Copyright (c), Ralph Holland, Mar 1997

Introduction

I have seen several Sailor 1500 Antenna Tuning Units circulating in the second-hand market; this article describes how they can be adapted for use with any amateur h.f. transceiver.

The Sailor 1500 ATU is a 400 Watt P.E.P. h.f. antenna tuning unit which was designed to operate from 24 volts and be semi-automatically controlled by the Sailor series of H.F. transceivers. This model ATU was designed to feed wires such as the backstay on a ship and is still current shipboard equipment. The ATU also operates as a dummy load when the higher inductance coil, which contains a lossy laminated-iron core, is introduced into the series arm of the "L" network and tuned to resonance.

With a few hours work the ATU can be modified and extended so it may be manually controlled to tune a 3 metre whip at 1.8 MHz, or upwards, via switched control wires driven from a 12 volt supply.

Summary of Modifications

Several modifications are necessary to adapt the ATU for stand-alone operation.

12 volt operation

The relays must be "adjusted" so they pull-in reliably with 12 volts. The relay operation is adjusted by applying 12 volts to each in turn and adjusting the tension of the spring and the contact travel by the adjusting screw until relay action is satisfactory.

The d.c motor will operate on 12 volts, so no further modifications are necessary.

Mechanical

The variable inductors are driven via a shaft connected to the d.c. motor. This shaft is free-wheeling in one direction because one finger has been removed from the driving arm. A replacement arm can be fashioned from PCB material by modelling it after the other driving arms. This modification locks the central tuning shaft to the motor preventing most platform movement from accidentally detuning the ATU when it is operated on the bull-bar of my four-wheel-drive (4WD) Landcruiser .

The unit must have the tuning shaft extension removed so the hole can be blocked with with suitable sealing compound in order to ensure the unit is waterproofed. Waterproofing will also be necessary around the control plug inputs and any unused access holes. Now the unit can be used in any position, not just the recommended vertical position and can be mounted horizontally on the front or back of some mobile platforms (ie 4WD).

Electronic

The "L" match configuration is inverted from the expected configuration with the shunt arm towards the load - this configuration is normally used to match long length elements, but the ATU has a 4:1 transformer at the transceiver side allowing it to drive shorter antenna lengths.

The ATU configuration should be modified to increase the flexibility. The "L" can be configured to operate either way around. The iron core in L101/L102 must removed so this maximum inductance coil can be used at 160 and 80 metres. One series capacitance element must removed so an additional shunt capacitance element can be added to permit the ATU to operate on the lower frequeinces. The 4:1 input transformer must be removed because the turns are insufficient for operation at 160 metres and barely sufficient for 80 metres.

Tuning

Manual tuning is offered with twelve (12) switches to control the series and shunt reactance elements, one (1) switch to select the direction of the "L" network, one (1) DPDT centre-off switch to operate the direction of the d.c. motor and one (1) master switch to turn off the system power.

Figure 1 Original Circuit

Figure 2 Modified Circuit

Modification Steps in detail

Figure 1 shows the original ATU configuration while Figure 2 shows the new ATU configuration. Perform the following steps to take the ATU from Figure 1 to Figure 2.

1. Remove all the stainless steel control plug and cover screws. Remove the control plug and the cover.
2. Check the operation of relays RE101 to RE112, by applying 12 volts using negative ground. See Figure 1 for original circuit details. Adjust the return spring tension with a pair of long-nose-pliers and adjust the offset screws until the operation is deemed satisfactory.
3. Remove board 200, the one above the d.c. motor, taking care to disconnect the antenna input from RE112.
4. Unsolder both small coaxes from this board, snip diodes D204, D205 and resistors R205 and R203.
5. Bridge diode D203 with a wire strap. Wire the Violet/White wire, pin 12, to drive RE112 and check that the wiring harness is connected as follows:
   

