CB4M DUO – Control Box for magnetic loop antenna tuning
The CB4M unit was developed specifically for remote tuning of magnetic loop antennas. Tuning is controlled by microprocessor using PWM (pulse width modulation) and was optimized just for the specific need of remote MLA tuning. The classic solution of remote antenna tuning is mechanical (using special tuning capacitors), those are used, for example, by army radio communication systems, it would be a costly solution for civilian use. The PWM tuning method was used and is proven to be working by several hundred operators of remotely tuned MLAs. Compared to other remotely tuned antennas, the CB4M has one main advantage. It sends the tuning pulses through coaxial cable and there is no need to run extra cables to tune the antenna. CB4M DUO expands the original control box ability and can control two motors over the coaxial cable that is used to power the antenna. With new MLA types, that include integrated rotator, the CB4M DUO is used for remote tuning and rotating the antenna.
Power adapters that used linear transformers were still common on the market when CB4M was developed, however they were pushed out of the market within ten years and replaced by smaller switching power supplies. However, switching power supplies create strong or very strong interference, especially on the lower bands. For this reason, use only power supplies that do not cause interference. For user comfort and no noise and interference battery pack made of rechargeable cells is supplied, as no noise and interference comes from batteries. Batteries need to be recharged from time to time, to make the recharging interval as long as possible the CB4M DUO automatically enter sleep after ten minutes of inactivity. This reduces the battery drain by about 90 %. Because of this it is not necessarily needed to disconnect the battery when not using the CB4M DUO. Battery pack with fully charged batteries should last about one month. Battery pack not connected to the CB4M DUO may even hold charge for multiple months.
Important warning: The CB4M and CB4M DUO cannot be powered from the same power source, that powers the TRX. Connecting both TRX and control box to the same 12 V source will damage the control box.
After connecting the 12 V connector to CB4M DUO, middle green LED (2) lights up. Depending on the state of the switch (6) the green LED (2) is permanently lit or flashes with a frequency of approximately 1 Hz. If lit permanently, the control box is set to control the rotator motor. If green LED (2) blinks, then the control box is set to PWM tuning control. Tuning speed is determined by how long you hold the tuning button, tuning speed is then indicated by the frequency of red LED flashing (1) or (3). After pressing the left button (4) the left red LED (1) starts flashing, when the right button (5) is pressed, the right red LED (3) starts flashing. You might be able to hear the motor working if you are close to the antenna. Red LED flashing speed is only indicative. The flashing speed of the red LEDs is not identical to the motor speed! When you want to switch the mode of CB4M DUO to tune or control the rotator, press switch (6). Once the middle green LED (2) lights up permanently it is set to control the rotator, after pressing the left button (4) the left red LED (1) lights up, when the right button (5) is pressed, the right red LED (3) lights up. That means the rotator motor is working. After ten minutes of inactivity, the CB4M DUO enters sleep automatically and the green light (2) will turn off.
Short pressing the left or right button (4) or (5) brings the CB4M DUO to normal operational mode from sleep. After connecting the coaxial cable to both HF connectors and turning on the TRX on the desired band that the MLA is set for with JUMPERS, we adjust volume on the TRX so that we can hear band noise on the desired band. Then we press one of the tuning buttons (4) or (5) and as the capacitor turns, at some point we will hear increased noise for a short period of time. Peak noise can only last a fraction of a second when the capacitor is turning fast. Then we fine tune until we have tuned the antenna for the loudest band noise. After pre-tuning the MLA in RX mode, it is advisable to make more precise tuning in TX mode using an SWR meter.
Control box CB4M DUO allows you to tune the MLA very quickly and accurately after a short practice. Capacitor tuning speed changes automatically in four steps depending on how long the tuning button (4) or (5) is pressed. The tuning capacitor does not have end stops, each tuning button turns the capacitor in one direction infinitely. This means, that button (4) is not always increasing the FRQ and button (5) is not always decreasing the FRQ or vice versa.