I want a lightweight homebrew 6m antenna for SOTA and Parks activations with a little more gain than the homebrew 6m flowerpot. I know which direction the SOTA chasers or Park hunters operating positions are therefore I’m an not concerned with losing the omnidirectional radiation pattern of the 6m flower pot antenna. Having 2.6 dB directional gain perpendicular or broadside to the plane of the wire loop should improve reported signal strengths.
Edge on or side on, the oblong loop has a deep null. If you point the edge or side of the loop towards the transmitting station you will observe a point where the signal is almost unreadable, this is the direction of the transmitting station. Rotate the antenna so that the loop is now broadside on the imaginary line to the transmitting station, this is the axis of maximum gain.
Before you start, be aware the polariation of this antenna is horizontal, not vertical. The majority of VK1 chasers working the 6m band use horizontal yagi antennas. The one or two who use a 6m vertical antenna will still hear me 🙂
The oblong loop is scalable to any frequency (perhaps up to the 70cm band), where the antenna is 1/6th of a wavelength wide and 2/3rds of a wavelength tall. In my arsenal of portable SOTA antennas I have a 2m and 70cm oblong loops.
The formula (metric) for a full wave loop is 306.3 / frequency, therefore 306.3/50.2 MHz = 6.102m (starting position, without adjustments for wire velocity factor).
Antenna dimensions after tuning
- Loop circumference: 6034 mm
- Width: 1017 mm
- Height: 2000 mm
- 1030 mm length of 16mm dowel
- 1030 mm length of 12mm dowel
- 6.2 metre length of 18 AWG copper multi strand insulated wire. Wire size is your choice.
- BNC female to Binding Post (banana) adapter
- Two brass screws 18mm long
- Length of Mil Spec RG58AU 50 ohm coax, mine is 5 metres
I purchased two 1.8m lengths of Tasmanian Oak dowel from a local hardware store, the surplus material will be used in another project. You may wish to substitute the dowel rod with plastic pipe. I found dowel weighs less for the same length and is more sustainable.
16mm dowel cross arm
Drill a 3 mm hole, at 90 degrees in one end of the dowel rod, approximately 5 to 7 mm from the end (the distance is not important).
From the first hole measure 1017mm and mark the dowel with a permanent marker, this is the position of the second hole. Drill a 3mm hole at 90 degrees, make sure the second hole is in then same vertical plane as the first hole. Next cut the dowel 5 to 7mm beyond the second hole. You should now have a length of dowel with two 3mm holes spaced at 1017mm. While you have the dowel in your hand mark the center position of the dowel.
Next drill a pilot hole in the center of the dowel then a series of larger holes until you reach a hole diameter which will accommodate your telescopic pole size. Take your time drilling the hole, I suggest using 1mm increments with the dowel mounted in a vice, you don’t want to break the dowel. Deburr the hole to remove potential splinters. I finished with a 9mm hole.
12mm dowel cross arm
Repeat the steps for drilling the 3 mm holes and cut the dowel to length. Mark the center position of the 12mm dowel, measure twice and mark.
Take the BNC/Banana adapter and position it in the center of the 12mm dowel. Mark the position of the two adapter holes on the dowel. See photo.
Drill two 2mm pilot holes through the dowel to accommodate the brass screws. Secure the BNC/Banana adapter to the 12mm dowel rod. The adapter should be in the center of the dowel.
Take the two ends of the wire in one hand and draw the wire through both hands evenly until you find the center of the wire. Mark the insulation with a permanent marker.
Commence feeding the ends of the wire from the top of the 16mm dowel cross arm through the 3mm holes, passing from top to bottom. Make sure the marked center position on the wire insulation is now over the 9mm hole in the 16mm dowel. Adjust each side as required.
Mount the 16mm dowel cross arm on your telescopic pole or other device and elevate the antenna. Where possible have the bottom of the antenna at least a half wave length above ground level and clear of any metal objects which may detune the antenna. Keep in mind to replicate the antenna height as you would during an activation. In the photo the antenna is mounted on a 7 metre telescopic pole elevated to ~6 metres.
Next feed the wire ends through the 12mm dowel cross arm, again from top to bottom. Now remove ~10mm of insulation from each end exposing the copper wire. Place the exposed ends, one in each side of the the adapter binding posts and seat each post so that the wire ends are clamped and cannot move. (see photo)
Connect a length of 50 ohm coax to the feedpoint. Using an antenna analyser check the resonant frequency of the loop, it won’t be 50.2 MHz. Remember I started with a 6.2 metre length of wire, the antenna’s resonant frequency before trimming is 48.5 MHz. Next adjust the loop circumference by removing wire (evenly on each side) I suggest 30mm from each side at a time until you hit the resonant frequency of choice. Take your time adjusting loop, don’t cut too much wire off in one step. My 18 AWG wire loop length finished at 6034mm for 50.2 MHz.
You may wish to add a 1:1 current choke as required.
Have fun building your own 6m oblong loop.