VK2ABQ Wire Beams

KQ6RH

(C) 1998, 1999, 2000

Ray Jurgens

(Up-Dated 2/25/2000)

VK2ABQ Wire Beams

  The VK2ABQ wire beam is described in Les Moxon's HF Antennas for all Locations which also contains the original references, but they are rather difficult to find. The VK2ABQ antenna is a planar wire beam that can be constructed from a single hub and four spreaders. The antenna is completely symmetric, i.e., the lengths for the driven and reflector elements are equal and form a square. The length of each side of the square is given approximately as 248/fMHz. The antenna modeling programs indicate that this antenna has 4.5 dBi gain , 10 dB F/B ratio, and an input impedance of roughly 170 ohms. I made some attempt to further optimize this design by permitting the driven element to differ in size from the reflector. Although slight improvements can be made, it appears that the extra trouble doesn't buy much. Although the feed impedance is somewhat inconvenient, you can successfully use 300 ohm twin line and an antenna tuner to feed this antenna with rather good results. I tried to find a source of 75 Ohm twin line which provides a quarter wave matching network as indicated in the Moxin's Figure 12.3, however, this material appears to have vanished from the market. So I tested several brands of the transparent speaker cable and lamp cord. These all had a characteristic impedance of approximately 110 Ohms. These cables will provide a fairly good match, but the losses were very high. Even a quarter wave matching section lost about 30% of the power. The original design also suggests using coat buttons for insulators between the radiator and reflector elements, however, coats with large buttons are not very common in California, so I used the standard "dog bones" and assumed that the insulators spaced the wires by two inches. You can use 1/4" Plexiglass plate to make the insulators. Below is a picture of a ten meter version of this antenna supported on my test mast. The spreaders are actually 8' in length, but the wires are attached at about 6' from the center of the hub.

10 m VK2ABQ Wire Beam

 The support structure is constructed of a central hub that supports four 1/2" fiberglass tube spreaders. The spreaders are pulled up about one foot using monofilament fish line. The formula 248/fMHz is close to correct length for the length of the wires on each side, but be sure to leave the proper extra length for connecting to the insulators and the feed point spacer. Normally about 2 to 3 inches is required for the "dog bones." I've used several types of copper wire so far with good results. Solid copper #14 or #16 can be found at any electrical store, but it is a bit stiff for very light weight spreaders. A better choice is the #14 flex-weave wire that several of the cable venders advertise in the ham radio magazines. This material stretches out easily even with the low tension that can be maintained by light weight spreaders. Don't attempt to use copperweld for light weight designs, it is entirely too stiff. But, you can use smaller wire sizes. The AO modeling program indicates that this design can be built with wire as small as #20 without serious wire losses. This is because the driving impedance is high, and the currents are low on all parts of the wire. I have used Radio Shack's #18 black insulated hook-up wire quite successfully for several wire beams.

  Typical Performance characteristics are given in the pattern figure below:

20 m VK2ABQ As Defined, 248/fMhz per side

  As a reality check, I built a 10 meter version this antenna cut for 29.0 MHz, and then made a set of SWR and impedance measurements spanning from 27 to 30 MHz. These measurements indicated that the resonant frequency to be about 28.6 MHz with a driving impedance of 170 ohms. This is almost exactly the value predicted by the AO modeling program. Fortunately, 10 meters has been open during the day, so I was able to make several contacts into South America, Australia and New Zealand. The antenna appears to achieve the 10 dB F/B ratio, and the signal reports indicated a full 10 dB improvement over my Hustler 10 meter vertical whip. Obviously, my 10 meter vertical is not an ideal reference dipole, but I doubt that it is more than 3 or 4 dB poorer than a dipole and most of that loss is due to inadequate grounding. The whip also had the advantage of being nearly 15 feet higher than the wire beam. So this simple antenna works fairly well and has only the driving point impedance as a drawback.

  If you are interested in trying to improve the performance, the table below gives the critical information. In this table, all lengths are measured relative to the spreader length, and the structure is to be a perfect square. The spreader lengths are actually the locations for the corners of the square on the spreader, so you can mark these locations on the spreaders with a touch of nail polish. The driven element length and reflector element lengths refer only to the sides, not the entire wire length.

20 m VK2ABQ Beam with Minor Optimization

As can be seen from the pattern plots, only very slight improvements can be made with respect to the original design. Attempts to bring the impedance down to match 50 Ohms severely reduce the front to back ratio. A slightly greater range of improvement can be had by allowing the radiator and reflector to have differing lengths. I allowed the size and the locations of the insulators to vary such that the reflector became a few percent longer than the driven element and the the size becomes slightly smaller, i.e., 243.4/fMHz. This also results in a lower driving point impedance of 150 Ohms with some reactance in the order of -20 ohms. The antenna pattern for this design is shown in the figure immediately above. Optimization beyond this is problematic, since improved gain and better F/B ratios are in opposite directions of the resonant frequency. The design above is about the best compromise that can be arranged without moving off resonance. The spreader length given in the table below is the distance from the center to the attachment point on the spreader. Note, (Side) is the length of wire on each side. This length is slightly smaller that that given by 248/fMHz (actually about 243.4/fMHz).

Optimized VK2ABQ Wire Beam

Note that the wire length for the director is Side + 2*Ld and the reflector is Side + 2*Lr. This will be just short of 4*Side for the total wire required.

  Most of the parts required to construct this antenna are available from MGS. What you will need depends upon what band(s) you are interested in covering. The 10 and 12 meter bands require no extensions to the 8' spreaders. The 15 meter band requires only a few inches of 1/4" tubing as extenders. The 17 meter band requires extenders of about 2' including the proper overlap. Although it is possible to cover the 20 meter band with this light weight construction, the spreader length exceeds 12' and the design of the extensions and guys is difficult and not suggested. In all cases, you will require the following:

 Normally Required Parts

Extensions to 15 meters (add)

Item	Quantity	Description
8.	1	8' 1/4" OD fiberglass tube (spreader extenders, cut in 4 equal parts) (MGS)
9.	4	1/2" stainless steel hose clamps (hardware store)

Extensions to 17 meters includes all of the above (plus)

Item	Quantity	Description
10.	4	GT 4-025 1/4" Guy Ties (RFJ Comm. Systems)

Obviously, you also need wire and feed line. Number 20 stranded hook-up wire is fully adequate for this antenna. Multi-band operation is possible by tying the feed points together with short segments of feed line. The construction of guy lines is covered separately (see our section on construction). What you choose to do here is greatly dependent upon whether this is a portable or temporary installation or if it is to be permanent.

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