Table 1 Existing Wiring

6. Unsolder the coax and relay grounds from the subassembly board. Unsolder the copper shims and leads on the relay contacts and remove relays RE101 to RE106 to permit access to the subassembly board containing the series capacitors C101, C102 and C103.
7. Remove T101 (4:1 transformer)
8. Remove C103 (30pF)
9. Remove C101 and C102, replaced C103 (100pF) with C103 (30pF) and reconnect to the subassembly board.
10. With the relays at the top and inductors at the bottom, place a solder lug on the lower right-hand screw of the capacitor subassembly board.
11. Remount the capacitor subassembly board.
12. Solder approximately 2350pF / 1KV rating (5*470pF) or so between the lower contact of RE106 (old C103 / 30pF relay) and the solder lug on the right-hand screw. This capacitor is required for 160 and 80 metre operation and shall be designated Cx.
13. Connect inductor L103, with a shim or copper strap, to L104 (the small rotating inductor).
14. Remove L101 and then remove the iron cores from the bottom of L101 - they are no longer required.
15. You will need to relocate and add new turns to the lower part of L101. Remove the copper shim at the bottom and keep for the new winding. Relocate the lower turns as high up the form as possible so you can add more turns at the bottom. The relocated winding in terminated in a new hole. The turns are extended by drilling a hole near the end of the relocated winding for the start of the new winding. The old and new windings are then joined by a bridge of solder across the gap and the shim is placed at the bottom of the new winding, and the new winding is terminated in a new hole. Make sure the ends of the winding do not have sharp points and are truncated close to the inside of the former. I also coated the coil with estapol and then baked it in the oven at 80 degrees Celsius for an hour. This prevents tracking between the turns (take care not to coat the spindle contacts).
16. Construct a PVC or Teflon insulator that will be placed under L101. I made mine from the top of an ice-cream container. You will need to drill two holes for the L101 mounting screws and you cannot make this insulator too thick as it will prevent the rotation of the tuning shaft. This insulator is necessary when the ATU is operating into highly reactive loads on 160 or 80 metres. (Ideally L101 should be mounted further from the case but we have no choice.)
17. Remove L104 and construct a new drive arm from a piece of insulated PCB material. This drive arm should be double-fingered in the same profile as the other coil drive arms; this will prevent coil movement when mobile and will enable you to use the reverse the direction of the motor for fine tuning adjustments.
18. Replace the small teflon coax running between L101 and the input board 200 with RG58 U/C or preferably RG8 to prevent voltage breakdown when operating on 160/80 m. Ensure the coax is routed so it clears the relay contact arms and return springs - use the restraining strips provided on the chassis.
19. Note GL201 is removed from the circuit as it will probably fire when you are transmitting on 160 or 80 metres, particulary using short antennas which mean highly reactive loads. RE201 is also removed as it is entirely inadequate for the voltage levels expected at the ATU load.
20. Reassemble all relays and wire them via shims or braid to the appropriate components. (See Figure 2.) Note the modifications around RE106. L104 must be bridged to L103 and the RE106 contact is between the shunt arm and the new 1880pF capacitor Cx.
21. Remove RE112 and modify it so it is a double-throw double-pole relay; i.e cut the top, bottom and centre contact shims - do not cut the circuit board though! The top and bottom shims are riveted and can be cut with a hack-saw, while the centre (moving) contacts can be unscrewed and cut with a pair of tin-snips. These contacts can be secured by a small amount of super-glue - but do not allow any glue to cross between the contacts as it offers a path for rf! Wire this relay so the antenna is connected to L101 on the series arm and so the transceiver is connected to the capacitor relays of the shunt arm when RE112 is off - as this is the default configuration for mobile operation. Ie place the antenna and transceiver coax centres on the moving contacts, the shields on the earth contact at RE112 and connect the capacitor rail to one side and the L101 coax on the other side of the stationary contacts. Construct loop-over wires from the insulated centre conductor of RG58 or similar and loop one over the antenna relay and one under the realy so the sense of the antenna and transceiver connection changes when RE112 is switched on and off. Dress the loop-over leads so they clear any metal contacts and each other - otherwise they will arc and burn!
22. Isolate the d.c motor from the chassis ground so it can be reversed. Connect the former ground-pin to the control cable pin 16. Remove the former motor end of R204 (12K) and connect it to ground.
23. If you want the current sensor output then R204 must be grounded as above and you must connect the sensor output to the control cable pin 15. Note this senses load current and not transceiver current. If you want to sense transceiver current you will have to move the sensor transformer to the input terminal. (I omitted the transformer as I use the SWR and power meter on my rig.)
24. Make a switch box to organise control of the ATU components. You can mark-up Table 2 for your future reference to aid cable wiring, while table 3 shows the layout and function of my control switches. The motor is connected to a DPDT centre-off reversing switch to permit rotation in either direction and the power for all switches passes trhough a master power switch (neither shown in Table 3).
    

Table 2 New wiring

Table 3 My switch layout and function

25. Remove the tuning shaft extension and plug the hole with suitable sealing compound. I have found that liquid nails works but silicon compound (silastic) may also be adequate.
26. After wiring in accordance with the above table and the coax connection, seal the control plug with suitable sealing compound around the cable entries and any spare holes to prevent moisture ingress.
27. You may now reassemble the unit.

Table 4 contains the reactance values of the series components.

Table 4 Series component reactance values

Table 5 contains the switch settings for operation on my 2.64 m whip mounted on the bull-bar; the switch values are designated in hexa-decimal (one bit per switch) collected into capacitor, inductor, extension capacitance and the Long / Short switch positions. (I keep table 5 as my cheat sheet.)

If you are having trouble matching on 160 m you can increase Cx until it is 3290pF which is 7*470pF capacitors in parallel. This value is approximately twice the value of all other capacitors so the switches will work in powers of two or octaves.

(1 is down and 0 is up so 33 for C is the pattern 011011 which corresponds to only C4, C3, C1, C0 connected and 2 for L represents 010 which is only L1 connected; the * represents the preferred switch values for the nominated frequency.)

Table 5 Settings employed for my 2.64m whip