This is a design I put together sometime ago but, I don’t have the space to put it up here at my UK QTH.
The design is based around two vertical Delta Loops 7.5m apart with the lower horizontal wire being just 1m above the ground.
The reflector loop (shown on the left in the image above) has a 70 ohm non-inductive resistor placed just above the lower corner of the loop (Denoted by the red square).
The driven element (shown on the right in the image above) has its feed point in exactly the same position as the resistor on the reflector element (Denoted by the red circle). The antenna presents an almost perfect match to 50 Ohms and therefore won’t require any additional matching circuit. Both the resistor and feed point can be directly at the corner of the loops for convenience without affecting the performance of the antenna.
Ideally a 1:1 balun should be fitted to the loop at the feed point however, it can be fed directly with 50 Ohm coax if needed.
The 3D field plot shows the overall radiation pattern of the antenna with a fairly wide beam width and excellent front to back ratio.
The 2D field plot shows a max gain of 5.16dBi at 22.5 deg and 1.6dBi gain at 7.5 deg and a front to back ratio of 13.03dB. This antenna should be great for working medium and long distances in one direction with excellent signal rejection on the back of the antenna.
The SWR Curve favours the lower section of the 40m band but is useable all the way up to 7.094Mhz with an SWR of 2:1 or less.
The point of resonance could be moved up slightly in frequency by raising the loops up, perhaps 20cm at a time until the antenna resonates where you want it. The lower wire must always be horizontal regardless of height above the ground.
With the ATU that is in most modern radios these days you should be able to use this antenna across the entire 40m band without issue.
Summary:
Horizontal Wire Length: 18m @ 1m above ground
Diagonal Wire Lengths: 12.6574m
Wire Dia: 2.5mm
Height at Centre: 9.9m
Distance between loops: 7.5m
Feed Type: 1:1 Balun in bottom corner (Can use coax if necessary)
Impedance: 50 Ohm
SWR: <2:1 7.0 – 7.1Mhz