Hours of planning, modelling and experimentation both in software with MMANA and also real-world testing has finally produced an excellent and extremely economical method of producing excellent DX gain in one direction with reasonable front-to-back ratio with fast set-up time and potential for /P operations for the heavy duty operator.
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IN DETAIL: 40m vertical yagi antenna centred on 7.1 Mhz. This is a vertical antenna array with ground radials, complimented by a pair of parasitic verticals; a reflector and a director. It will deliver around 6db of additional gain over a standard vertical with a beamwidth of around 70 degrees when compared to a standard vertical at 5 degrees off the horizon. I used to build mine pointing 300 degrees so that I can work the US easily at night. Alternatively, build it at around 270 and get both North and South America as well as ZL etc in the morning.
This requires no other components other than some poles and my water-jet cut components. It is not a phased array. If you wanted to switch directions, you can lengthen or shorten the reflector and director with relays, or switch them out for omni-directional gain.
DX Commander M0MCX VPA (Vertical Parasitic Array). Click for plot.
Listen to me on 40m into ZL one morning last year on this antenna.
This is complete kit ready to go. What’s in the bag:
- 3 x DX Commander 10m poles
- 1 x Regular DX Commander Ground Plate with SO239 for driven element
- 2 x Parasitic Ground plates (for director and reflector)
- 1 x Radiating plate for driven element (which you can convert to a regular DX Commander Multi-Band vertical with elements for 30m, 20m and 17m simultaneously whilst running this as a 3 element vertical yagi at the same time)
- 3 x Guy plates
- 200m Antenna military comms wire, enough for 3 x vertical elements and 54 x 5m radials (34 radials for your centre radiator and 10 radials each for director and reflector)
- Complete set of nuts, bolts and washers for all elements
- 12m Paracord for guying (just supply your own heavy duty camping guy stakes)
- 3 x Stainless Hose-Clamps for holding the lower plates in place
The construction method is very easy and each element is based on a spacing of 11m between each vertical element. The following dimensions produce a perfect match at 7.1Mhz.
Originally, I achieved excellent results with 10m long radials as follows:
- The driven element has 16 (use 30) ground radials and is cut (tuned) for 7.255 Mhz (I measured at 9.36m)
- The front (director) element has 4 (use 10) ground radials is tuned for 7.395 Mhz (I measured at 9.16m)
- The rear (reflector) element has 4 (use 10) ground radials is tuned for 7.035 Mhz (I measured at 9.76m)
However, I have doubled the numbers of radials recently, but halved their number. I therefore suggest you use 5m long radials.
Looking back at these numbers, I am amazed these lengths are so small. I suggest you cut your elements at least a metre longer and fold back the top of the wire back down the pole. Cut until you achieve resonance.
TUNING: Initially, build a single vertical and get it tuned for 7.255 MHz on your analyser. Lay it on the ground and do the same for the rear (reflector) and tune that for 7.395 MHz. Lay that down and again do the front (director) and tune that for 7.035.
Original prototype with choc connector to allow temporary connection for tuning.
Note: To tune the reflector and director, make up a pigtail so you can connect between the element and the radials on a temporary basis, then once you have trimmed the length to appropriate resonance, ground the element back to the radial plate.
Once complete, you should achieve an impedance of around 40-50 ohms at resonance. With the above lengths, I achieved a perfect match at 7.10 Mhz (the MJF analyser actually says SWR of 1.0). I am achieving better than 1:1.2 SWR across the whole band. For those world-wide amateurs with access to the 7.2 – 7.3Mhz, you will still achieve better than 1:1.4 SWR even at 7.3Mhz. At 7.0mhz, I am showing an SWR of 1:1.2.
Coax feed for the scientists: Because impedance repeats every half wavelength, I had pre-cut a full wavelength of coax, less the amount of the Velocity Factor (VF). This way, I can make measurements in the shack, accurately without having to venture outside. I used a 50 ohm coax with a VF of 0.82. The length of my coax is therefore around 34.5 metres. Of course, you could also use a wavelength (or multiple of any half-wave length) of 75 ohm coax too, as long as it’s cut accurately according to the VF since the impedance at the feedpoint will be the same at the termination (rig) end.
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