LDG make fantastic antenna tuners. An antenna tuner is needed to tune the impedance of your radio (50 Ohms) to that of the antenna, which may be considerably more depending on it's resonant properties.
These little devices work fully automatic. Just a touch of the tune button is enough for the small processor inside the unit to automatically add capacitive or inductive loads to the line to balance the impedance. Once the correct setting is found, it is stored for the next time. Each setting, per band, is stored and recalled so that the tuning time is reduced which switching between frequencies.
The old man that brought this unit in, said that it would continually hunt for a matching setting that would work but was unable to find anything that matched. So let's crack the case and look inside.
The picture above, shows the various modules that make up the tuner. As the tuner was hunting for a match, that told me that something was wrong with the SWR sensing section of the tuner. It stands to reason, that if the tuner cannot read the correct SWR, it will continually try to rattle through all permutations and combinations of inductance and capacitance, in the hope of finding a match. All for naught.
The SWR bridge is shown below and is common to very many radios. The two 1N4148 diodes are key to making the circuit work. A portion of the incoming signal is tapped off over a center tap transformer. One vertical leg of the circuit measures the forward power (FWD) and the other measures the reverse (REV) or reflected power due to the impedance mismatch. These DC voltages can then be displayed in an SWR meter to show the SWR.
Now a common mistake that we amateur radio operators make, is that we connect the tuner the wrong way around. So the transmitter to the antenna output and the antenna to the transmitter input. Ouch! Doing that, causes the SWR sensing diodes to become reversed and they give up their magic blue smoke.
A quick probe with my diode tester, showed that they were in fact toast. Both were open circuit. The diodes were replaced and the tuner sprang back to life once more.
Another happy old man.
