The transmatch, or ham radio antenna tuner, is a variable impedance matching device. It is used between the transmission line of an HF antenna system and a transceiver or transmitter.
Its purpose is to make the antenna system "look like" a purely resistive load - usually 50 Ohms.
The ham radio operator varies the inductance, along with input and/or output capacitances, in an effort to "tune out" (cancel out) the capacitive and/or inductive reactances that may appear at the transmitter end of the transmission line.
Why match the antenna system impedance to the transmitter's output impedance?
Because maximum transmitted energy is transfered to the load when the load matches the characteristic impedance of the transmitter output (usually 50 Ohms).
That is what we all want ... that all the RF energy generated by the transmitter reaches the antenna, instead of heating the components of the final amplifying stage and risk damaging them.
Please note that the transmatch - when installed between the transmitter output and the transmission line . . .
will not eliminate the standing waves that may appear on the transmission line and will not eliminate the signal lost as heat within the transmission line due to the standing waves.
will not tune the antenna to resonance, at the other end of the transmission line (an unfortunate popular misconception).
T-Networks VS L-Networks
Most inexpensive "antenna tuners" out there today are T-networks. The MFJ-940D pictured above is an example.
You should know that T-network tuners are the least efficient of all possible network configurations.
Why do manufacturers offer them? Because they do not cost much to manufacture ... and, consequently, cost less to buy!
Mind you ... the T-network configuration will do the job of transforming complex antenna system impedances, appearing at its output (antenna system side), to a 50 Ohm impedance at its input (transmitter side).
However, in the process of cancelling out unwanted antenna system reactances, the T-network will waste a prohibitive portion of the RF energy as heat! (ref: ARRL Antenna Handbook, chap.25).
Automatically Switchable L-Network Transmatch
L-networks require very large capacitance values ... in the thousands of picofarads (pf) at low frequencies! Variable capacitors with this much capacitance are extremely expensive.
The AT-600Pro Autotuner by LDG neatly solved the problem of cost while preserving full flexibility, efficiency and effectiveness of the L-network configuration! :-)
The AT-600 automatically chooses the proper L-network configuration, to adapt itself to high or low impedance situations, and switches fixed values of capacitance in or out to fine tune for a match.
Very clever! That's why I chose the AT-600Pro transmatch instead of the MFJ 940D for multiband operation with my Carolina Windom antenna.
I modified the 940D for 160 meter work by adding fixed capacitors in parallel with the input and output variable capacitors. It's a compromise but it lets me go on 160 with a second, homemade antenna that I also use as receive antenna for my QS1R SDR receiver.