Ever wanted a high gain (ish) directive antenna on the cheap ? Tower + antenna + rotator is too expensive ? Well, it is about time to build a directive antenna using wood poles, some wires and ropes, 3 plastic boxes , 4 SPST 12V relays, 2 T92-2 toroids and some electric cable.
Construction info :
- Using 4 meter long wood poles, create and erect 2 x 9 meter height poles and stabilize it using guy ropes, poles spaced exactly 6 meters apart, along of interested axis (East – West in my case).
- Each pole has a small pulley ontop and looping rope down to the bottom .
- Assemble another 6 meter pole and mount 3 small pulleys spaced @ 0.1 , 3 and 5.9 meters along.
- Use looping ropes to for elements to be lifted up from the ground for each pulley.
- Horizontally erect this pole using the pulleys ontop of each tower.
- Build a 1:2.4 balun in order to adapt low antenna impedance to coax 50 ohms.
- Using waterproof insulated plastic box and UHF female connector , build central element coaxial joint.
- Using same type of plastic box, build a relay box for 1’st and 3’rd element , where in my case Western box relay is shortcuting wires on Normal Close , and 0.8 uH inductance on Normal Open contacts. For Eastern element Shortcut and Inductance is reversed.Each boc has electric 2×0.5 mm electric wire along the mast in order to energize relays for East , and no energy for West
- Using 1.5 mm insulated electrical wire, measure and cut 2x 5.3 meter wires to use for Dipole.
- Using 1.5 mm insulated electrical wire, measure and cut 2x 5.15 meter wires to use for Reflector.
- Using 1.5 mm insulated electrical wire, measure and cut 2x 5.15 meter wires to use for Director.
- Extend the wires with some light rope to secure elements to the ground @ 10.5 meters along North / South axis.
- Erect all boxes with elements connected , using the pulleys mentioned for horizontal poles.
- Elevate Dipole Only , secure it to the ground and measure impedance.Simulation tells us that inverted vee 1 element only should have about 50 ohm and 0 reactance , but the 1:2.4 balun on top of it should transform it to 120 ohms , but in my case only 90 ohms was seen .Trim wires until resonance at 14.030 in my case (resonance is not where impedance is 50 ohms but where reactance is 0).
- Elevate Director element with shortcut between wires and remeasure dipole.Trim Director until resonance is at 14.150 Mhz, yes 14.150 for CW segment. Set analyzer back to 14.030 and reactance should be about negative 10 ohms. Energize the relay and reactance should drop to negative 30 ohms.
- Elevate Reflector element and trim wire until almost 0 reactance. Connect both 12 V relay cables together and alternating East / West observe impedance variance , eventually fine trim last element until both directions show just about same values, in may case SWR 1.07 West and 1.13 East.
- 3 elements yagi have a resonator, director and reflector. By inserting a 0.8 uH inductance to one wire, it will be electrically longer so it will act as reflector.
- By switching inductance / no inductance , 1’st and 3’rd element will act as Reflector /Director and viceversa if relays are energized
- Maximum care is to be taken to wire parallelism.
- Due to extra weight on the middle of horizontal pole, the dipole box would be slight lower than others so using pulleys and ropes align the height to be the same on all wires
- Simulation using MMANA GAL software shows 10 dbi gain and 22dB F/B ratio.
- In real word I’ve seen about 5 S units so about 30 dB F/B ratio is expected, but only along West /East axis , for North / South there is no difference but no gain there.
- This is a compromise but for biggest CW contests should be enough, most of it you can flip beaming just with a switch and bang ! , instant rotate.
- Testing just 100 W I easily did break pileup for Chile, Canada Japan and Australia, so beam width is large enough to cover most of the useful directions.
Good luck building and comment here for extra informations. Cheers all, Gabriel yo8rxp.
mmana design link here