Construction Practices
KQ6RH
(C) 1998, 1999, 2000
Ray Jurgens
(Up-Dated 2/25/2000)
Introduction
The construction of light weight structures that support wire antennas requires some definite insight into the behavior of the materials. I shall consider only constructions using fiberglass tubes that telescope and the guy lines that are required to stiffen the structures. In some cases, the guy lines are actually the antenna wires, to the mechanical properties of the wires is also important. To understand the structural properties of fiberglass tubing, you should read Properties of Fiberglass Rods and Tubes. This section should give you a first hand understanding of the mechanical properties of this material and a good feel for the amount of bending that can be expected for various diameters and lengths of tubing. The total bending of composite telescoping spreaders made from the standard sizes of tubing can be estimated by considering the loading of the outer most sections at the beginning of the next larger section and working inward so long as the total angle of bending remains small. Small for practical purposes is about 20 degrees. In the case of most light structures, the bending will be more than is desirable, and guy lines will be required to take some the load thus reducing the total bending moment along the beam or spreader. In the end, the maximum loading is controlled by the tendency of the spreader to buckle due to the total force of the guys along the axis of the tubing or rods. This limiting force is easily computed given the geometry and the modulus of elasticity of the material. The remainder of this section considers mostly construction practices that have been found to work well and save time in assembly and disassembly of the antennas. As most of the antennas considered in this web site can be considered as portable, assembly time is often an important factor.
Telescoping Spreaders
The standard tubing sizes available from Max Gain Systems begin at 1/4" OD and work upward to about 2" in 1/4" increments. The walls are slightly less than 1/8" thick allowing the sections to telescope. In the case of the 3/4" tubing, the OD is slightly too large, and it is not possible to insert this size into the 1" material more than about a foot without getting it seriously stuck. This works fine as a fixed length spreader, but you can not vary the length. I shall focus on the construction of light weight antennas using only the 1/8", 1/4" and 1/2" sizes for the spreaders and the 3/4" and 1" sizes for boom and upper mast sections. These materials are sufficiently light that all antennas I have considered here can be easily constructed and erected by a single person.
The standard procedure used to assemble telescoping aluminum sections works equally well here. Six inches to one foot of overlap is always desired between sections. The sections are locked in place by using standard hose clamps to compress the tubing at the tip. For this to work, a slit must be cut at the tip of each section to be compressed, and the depth of the cut should be about twice the diameter of the tubing. So, for 1/2" material the cut should be about 1" and the hose clamp mounted exactly flush at the tip. The cut can be made most easily using a hack saw. you should be careful not to breath the dust from this operation, i.e., I suggest you use one of the paper filter masks while doing the sawing. You may find it difficult to find hose clamps smaller than 1/2" in most hardware stores, however, the 1/2" size will normally compress down to 1/4" but that is about the limit. The 1/8" solid rod can be used for micro extensions to the spreaders, but these are normally limited to about 6" in length. As a result, the maximum length of the spreader is limited to about 16' when using 8' sections of 1/4" and 1/2" materials. So the largest practical structure might support a full-size rotatable 20 meter dipole. A more useful limit for quads and planar wire beams is about 12'.
Supporting Spreaders
After considering many standard procedures for supporting spreaders, I decided that quick assembly was more important than cheap. There are several inexpensive ways to mount spreaders, for example, you can use a 1 or 2' section of 3/4" fiberglass rod in the center that can be U-bolted to a plate or hub. The plates and hubs are normally made of aluminum or hardwood. I experimented with both materials and found that neither suited me for this application. After discussing the problem with a good machinist, he suggested that we try PVC. We initially constructed four hubs from this material for testing, and found that it is sufficiently strong, light in weight, dark in color, easy to machine, and holds tolerances well. Currently, we are making these available in one size only, that is, for 1/2" spreaders with a 1" boom or mast. Stainless steel set screws are used to lock the spreaders into the sockets and to lock the hub to the most or boom. The initial hubs have been in use for two nearly years at this point and show no degradation of the material other than a slight change in color. See our Products Page for details and cost. These hubs permit rapid assembly and disassembly using a small Allen wrench. We have paid considerable attention to the accuracy of the machining. The spreaders are held accurately at ninety degrees and the length from the center of the hub to the tip is exactly one inch longer than the spreader when the spreaders are fully inserted in the sockets. These tolerances are important to maintain so that the lengths of the guy lines can be accurately calculated when they are needed.
