The ATAS 120A is a motorized omnidirectional antenna, originally meant for the Yaesu series of HAM radios, and it is an antenna that is highly praised in the community due to its wide coverage of bands, working on all HAM bands up to VHF and UHF that are only supported using an additional counterpoise.

The working principle of the ATAS antenna, follows the wavelength equation where the wavelength varies with the length of the antenna, using a motor that can be engaged from the radio that will lengthen or shorten the antenna in order to match the desired frequency. In order to tuned the radio to a specific frequency, the following steps are carried out automatically by the radio or the antenna controller:

• the radio is set to emit a carrier, typically CW or AM of about power,
• the SWR (signal reflection) meter is engaged in order to display the current SWR,
• the radio sends a signal to the antenna to shorten or lengthen the antenna whilst measuring the SWR,
• the radio stops tuning when it reaches an SWR that is under an SWR of or

All of the former can also be performed manually by the operator by using the radio controls to lengthen or shorten the antenna whilst emitting a lower-powered signal. On the other hand, the ATAS antenna is self-standing equipment, and it is very plausible to use the ATAS antenna with radios other than Yaesu brand radios. To that end, the MFJ-1925 ATAS-120 antenna controller is powered by via a molex connector and provides an up and down button to lengthen or shorten the antenna.

Regrettably, the MFJ-1925 controller is unbelievably expensive for what it does such that this documentation will provide the schematics and build instructions to create an ATAS controller for a tenth of the price of an MFJ-1925 controller.

Furthermore, because it is possible, the antenna controller that will be created will benefit from a remote control and will be able to control the ATAS at a distance instead of just providing an up and down button thereby making the controller easy to use as a portable device.

The ATAS antenna controller is almost trivial for what it does and just follows the simple principle of operation:

• in order to extend the ATAS antenna, feed a voltage over through the antenna line,
• to retract the ATAS antenna, feed a voltage of through the antenna line

Everything else that the ATAS antenna controller does is to protect the radio from the voltage that is supposed to be fed to the antenna via a capacitive bridge that is placed in front of the connector jack that leads to the radio.

• 1 remote controller with two relays (can be just bought for a few bucks)

It is entirely possible to use an ESP8266 and some relays, perhaps a non-latching version of a low-power featherwing relay, along with an RF module and a generic RF remote to build this module from scratch as well but it is not necessary and ultimately it does not bring anything new to the build.

• L7805 voltage regulator,
• two capacitors (or ) for the voltage regulator,
• three ceramic capacitors rated at to create a voltage blocking bridge for the voltage to not flow into the radio,
• two resistors,
• one resistor (a small surface-mounted potentiometer would be good as well for calibration),
• an SPST micro-relay, with a a coil voltage ranging somewhere between and ,
• a PCB mounted jack to power the build with ,
• a inductor / RF choke,
• a basic diode that can sustain up to of current,
• antenna jacks (PL),
• a mounting box,
• the usual building materials; PCB, soldering equipment, copper thread, etc.

Given the very simple mode of operation ( for retract, to extend) there are numerous schematics that can be built. For example, K5LXP builds an ATAS controller with two voltage regulators L7812, that provides to extend the antenna, respectively an L7808 that provides to retract the antenna. There's a more interesting schematic, by DG1SFJ that uses a single voltage regulator which is neither better nor worse but somewhat cooler given the principle of minimization of circuitry.

The basic schematics provided by DG1SFJ are also very easy to realize, and are not much more than the schematic mentioned by the voltage regulator but with the extra addition of a feedback resistor that allows to control the output voltage of the voltage regulator by draining the voltage regulator using varied resistors. In fact, the lengthier task is to calibrate the voltage output by varying the resistors and ensuring that , respectively can be generated selectively just by commuting between contacts. If that part is realized, then the remote control commuter with the relays can just be interposed and made to switch between voltages in order to extend or retract the ATAS.

Using CircuitsLab, the schematic can be simulated in order to ensure the proper voltage output required by the antenna based on one single L7805, voltage regulator.

The circuit is the recommended circuit for a voltage regulator, the capacitors C2 and C3 being added as recommended by the L7805 data sheet, and with a voltage source on one side and a voltmeter on the other side.

The addition of a feedback resistor R1 allows the voltage of the regulator to be surpassed and by commuting the drain through R2, respectively R2 and R3 via the SW switch, the voltage regulator is made to output in order to retract the antenna, respectively to extend the antenna.

It is important to remember that the car battery voltage is nominally and not the theoretical such that the schematics are calculated using instead. Even though the controller will not be used with car battery, one previous project involved modifying an HP server power supply to provide the necessary to a HAM radio, such that the voltage will indeed be set to instead of just .

When attempting to simulate the original circuit, the correct voltages were never obtained, and this is due to the notations that are wrong on the original schematic, namely that are not necessary and the ATAS is in fact happy with any voltage over in order to extend the antenna.

For example, K5LXP presents a schematic with two voltage regulators L7812 and L7808, which is the most straightforward solution that takes care to downscale any voltage exactly to , respectively .

