A Simplified Directional 40m Antenna (Part 1)
Bob Whelan G3PJT describes a gain antenna for 40m that can be put up in many gardens.
Bob Whelan G3PJT describes a gain antenna for 40m that can be put up in many gardens.
Effective antennas for 40 or 30m present somewhat of a challenge to the DX operator. Gain and directivity can confer big advantages in being able to hear and work DX when competing with other stations, especially those in Western Europe. But antennas with gain and directivity at 40m can involve significant amounts of space and engineering. I have built two types of directional antenna, a four-element parasitic array for 30m [1], Fig. 1, and a two- and four-element driven array (4-square) [2]. Both of these antennas were electrically rotatable and both showed good directivity and some gain in the forward direction. Using these antennas, I quickly realised that themajor advantage of these antennas over omnidirectional vertical antennas was the significant improvement in received signal-to-noise ratio.
This article covers some experiments undertaken in 2018/9 with the objective of seeing if computer modelling and better measuring equipment might allow some new insights and maybe some design simplifications.
Initial Thoughts
In early 2018 I constructed a three-sloper dipole array [3] suspended from an 18m tower. These slopers were also backed up with reflectors to provide better rejection of signals from Western Europe when beaming to the west. Results were mixed and did not meet expectations. But still in my mind the proven performance of the 4-square was offset by the significant effort required to construct a new one, especially the need for four new ground radial systems! Yes, elevated radial systems represent an option but there is still debate as to their efficacy especially when close to ground.
The antenna described in [1] if scaled for 40m was going to need a new 20m mast – I was sure this would not get past the local planners. This antenna, Fig. 1, had bent dipole elements, each element being centre fed with parasitic tuning using variable capacitors. EZNEC showed it had a rather similar pattern and gain to a 4-square though estimates had to be made of some dimensions as my experimental notes were lost.
The complexity of the 4-square − four matched ground radial systems, the need to drive all four elements, use correct feedlines and construct a switch box system − arises because of the need to feed each element with the correct phase and current amplitudes. Yet HF antennas use parasitic coupling to achieve the correct element currents and there is no reason why the same approach cannot be used on lower frequency antennas too with a potential simplification of the system.
G4DKG [4] described a rather simple way to add directivity to an 80m vertical and more recently VE1ZAC and VE3VN [5] have also described vertical antennas with parasitic elements to add directionality. A 3-element in-line 40m antenna is available from M0MCX in [6], but that antenna is fixed in one direction. Another reference worth consulting is in [7], the section on 80 and 160m antennas especially.
An EZNEC model of the G4DKG antenna, Fig. 2, shows that this would have the directional performance of a 2-element antenna even though three parasitic elements are used.
At 20° elevation over average ground the gain is 5.8dBi, about 3-4 dB over a single vertical (1.98dBi). (All models are at 20° elevation). There is ~120° sector in the rear direction where signals are attenuated by 10dB
or more. Unusually, this design uses a single ground radial system and the ends of the three parasitic reflectors and the driven element are bought back to a switch and tuning box at the centre of the ground radial system. No feedlines are needed. Thus, by selecting one element from four as the driven element, four directions can be covered.
VE3ZAC used a bent element director as an addition to a simple vertical and he changed direction by moving the director so it ‘pointed’ in the direction he wanted. VE3VN describes a two-element array with elevated radials, the base of each vertical being at 10m – I judge this to be a significant engineering exercise!
Summarising the conclusions at this point. A simplified parasitic vertical array seemed feasible based on a single radial system and using wire elements. A 10-11m support would be needed and the likely performance would be as good as a 2-element vertical.
Experimental Designs
As a first step the dimensions used by G4DKG were roughly scaled to 40m. Each element looked like a tilted-L with the long leg 8.9m and the short leg 5m. The junction of wires 1 and 2 is at 1m and about 5m from the centre. A single element of this shape has a modelled resonant frequency of around 5.8MHz. According to EZNEC it has a circular azimuthal radiation pattern at 7MHz almost exactly the same as a full-size quarter-wave 40m vertical.
2, 3 and 4 element antennas were modelled and combinations of driven element and parasitic element tried at various orientations around a central support mast. As the parasitic elements were always inductive at 7MHz, they were tuned using a series capacitor. The most interesting arrangement was three elements disposed at 120° as this allowed three directions to be selected and if reversible, a further three, making six directions in all, Fig. 3(a).
In the case of a three-element antenna a number of different arrangements were possible, namely,
• two driven elements fed in parallel with a single parasitic director or reflector
• one element driven with two parasitic elements in parallel as either directors or reflectors
These two arrangements beam in opposite directions but have very different electrical characteristics and therefore need a more complex switch and tuning box.
• a pair of elements, one driven and one as reflector or director with the third element floating or grounded. As the selected driven and parasitic elements can be reversed easily, this combination allows for six directions and was the arrangement selected for further study. Modelling showed that performance was best if parasitic elements were tuned as reflectors.
A typical azimuthal pattern is shown here, Fig. 3(b), almost identical in shape to the G4DKG design but slightly less tight in the reverse direction. The outer ring is 5.4dBi giving a gain of about 3dB over a simple vertical. The −3dB beamwidth is 143°. By inserting various values of series capacitance into the parasitic element the performance could be optimised for gain and front-to-back ratio.
EZNEC predicted that at the feedpoint the current in the driven and parasitic element would be almost equal and the phase difference would be ~135 to 140°. The parasitic