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Bias-T

How to control the motor driving the VC of a loop antenna, remotely and without adding wire ... With a Bias-T. In fact, it takes more precisely two and it is a matter of passing a DC voltage through the coaxial connected to the antenna.

The Bias-T is very simple. A capacitor is placed in series between the input and the output. And a choke allows the insertion of a DC voltage, that which will pass through the coaxial. In the station, the capacitor is placed on the transceiver side whereas at the closer of the antenna, the capacitor is placed on the antenna side. Thus, the DC voltage injected at the station and recovered at the antenna never goes to the transceiver or to the antenna, blocked by the capacitors.

Both Bias-T can be built identically, one is placed in one direction and the other in the opposite direction, simply.

Note that the pair I am going to describe here is used to transmit a power supply sometimes positive, sometimes negative, depending on the direction of rotation chosen. There are no diodes or polarized capacitors. Moreover, for this to be possible, it is necessary to use a completely independent source of power supply.

I use these Bias-T as follows:
An independent 12v 600mA power supply is the source of a PWM 1203BB module, which has a variable output that can also be reversed. It is this output that is connected to the first Bias-T and this voltage is recovered on the second to directly supply a 12V 30RPM motor whose axis controls the CV of the antenna.

The schematic diagram

With three 10nF capacitors in parallel, you can reach 160m band. There are ceramic models supporting 3kV that will fit very well and support well over 100W.

The selected inductor 100µH has a closed housing and does not take up space, it supports a current of 1A.

As for the decoupling capacitors, the 10 and 100nF are 500V ceramics and the unpolarized 1μF is a mylar 50V. Nothing really complicated finally.

Assembly

The small boxes of 50mm of side and 30mm of height are perfectly suitable. The connection between the two SO239s must be as direct as possible and the inductance directly soldered on the latter. The rest of the layout is at the discretion of everyone.

Testing both Bias-T in series to a dummy load shows that they do not bring ROS from 1.5MHz up to 50MHz but are not usable in VHF. Perhaps a smaller housing allowing a very short distance between the two central cores of the SO239 would have allowed to climb higher frequency

In use

A Bias-T is placed near the station and receives the voltage produced by the PWM power supply. The other is placed closer to the antenna and receives the link going to the motor.

It is simply necessary to ensure that the boxes are oriented correctly so that the voltage that will pass through the coax is not blocked on the one hand, and it goes neither to the transceiver nor the antenna, on the other hand.

It is therefore necessary that the capacitors begin and end the circuit, as in the diagram opposite.

The setting is finally quite practical because of the use of a PWM module. Indeed, this kind of power supply producing noise perfectly audible on the transceiver outside the tuning frequency of the antenna, it suffices to attenuate the reception enough for this QRM to have only a weak signal.

Then, during the rotation of the VC, the passage on the desired frequency will result in a net rise of the noise, the maximum corresponding to the best tuning. A simple fine tuning while controlling the SWR during transmit and the antenna is set.

Conclusion

When you look at how a business model is built and how much it costs to get one, it's worth building two for the same cost or less. It's convenient and it works.