5/8 wave vertical for 2m band using 3D printed parts

photo and diagram of the 5/8 wave vertical photo and diagram of the 5/8 wave vertical


I used the 3D printer to make the base / center for a quarter wave ground plane (GP) antenna. This worked well but I felt I needed a bit more gain. Browsing through Carrs book on Antennas (Practical Antenna Handbook (2nd Ed), J J Carr, 1994. ISBN 0 07 011105 7) I decided to try out a 5/8 wave vertical. I used what I had learned making the GP antenna in this design. I increased the size of the coax connection box so that there would be more room. The 5/8 wave antenna has a inductor matching coil so we need more space for this (see diagram). I had a number of flat aluminium bars (1m x 10mm x 2mm) that I decided to use in this design for the ground planes. These simply form a horizontal X below the vertical radiator (unlike the GP antenna they do not droop downward at an angle).

photo and diagram of the 5/8 wave vertical


I printed rectangular slots in the device so that the aluminium strips would easily and conveniently slide into the base. A self tap screw attaches to one of these inside the box and allows a solder tag to make electrical contact with the coax screen. The other ground plane attaches a few cm offset from this earth-point using a 3M nut and bolt through a plastic pillar in the 3D design.The coil is simply a 1cm long 1cm diameter 2 to 3 turn copper wire coil (ca. 1mm diameter wire) wired across the center and screen of the incoming coaxial cable. The coil can be adjusted a little to get a good SWR but it is more convenient to simply lengthen or shorten the main radiator. The main vertical radiator tube simply slides into the top of the connection box and is secured using a self tap screw from inside. There is a screw driver hole on the side so that you can screw this in. I included pilot holes for all the self tap screws. A solder tag allows connection to the inner of the coax. The tuning inductor / coil can be attached between the GP and radiator solder tags. I used a 3m length of mini 8 coax to a PL259 plug as a short length of feeder to the antenna. This can of course be connected to a longer length of feeder.

Strengthening the base
In this version I extended the gap a little between the two U-bolts for stability as we have a much longer radiator in this antenna. I also added a strengthening bar along the length of the base to stiffen the structure.

Tuning and results
Once the coax cable has been feed through the box from outside and then connected to the coil, the GP and the radiator the U-bolts can be added to the base and the antenna fixed a few meter above ground for testing. The main radiator tube can be made from a 10 mm diameter (ca. 100 cm) tube and then a smaller diameter tube (say 8mm ca. 100 cm long) can be slid down into this. I made the total length a bit long (ca. 160 cm) and then adjusted the smaller tube position for best SWR. The total length of the radiator required to get a low SWR was a little longer than I had expected to be (5/8 wave should be 5 x 200 / 8 = 120 cm, 48") but my antenna gave best match for a 150 cm (59") radiator (more like a 3/4 wave ?). This maybe due to the rather random length of the GP radials I used which were 1m as this happened to be the length I had available. Once the correct length had been established I used a small self tap screw to fix the two tubes and taped around the join to stop water getting in. I put glue gun or sealant in to the top end of the tube to stop water getting in. Then its worth taping the screwdriver hole on the outside and fill the box up with sealant to water proof all the connections and the tuning coil.

Results
As expected the 5/8 wave give better performance than the smaller GP antenna perhaps 1 or 2dB. If you are only going to work local stations or the local repeater, I would actually recommend the smaller, easier to make and tune Ground Plane antenna. The advantage of the 5/8 wave is that the tuning coil shorts out the inner and outer of the coax to DC (or off frequency signals) helping to reduce static pickup and voltage pulses from nearby lightening etc.

photo and diagram of the 5/8 wave vertical



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THE CREATIVE SCIENCE CENTRE


Dr Jonathan Hare University of Sussex, Brighton.
e-mail: j.p.hare@sussex.ac.uk

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