The general arrangement of the Delta Loop is shown below, as well as the feed method which has been found to be the most successful.
For maximum vertically polarized radiation, the feedpoint should be 1/4 wavelength from the apex (whether pointed up or down)
The antenna should be relatively close to the ground. For a 40 metre model, 15 to 30 ft up to the bottom is max, and models suggest that this scales for other bands. If higher at the bottom than about 0.2 wavelength, then a second high angle lobe develops, and stronger, closer in stations can overide more distant stations, even if the lobe is not dominant in the pattern.
The Delta is at its limit on 20 metres of being a better DX antenna than a dipole, you will not get more gain from a Delta Loop over dipole, but it may exibit a better signal to noise ratio - hence more comfortable listerning.
THE DELTA LOOP IS A MONOBAND ANTENNA
Although the vertical-plane delta loop either pointed up or down and fed virtually anywhere will radiate well enough to provide contacts, it is a relatively poor performer in any configuration when stretched to other bands, compared to even the simplest substitute. Modelling of Delta Loops and similar antenna's has proven to be a quite reliable indicator of performance, since nothing in the antenna type even approaches, let alone exceeds, the limits of the software for accurate modelling. So the following notes may be useful. These notes apply to vertical-plane equilateral delta triangles, although with some variation, they apply equally well to vertical-plane right angle deltas. Modelling was done on NEC 4.
First, the antenna. A vertical-plane equilateral delta for 40 metres will be about 48ft 5in wide and 42ft high. This places limits on the height above ground of the lowest point, since with a 35ft low-point height, the high point is 77ft up. For the examples below, a standardized 15ft bottom point is used, with a 57ft upper point.
(a) When fed as a self contained vertical, maximum bottom heights of 20 to 25ft yield the highest gain (about 1.45dBi) with an elevation angle of maximum radiation of between 16 and 17 degrees. The first signs of a secondary higher angle lobe just make an apperance, but do not constitute a problem relative to the antenna's use as a low-gain, low-angle DX antenna with superior signal to noise characteristics due to insensitivity to higher angle radiation.
(b) As the antenna is raised by another 5 to 10ft, the secondary lobe increases, so that with a bottom height of 40ft, the antenna's secondary high angle lobe at 60 degrees is down by only 4dB or so. As the antenna is lowered, gain decreases very slowly (imperceptably), and the elevation angle of maximum radiation increases very slowly (about 1 degree for each 5 feet of lowering. A 15 ft lower wire or point height provides good performance within this class of antenna.
(c) Moving the feedpoint to a mid-side location increases gain by increasing the amount of horizontally polarized radiation produced by the antenna. This increases the elevation angle of maximum radiation as well as spreading the main lobe vertically to permit considerable amounts of higher angle radiation. In effect, moving to far from the feedpoint prescribed for use of the vertical plane delta as a vertical antenna tends to defeat that purpose. However, the vertical-use feedpoint itself is not critical within say 5% or so of the recomended point.
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