Guy Lines
We have experimented with and tested guy lines made of three materials at this point. The decision of which to use depends upon a number of considerations such as visual impact, the amount of stretch that can be tolerated, the degradation over time related to weather and ultraviolet, and the expense of the material. The following materials have been tested:
|
Material |
Diameter |
#Test |
E, Modulus of Elasticity |
|
Nylon Fishing Line |
25/1000" |
40 |
|
|
Black Dacron Rope, DR33225 |
3/32", ~ 105/1000" |
>600 |
|
|
Bonded Kevlar, Size 207 |
25/1000" |
> |
|
Table 1 |
|
Properties of Guy Line Materials |
Of the three materials, the Black Dacron is most visible due to its greater diameter, the Kevlar is next due to its gold color, and the Nylon is least visible of the three if the material is clear or lightly tinted. Of the three materials, the Kevlar stretches the least for a given force and does not creep with time. In general, it is difficult to measure the tensile strength of these materials, as it depends upon how the materials are tied or bonded to the supports. These materials will always break at knots, so, that tells you that tying knots in the material is not the best way to form the tie-loops. After considerable testing we found that crimp fittings designed for fishing leaders worked well for the monofilament Nylon lines. These work less well for the multi-filament line, because the line flattens easily and requires the crimp to compress fully which is often doesn't. This results in slippage under tension. In the case of Kevlar, we found that most of the failures could be prevented by double crimping and tying a double half-hitch behind the crimp. The the larger diameter Dacron material can be handled with standard aluminum crimp fittings designed for wire cable. Again, the crimp fittings have to be compressed evenly across the tube. It is a good idea to do a few practice crimps and test them under tension just to be sure you have the technique under control. Once you are sure of this, it is time to make some guy lines of specific length.
The easiest way to make guy lines of accurate length is to fabricate them on a wood board about 8' feet long using two finishing nails separated by the desired length if the lengths are less than 8'. Longer spans can be looped back one or more times, i.e., the nails are spaced at some sub-multiple of the required length. We have found that stainless steel leader clips are very useful for attaching guys to the eye-bolts. The leader clips permit easy assembly and disassembly-assembly of the structures and provide a simple method of attachment that is stronger than most other methods. The combination of the crimp fittings and the leader clips permits the guys to be assembled under tension. As shown in the figure below. The second figure shows
The computation of the length of guy lines is normally not very difficult in the case that the assembly consisting of a hub and spreaders is planar. However, even in that case, one must make allowance for the distance of the attachment points form the spreaders, guy posts, and any other attachment structure that changes the distance between the two points of attachment. If you use snap clips, you must include them in the length of the guy. As an example, consider calculating the length of a guy line that spans the distance from a central guy post to a point near the tip of an 8' spreader. The lines are to be secured with snap clips to eye-bolts that extend perpendicular to the center post and to the spreader. Suppose the guys are attached to the center post at a height of 36" above the plane of the antenna. The eye-bolt extends 1.5" from the center of the post. The attachment to the spreader is set at 93 inches form the center of the hub, and the eye-bolts extend 1.25" above the center of the spreader. The spreaders are to be planar (not bent). The offsets due to the eye-bolts define a slightly smaller right triangle whose length, L, and height, H, are as follows:
L = 93 - 1.50 inches
H = 36 - 1.25 inches
So, if S is the required length of the guy line, the required length is:
S = sqrt (L^2 + H^2) = 97.9"
In the case where the spreaders are bent out of the plane, the computation of the guy lengths is a little more complicated. This is because as the spreader is bent by pulling on the guy point near the tip, the length of the guy point moves up and out of the plane and closer to the center guy pole. If you choose the amount to bend upward out of the plane, you then must know how far that point moves toward the center as well as the angle of the bend at the guy point on the spreader. Calculating these quantities is possible but a bit complicated, so it may be simpler just to bend the spreader and measure the length from the guy point to a point perpendicular to the guy pole or directly to the attachment point on the guy pole. However, the table below gives the approximate length of the tip of the spreader perpendicular to the center of the guy pole, the elevation of the tip above the plane of the hub, and the approximate force required to pull the spreader into this shape when guyed to a 3' guy pole.
All materials used for guy lines stretch, and the amount of tension in the line determines how much it stretches. Different materials of various weights stretch different amounts, so you may want to measure the amount of stretch. Nylon fish line stretches quite a lot while Kevlar thread hardly stretches at all. The stretched materials also grow in length under tension, for example, a Nylon line cut initially to 100" may creep to 104" when to about 3 kg or 6.6 lbs. It will also almost return to its original length when left un-tensioned. Because of this behavior, it is very difficult to measure the modulus of elasticity except by dynamic means. On the other hand, a similar length of Kevlar thread size #207 will stretch only about 3/8" when tensioned to 3kg and returns to the same length when the tension is removed. However, even the Kevlar deforms slightly over a period of weeks by about 3/8" to ½" for a 100" length. This means that you should provide for some room for adjustment when setting the locations of the guy points. You should assume that line lengths will grow a bit and provide space to extend the guy points. It is also possible to alleviate the stretching by pre-tensioning the material before cutting the lengths, and this is especially true for Nylon. In the case of Kevlar, the tension may take several days to weeks to stretch the material. Pre-tensioning also is commonly done with copper antenna wire, as it also grows in length in a similar manner. So, it is a good idea to pre-tension all materials before cutting. The only exception to this rule might be the Kevlar.
Fortunately, it is easy to compute the amount of stretch for a given material with simple tools. If you have a fishing scale, a pulley, a long board and a few nails, you have every thing you need. Or you can use the same materials that we use for which the numbers are known. (to be completed)