Here is my 2-meter Delta-loop in its Mk1 form; the original idea for this antenna came from Paul M0PVN to build a 2-meter antenna that can be used on field and or portable days. A very cheap, interesting little project with surprising results:
- 35mm drain pipe
- 20mm plastic conduit
- Copper wire (I think it is about 26 gauge)
The first problem I had was how to find 120° marker points on a round pipe, I achieved this buy using a CAD program and printing out the circle with three lines at right angles to each other. Placing the pipe upright and marking out (but there are other ways you could try). In other words I needed to ensure accuracy and consistency across all parts, ensuring alignment.
Placing a length of roofing lath; marking along the pipe achieving three lines as guides. Now it a simple case of marking where you need to drill the holes for the stretchers, well almost because it was at this point I decided to cut the boom into three (so can be transported).
I ended up putting a tee piece (see image 2) just after the driven element and kept my fingers crosses that it would be balanced when it was all put together.
The next issue was how to get the 20mm pipes to stay at the correct angle (120 degrees) see image 3 to form the triangle, this caused me a few problems. With help and ideas on how to solve them I tried a couple of things.
After trial and error, it was at this point I realised I needed to get the wire in the middle of the pipes so they would be in-line on all three parts of the beam (see image 4). It’s important to note that the each delta needs to be the same and in-line. The tie wrap in the image is used as a method of tensioning the wire. Prior to the above I attached elastic bands as a temporary fix to calculate where the centers of the wire would be to ensure the length of each side of the wire was the same. Using chocolate blocks as a guide this mix mash was now beginning to look like it might just work (see image 5). But I had to support the conduit to stop it moving off balance…
Idea 1 PVC sill board
Cut into triangles then drilled in the middle @ 36mm so as to fit over the boom and give the support needed and with a couple of holes at the right place to keep the stretchers in place job no good as the plastic just melted under the heat generated by the drill.
Using some nice pine, drilled to fit with a couple of holes for wire ties to strengthen and keep the stretchers in place.
Now the next thing to do was to align the wires, to do this so they were all in the right place I used my CAD drawing to find the right distance from the outside diameter of the boom to the point where each wire passes through the 20mm pipe at equal sides.
To feed the antenna there is a ¼ wave length of 75 Ώ coax balun, this I wrapped a few turns around a 25mm PVC pipe, connecting it together using chocolate blocks.
Once this was all built I put the antenna on my buddy-pole stand and checked the SWR, to my surprise it seemed to be very good at 1:1.3 average throughout the 2meter band. Very nice and ready for a proper test under portable conditions.
So with my trusty old 2w FT290 rig off to Barr Beacon (about 620′ ASL) pointing north on the first CQ call I got a GW station, on about 5/5 with low audio, turning the antenna round towards the approximate heading and viola 20 over 9! I knew the antenna was working, the side rejection and then from the front to back rejection was very good. I then tuned onto the GB3VHF 2 meter beacon located in Kent and once again I got received a good signal from that as well, by moving the antenna from side to side I found the antenna to be quite directional.
Checking the antenna on a MFJ analyser I have decided to stretch the wire about 2% and just increase the balun to match the UK bandwidth.
This antenna is horizontally polarized but needs rotating by 90º it for vertical polarisation (not yet tested).
- Reflector element 73cm x 3 off
- Driven element 70cm x 3 off
- Director element 67 x3 1 x off (these dimensions are as I built the aerial, allow an additional 10% for uk frequencies)
- Matching balun; 325mm length of 75 ohm coax on 40mm diameter former,
- The boom is 70cm long, driven element is 255mm from reflector and the director is 566mm from reflector.
2E0MVF Mick Findon