Despite it’s name, an antenna tuner unit (ATU) doesn’t actually tune the antenna, or indeed any part of the antenna system. Now you well may be asking yourself, “If it doesn’t tune the antenna, why use one?”
The reason you use an ATU is to “fool” the radio into thinking that it’s delivering the RF output into a nicely matched load. When modern solid state radios are trying to send RF into a mismatched load, the built-in protection circuits kick in and lower the output accordingly.
When transmitting, RF power sent to an antenna will always have a small proportion reflected back to the transmitter.
Imagine a ripple from a stone dropped into a bucket of water. The initial point where the stone hits the water is the transmitter and the side of the bucket is the antenna. When the ripple reaches the edge of the bucket it bounces back and so on…
As the antenna mismatch increases, so does the amount of reflected RF power which is wasted in the form of heat within the antenna feeder. Too much, or long periods of, reflected RF power can also destroy the output stage of the transmitter!
What does the antenna tuner actually do?
The ATU simply fools the radio by adding inductance and capacitance in the feedline between the transmitter and antenna. The transmitter “sees” the antenna system as a matched load and delivers it’s RF power accordingly.
By adjusting the amount of inductance and capacitance, the ATU essentially forces any reflected RF from the antenna back on itself again and into the antenna.
In practice, only some of the RF power forced back to the antenna will get to be radiated as part of the signal, the rest will be lost in the feeder.
It must be noted that if the antenna is no better than a piece of damp string, the ATU will not make the antenna any better than it actually is, thus it will still work like a piece of damp string.
Using the MFJ-949E ATU
The MFJ-949E has an antenna selection switch, RF power range and peak or average reading switches and a switch for the meter illumination. Most ATUs, like the MFJ-949E, have an inductance rotary switch and two variable capacitors. The capacitors are often labelled ANTENNA and TRANSMITTER. Download the user guide here.
Let’s go through using the controls to match the antenna:
Start by rotating both capacitor knobs to the midway position. Unless the knobs have been removed from the ATU, this should set each capacitor to its mid value. Depending on the capacitors used in the ATU, they may have stops at each end of the 180 degree arc, or they may rotate continuously.
Next tune the radio to the frequency you wish to use and LISTEN. Rotate the INDUCTANCE control until you receive the strongest signal or maximum noise. When you’ve found the setting that gives the maximum noise or signal, slowly rotate the capacitors in turn, again listening for maximum signal.
At this point, the ATU should be quite close to the final settings. To peak the settings, make sure the frequency is clear and set the transmitter to a low power level.
For the next step you may use AM, FM or CW. Note if using CW you must be able to send a continuous carrier, if the radio has a keyer, this will have to be turned off first.
Transmit a continuous carrier while you tweak the antenna and transmitter controls for the lowest reflected power reading with the highest output power as read on the meter. You may find that you have to change the position of the inductance switch, do not change the inductor switch while transmitting.
Remember, you will need to make adjustments to the settings whenever you change frequency by more than a few KHz. It may be beneficial to make a note of the settings for each frequency/band you use for quicker retuning at a later time.