The new MLA-T PRO v.6 replaces a well-known model of magnetic loop antennas – MLA-T PRO v.4, v.5, with approximately 100 units sold during the last years. The CB4M DUO control unit is the latest version of the control box for magnetic loop antennas.


998.00 €


in stock


The new MLA-T PRO v.6 replaces a well-known model of magnetic loop antennas – MLA-T PRO v.4, v.5, with approximately 100 units sold during the last years. The MLA-T PRO is a commercial magnetic loop antenna that was used and enjoyed by many HAMs. During the production period, the MLA-T PRO antenna was upgraded several times. The common advantage of all MLA-Ts is that with a relatively small diameter of the four loops, at only 0.8 m, the antenna also works on the 160 m band. Although the efficiency at a low antenna diameter to lambda ratio is not ideal on the 160 m band, the robust construction, the use of copper tubes and high voltage capacitors allows you to use the full power of 100 W, which most HF TRXs have. Because of this, the lower efficiency of the MLA-T PRO at 160 m is partially compensated. The external design of the MLA-T PRO v.6 remains identical to the MLA-T PRO v.4 model, the internals of the antenna are upgraded.

The main advantage of the new MLA-T PRO v.6 is the addition of a new usable band – 30 m band, and new tuning motor, that allows the use of rechargeable battery pack to power the CB4M control unit. Thanks to the use of rechargeable battery back we eliminated interference of "unknown origin" caused by most wall plugs. The previously used linear power supplies with a transformer were gradually pushed out of the market by smaller and cheaper adapters (switching power supplies), which bring new unpleasant phenomenon to the HF – QRM. Reduction of QRM when using AKU pack can be heard immediately.


MLA-T is for indoor usable with maximum power of 10W. For healthy: higher powers than 10W don't use it as indoor antenna nearer than 5 m. Operators with a pacemaker; best not to use it.
Caution: High frequency magnetic field passes through walls with low loss!


The CB4M DUO control unit was originally developed to control two motors on the MLA-S (RT/R) antenna. Experience has shown that the CB4M DUO is also useful for tuning the MLA-T PRO antenna.

With the new CB4M DUO, when the black button on the panel is switched to the "TUNING" function (green light flashes), it is switched to standard spread tuning using PWM, as well as was the case with the original CB4M controller. When the CB4M DUO's green indicator light is permanently lit (ROTATOR function), fast MLA-T tuning (originally handled by PWM) is achieved.

If an AKUpack is used instead of a mains adapter, the advantage of the CB4M DUO is automatic switching to standby mode. The residual processor current drops to just hundreds of microamps. This significantly saves the power of external batteries. The reason for replacing the mains power supply to power the CB4M DUO batteries was the fact that the previously manufactured linear 230 V/12 V 50 Hz power supplies were gradually being replaced by smaller and lighter pulsed power supplies.
However, these mains adapters cause high pulsed QRM/noise, and for radio communications purposes are these low-cost commercial pulsed power supplies - mains adapters - are completely unsuitable.

The standby mode will automatically switch on after approximately ten seconds of controller inactivity CB4M DUO. This is indicated by the green indicator light not illuminating. The standard operating mode The CB4M DUO switches to the normal operating mode by pressing any of the tuning buttons on the top of the cabinet.

Note warning alert

The CB4M DUO must not be connected to the common 12V power supply that powers the TRX. This will destroy the CB4M

Control Box for tuning magnetic loop antennas CB4M DUO

The CB4M unit was developed specifically for remote tuning of magnetic loop antennas. It is microprocessor controlled via PWM and has been optimized for the specific need of remote tuning of MLA. The so-called "spread tuning" function is also provided electronically. Solving this problem by the classical mechanical way (using special tuning capacitors), which is used for example i n military radio communication systems, is not economically sensible for civilian use. The correctness of the method proposed ten years ago has been confirmed by practice and experience of several hundred users of remotely tuned MLAs. Compared to other remote antenna tuning systems, the CB4M unit is advantageous primarily because no additional control cable is needed to tune the magnetic loop antenna. The high frequency power supply (coax) is also used to operate the tuning motor. The CB4M DUO extends the original controlbox with the ability to control two motors. For the new MLA SMART types. Which have an integrated rotator, the CB4M DUO is also used to control the rotator motor.

