Phil EI9KP has very kindly sent on details of the Delta Loop antenna that he built for the new 40 MHz / 8-metre band.
The Delta Loop is a very popular antenna on the HF bands and can be used with success at the lower VHF bands as well.
The dimensions of the Delta Loop for 40 MHz are shown below...
Some key points...
1) Up or Down....The Delta Loop can be orientated so that it is pointing up or down. The advantage of having the apex at the top is that it may be be more mechanically secure as the antenna hangs from a single point. The advantage of having it pointing down as in the diagram above is that it means the maximum current part of the antenna is slightly higher above ground level.
2) Impedance.... The Delta Loop has an impedance of about 100-120 Ohms. A quarter wave of 75 Ohm coax is used to transform the impedance down closer to 50 Ohms. If you are using coax other than RG-59 then make sure you correct for the velocity factor.
3) Choke Balun..... The 75 Ohm coax is wound onto a 50mm PVC former to create a choke balun. The Delta Loop is a balanced antenna and the choke balun helps prevent RF from going onto the outside of the unbalanced coax feed line.
4) PVC Mast.... The support for the Delta Loop should be a PVC pipe or a similar material. A metallic mast running through the centre of the Delta Loop will probably detune it.
5) Directional..... The Delta Loop is bi-directional like the dipole and has a figure of 8 radiation pattern. It has a small amount of gain, perhaps 1dB or so over a half wave dipole.
Some photos from Phil EI9KP are shown below...
Some notes from Phil..."8m Delta Loop. The first, experimental version was in upside-down ‘nabla’ shape but it was a bit top heavy. I rebuilt it a couple of times now for Delta shape, fed at the bottom for HP. Wire length 7.70m is same for Delta or Nabla shape. The wire I use is surplus military field telephone wire; it has an additional metal wire inside for strength. The impedance adapter is a quarter wave * VF of 75 Ohm coax (RG-59), which I have coiled on an 50mm ex-sealant tube. Actual length is 1.30m allowing 2cm either side for soldering. All bolts, washers, split washers and wing nuts are stainless steel M6."
VSWR...... The VSWR as measured by EI9KP is shown below. The 1:1.5 VSWR bandwidth is about 2 MHz.
In conclusion.... The Delta Loop is a pretty easy antenna to construct and most of the material can be obtained at a local hardware store.
It's an easy way to get an antenna with a small amount of gain well above ground level.