The schematic has some issues, namely that linear voltage regulators with a constant voltage output typically need about above their conversion value, such that the L7812 might have hard time providing and that is even if the power from the battery is . Next, it just seems that two voltage regulators is overkill if the same can be achieved with just one single voltage regulator from the point of view of circuit minimization but on the other hand it is not that bad of an idea either considering that the voltage regulators used have a fixed constant output that will ensure the voltage output.

Another problem is that FG1SFJ will only power on the device when the antenna needs to be retracted or extended, such that the derived schematics mentioned earlier will have to be modified in order to ensure that power flows to the regulator only when the button on the remote controller is pressed. In fact, the provided schematic has two separate commuters, the top-most switch that engages to feed the circuit with voltage, and the lower switch that couples the resistor in order to attain . This is where the additional micro-relay comes in for the derived and final schematic that, upon the need to retract the antenna, will both power the voltage regulator as well as coupling the R2 and R3 resistor to ground thereby attaining . In effect, the remote control circuit and the relays will just power the voltage regulator with the down button having the additional effect of stepping down the voltage even further to retract the antenna.

The final circuit can then be sketched up as:

with the following mentions:

• the relays RLY2 and RLY3 are the existing relays on the remote controller module and they are engaged intermittently whenever button A or button B is pressed on the remote control, button A (antenna up) engaging relay RL2 and button B (antenna down) engaging relay RLY3,
• the diode D3 is there to separate between the common that engages the L7805 voltage regulator such that pressing button A will not accidentlly engage the third relay RLY1 that is responsible for dropping down the voltage to (otherwise both A and B buttons on the remote would only generate ),
• the relays, RLY3, RLY2 and RLY1, on the schematic have the following role:
  • both RLY3 and RLY2 will power the voltage regulator L7805 in order to drop-down from ,
  • RLY2, will just power the voltage regulator and will output for the antenna to go up,
  • RLY3, will power the voltage regulator and also additionally will power relay RLY1 that will drop-down to in order to make the antenna retract,
• the capacitive bridge consisting of ceramic capacitors C5, C6 and C7 are rated at and each and are responsible for the generated current to not flow back into the radio when the antenna is operated, either retracted or extended

It is already observable that, given the current state of the circuit, the voltmeter VM2 reads . This is due to all relays RLY2, RLY3 and RLY1 being coupled such that the circuit generates the required to retract the antenna. If RLY3 were to be decoupled, RLY3 would additionally decouple RLY1 and assuming that RLY2 would be coupled, the necessary would be generated in order to extend the antenna.

With the theory established, the circuit can now be built and the project realized. Iteratively, the circuit is built around the L7805 voltage regulator since it will be the centerpiece of the circuit with clear left-and-right separation of "before stepping down" and "after stepping down".

On the back, traces are built using thin copper wire to connect the individual components together.

At some point it is decided to not solder the remote controller onto the PCB, but rather remove the provided wires on the remote controller and then add pins with a matching socket on the PCB such that the remote controller can be added and removed simply by plugging it onto the PCB.

The black jack is mounted on the side, two pairs of copper struts protrude from the PCB to which the antenna sockets will be soldered, both going to the antenna on the right-side and then going to the radio transmitter on the left side of the PCB. Even though it is not necessary, a small JST connector (white) is added to the PCB from whencefrom can be drawn.

The PCB is then finalized with some "wild traces" drawn as copper wires from the bottom to the top and by sliding the copper wire between the various components in order to not extend the height of the PCB nor build too many bridges for the circuitry on the bottom part of the PCB.

As it can be observed, there are four resistors and this is due to not having the exact value for the drop-down part of the circuit, such that two resistors in series are used instead to generate an approximate value that will be sufficiently good to retract the antenna.

Note that the capacitors used for the voltage regulator are polarized, but only incidentally so due to not wanting to use surface-mounted Tantalum capacitors. That is not a problem and the positive pole of the capacitor is connected to the pins of the voltage regulator.

On the other side, the blue ceramic capacitors can be seen, along with the inductor that form the voltage blocker in order to not damage the radio. The leads to and from the ATAS connector and the radio are realized out of a pure copper wire in order to maximize the connectivity between them.

Even though the voltage regulator has a heatsink, a smaller heatsink is attached due to the fact that the voltage regulator is not expected to run for lengthier periods of time but will rather be engaged whenever either button is pressed on the remote control.

The final step is to find or create a matching case, preferably made out of aluminum in order to minimize RF noise radiated towards the exterior of the circuit. A metal project-box will suffice but it will have be tall enough to accommodate for the antenna connectors that tend to be pretty bulky. One issue that might be encountered is that the remote uses RF such that the pigtail antenna might have to be removed from the remote controller PCB and relocated somewhere on the outside of the project box metal case in order to avoid the Faraday cage effect. However, it is expected that the project will be operated fairly close up without too large of a distance between the remote control and the realized circuitry.