The CB4M is a plastic box measuring 11 x 9 x 3.5 cm, which has two PL239 panel coaxial connectors and one power connector on the back, see Figure 1. The power supply of the CB4M DUO electronics is handled by an external special battery pack. On the top of the CB4M DUO there are two buttons designed to control the right/left direction of rotation of the two motors. The speed and direction of rotation of the tuning capacitor motor is indicated by two red LEDs flashing at speed. The rotation of the rotator is indicated by their permanent light (no flashing). The middle green LED indicates the switching on of the CB4M electronics and the operation of the device by its regular flashing or continuous light.

The original power adapters using linear transformers, which were common in the market at the time of the development of the original CB4M, have been phased out of the market within ten years and replaced by smaller size and weight pulse power supplies. However, these modern power supplies produce strong to very strong interference, especially at the lower bands. For this reason, a battery pack is supplied. To avoid unnecessary battery discharge when using the battery pack, the idle current of the CB4M DUO is automatically software limited by putting the processor to sleep during idle, which reduces the unit's idle current (without motor current load) by about 90%. A connected AKU pack with fully charged batteries will last about one month with the processor asleep. An unconnected AKU pack can last many months to several years).

Important note: The CB4M and CB4M DUO cannot be powered from the same power supply as the TRX. Connecting to the same 12V power supply will most certainly cause damage to the CB4M.

When the 12 V supply voltage is connected, the central green LED lights up. Depending on the status of the function switch on the front panel (ON/OFF - which is not visually obvious), the central green LED will be steady on or flashing at a frequency of approx. 1 Hz. If it is steady on, the unit is switched to rotator motor control. If it flashes then the unit is switched to PWM tuning control. The time the button is pressed affects the rotation speed of the tuning motor, which is indicated by the frequency of flashing of the two red LEDs. When the left button is pressed the left red LED starts flashing, when the right button is pressed the right red LED flashes. If everything is working properly, the motor steps can be heard when the ear is placed on the antenna housing. The blinking speed is only indicative. The blinking rate of the indicator red LEDs is not linear to the motor rotation speed In the rotator control mode, when the central green LED is permanently lit, t h e indicator red LEDs that indicate (in normal operation of the whole system) the rotation of the rotator are permanently lit/not blinking. When the CB4M DUO is idle, it will fall asleep and the green LED will go out. A short press of the left or right button will cause the unit to come to life in both modes (tuning/rotation).

The CB4M DUO connects to the TRX via a coaxial cable fitted with a PL connector. To the second PL connector on the rear panel of the CB4M we connect the coaxial cable - the feed from the magnetic loop antenna, to the first connector we connect the coaxial cable leading to the TRX. If using multiple MLA types switched to one TRX via an antenna switch, only one CB4M DUO is sufficient to control all MLAs.

The Control Box CB4M DUO allows, after a short training, to tune the MLA very quickly and accurately to the desired frequency. The speed of the motor in tuning mode is changed in four gears without using another switch by pressing the control button. In both polarities. Thanks to the PWM and especially to the sophisticated software, which has been optimized specifically for motors where the tuning capacitor has no stops, the direction of motor rotation is not a determinant of the frequency change sequence and it is not the case that button 1 means increasing FRQ and button 2 means decreasing FRQ. For the rotator, the right/left direction of rotation is maintained.

Instructions for use CB4M DUO

After connecting the coaxial cable to both RF connectors and turning on the TRX on the band that is set on the MLA using the JUMPERS, set the audible noise level on the desired band. When remotely rotating the tuning capacitor using the CB4M DUO, at some point the rotation capacitor will increase the noise for a short time. Peak noise may last only a fraction of a second. It should be returned, and the procedure repeated several times as necessary. After the MLA has been re-tuned in RX mode, it is advisable to refine the MLA tuning in TX mode using the SWR meter. After gaining experience - acquiring the know-how, remote MLA tuning is relatively easy. Quality MLAs tend to be so selective that detuning from the resonant frequency by even as little as 10 kHz can result in a 1 to 2 S drop in signal strength.

Technical parameters

  • Number of loop turns: 4
  • Loop diameter: 0,8 m
  • Antenna dimensions: 83 x 30 x 100 cm
  • Antenna weight: 7,3 kg
  • Shipping dimensions: 85 x 28 x 105 cm
  • Shipping weight: 12 kg
  • Maximum usable power: 100 W
  • Bands: 160m, 80m, 60m, 40m, 30m