Some Ideas for Portable Directional Arrays
While doing the work for the portable vertical antenna, it occurred to me that the use of directional arrays for portable/ backpack users would be a benefit to the QRP folks. So with this in mind, some simulations were run for both 1/2-wave and 1/4-wave spacing. The use of vertical arrays has been well documented down through the years and needs no further comment here except to say that they work well for DX. At QRP levels they will do even better at helping you make contacts. If you purchase two collapsible fishing poles the array may be carried with the other components in a backpack.
Figure 1 shows the bi-directional pattern of two vertical antennas with the antennas being fed with zero phase difference. The two red dots are the vertical elements. This is an end fire array, as the radiation is from the ends of the array.
The Figure 2 illustration is of the same antennas with 180-degree phase difference.
Figure 3 shows the vertical radiation pattern of both of the 1/2 wave spaced phased verticals. The major lobe falls between 28 and 25 degrees, which is fairly low and good for DX.
Figure 4 is a 3D representation of the end fire array which will further show the shape of the radiation pattern of this array.
Figure 5 is of a unidirectional array, consisting of two verticals spaced 1/4 wavelength. The red dots are the two antennas and the phase difference is 90-degrees.
Figure 6 is the 3D representation of the total pattern of this array and this shows that nearly all of the radiation goes to the front of the array. Again the vertical bars represent the two verticals.
Figure 7 illustrates the vertical radiation pattern and this indicates that the maximum angle of radiation is less than 30 degrees, usually around 26 degrees. This is a good angle for DX work.
In Figure 8, the basic construction of both the 1/4-wave antenna array and the 1/2-wave spacing array is shown, sans the radials and matching networks.
Backpackers who take their QRP rigs with them may take this array along by careful choice of components. The verticals are two of the same fishing pole type that was described in the multi-band vertical article in the previous issue (antenneX – July 2000).
RG-58 coaxial cable should be the minimum size used although RG-174 is much smaller and weighs less than RG-58. The phasing lines should be made of the same coax cable as the feedline. Correct lengths of phasing lines is determined by the following formula:
468/f x Vf where Vf is the velocity factor of the cable
Connection to the 1/4-wave array should consist of connecting the two elements together with the phasing line and then connecting the feedline to one of the elements. A small matching network will be needed for each element and for the transmitter connection point as well.
Figure 9 shows the array less radials. The antenna matching networks may be built based on whatever parts are most handy to you. The gain for this array is about 3 dB, despite what the patterns show. The null to the rear is about 60 dB in real life. The difference is that this pattern was measured at a radiation angle of 5 degrees and not at the maximum radiation angle. You should connect the feedline to whichever antenna is aimed at the direction of interest.
The other array is the 1/2-wave spaced bi-directional array. By changing the phase shift from zero to 180 degrees the pattern will shift 90 degrees. Knowing the direction of interest will allow you to aim both of these arrays with the use of a compass. The amount of gain these two arrays have is not much—only about 3 dB. But, having 3 dB more signal going the way you want it, makes a major difference. Figure 10 shows this array.
Figure 11 is a diagram of the switching for the 1/2-wave spaced array. In the diagram, S1 is in the zero phase shift position. When the switch is thrown to the B position, the phasing line is in the circuit and there is a phase shift of 180 degrees between the two elements and the pattern is shifted.
NOTE! The feed lines between the two elements must be the same lengths for this antenna to work correctly. An SPST switch will also work if you connect it to short out the phasing line. Almost any switch will do at the QRP levels found on the ham bands.
The components should be suited for the power levels being used. Since the elements are made of wire strung on the telescoping poles, these can be carried in marked pill bottles, as can the radials for both types of antennas. The matching networks may be encased in pill bottles and other lightweight plastic or wood cases. Tupperware is another good choice.
Photo 1 is of the 1/4-wave spaced array. This was taken in a rather high wind and the verticals are not quite vertical as a result. Not visible are the radials or feedline
Photo 2 shows the 1/2-wave vertical array. Notice how the pole on the left blends into the background and is an advantage that should not be overlooked. Many camping places frown on anything that disturbs the “natural appearance” so this is an ideal method of operating an antenna that is inconspicuous.
If you decide to use RG-174/U for your feedline and phasing lines for weight considerations, just keep in mind that there is more loss on all ham bands below 30 MHz than if RG-58 is used. I do not have an idea of how much, but the loss can be quite noticeable if a very long length of RG-174 is used. RG-174 is also rather fragile compared to RG-58.
My own personal experienced with phased arrays has been good on 40 meters. I had built a 3-vertical array from plans the now defunct HyGain antenna company had published. The antennas were arranged in an equilateral triangle and switched by relays to each pair of antennas, with one antenna not being fed at any given time.
This array allowed me to punch a hole to the East Coast through the many broadcast stations from Europe that dominates 40 meters. I had a Heathkit HW-101 and without the linear I would get 20/9 reports from all up and down the East Coast. In some cases, I could cover up Radio Moscow and the other stations that covered the 40-meter band at that time. If I turned on the linear, a Johnson Thunderbolt, I dominated the frequency.
By using this type of array you can increase your QRP contacts in a given direction. Every little bit helps when you are running 5 watts or less from a remote location that you have backpacked your rig and other supplies into. Besides, if the band is dead, then you have two poles to go fishing!
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 28th May 2024