocity factor of the coaxial cable (a decimal fraction) FW ■ the operating frequency in megahertz.
Coaxial balun transformer.
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