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Of course many amateurs do not ask for this approbation from the moment that their backyard or their roof is large enough to erect a beam without disturbing their neighbouring. But this is at your risks if one day a municipality agent knocks at your door requesting your planning permission or showing you a legal restriction for the height of your tower or the wingspan of your antenna. Several factors are to be considered before installing a tower : - The size and weight of the antenna desired - The weather conditions (speed and frequency of wind gusts, risks of thunder, snow, ice, tornado, cyclone, etc) - Risk of earthquake and underground stability - The ease of assembling of the concerned tower - Any legal restrictions with local ordinances (about building, height, distances, etc) - Free space available in garden or on roof (to prevent that the antenna falls onto the neighbor's property) - The investment (the tower, the antenna, the rotator, the lightning protection, plus accessories) - The availability of providers or manufacturers for parts and support - Your ability to handle alone the project (or with a friend) Add several safety considerations : - Never place an antenna over or near a power line - Check the advantages but also drawbacks of each tower design - Always place the anchorage system on your property while respecting all distances (to tower, to neighboors) - Secure the access to the tower (create a safety perimeter, shield the base to prevent climbing, protect guy anchors) - Ground the antenna, the tower and install a lightning protection. Always interested ? Right ! Let's see now how to select, assembling and erecting an antenna system. We will imagine that you received all required authorizations, that your neighboors and family are not really irritated or affected by your project, thus it remains to buy the material, get some tricks of construction, and of course to find some money, your main limitating factor after the space available. Here are some basic recommendations if you want erecting an antenna on top of a tower, knowing that this subject can be developed on dozens of pages and has room for experimentation. We will discuss about usual installations, leaving aside the biggest ones that request the assistance of a a crane or even an helicopter... The light and stealth antennas Before to begin, say a few words about the light and stealth installations that are used by many amateurs living in cities or in areas subjects to strict regulations. If you live in an antenna-restricted neighborhood, you will have to make a compromise between the performance and the size of your antenna system.
Duing these short activities, these antennas are sometimes installed without guy wires, simply supported by a sturdy tripod, a fork maintained below a car tire or attached to the car frame. Such sticks, sometimes of large size, are first of all suited to erect a few meters high (up to 12m, 40') small ground plane verticals, whips, wire beams or dipoles but they will be unable to support much strain under high winds. At 5 m high, 5 kg vertical already bends under wind gusts and you have chance to break it apart if you don't attach it with guy wires on top. By definition these installations are portable and the activity does usually not last more than a week or so. For a permanent and durable outdoor installation, you cannot imagine working this way or you will quickly find your antenna bent or broken apart on the ground !
If you cannot invest in such an installation, you can already resolve partly the problem in installing the antenna in a lift system, also very practice to service the antenna. Remain to secure the tower, but what tower to select ? Selecting your tower The selection of a tower is a serious affair that should respect the professional standards and offer the highest security. Towers come with a triangular or square base from 50 cm to 1m wide (20-40"). A 10 m high (33') tower, 50 cm wide at the base displays already a weight of about 40 kg (80 lbs.) for the lightest made of aluminium. Depending the load and surface, a 10 m high tower should resist to a wind load up to approximatively 2m2 at 180 km/h (~20 sq.ft at 110 mph) and a tower half that size should support 75% of this value. These specifications are high and probably hard to meet by low and mid-range towers. These values or near ones are however supported by all high-end models. This is a prerequisit if you live in windy areas and if you want to set up a HF beam at about 10 m high in all security. But this is not enough, and the second criteria will be the size of the base. Using an oversized tower is not necessary useful and will be surely more expensive. But adding a security margin to your needs is more than recommended in this matter. Think seriously about the risks for your family and the neighborhood in the event of an accident. If the tower breaks by high winds, the top will fall down first, bending half the tower if it is not uprooted, and bending or breaking the antenna in the same occasion, as the next pictures show us dramatically. An HF beam or a large vertical is nothing less than a metallic tree. For kids this is rather a funny framework on which they can climb easily if its access is not protected. In all these circumstances, the falling of an antenna might produce severe damage if you don't plan seriously its assembling.
Towers come in many designs, each offering advantages and drawbacks. A free-standing tower for example plays with the wind to stay balanced as there is no guy wires to maintain it steady. When the winds blows on one side, this part of the tower is pulled upward while the opposite side is compressed downward. This movement reaches the base of the tower where it creates a huge moment that has a tendency to pivot, amplifying the torque. Therefore the base of a free-standing tower has two functions : holding the tower up and simultaneously prevent its rotation in one direction or another. The base of these towers requires thus to be driven in a deep hole, filled with much more concrete than a guyed tower where the guy wires restrain the tower, the mouting base being tightly bolten in the anchorage system. Each section is then bolten to the next. On the contrary the crank-up of telescopic tower must be stabilized with guy wires. Like any telescopic mast, the outer diameter of each section decreases gradually with the height, the inner diameter of the next higher section being smaller than the previous. Each section is attached to the previous with bolts and nuts, and for the heaviest, with a complex system of cable and pulleys. The height of each section can thus be adjusted or even completely retracted if you want to work closer to the ground. Most of them require to be guyed for security purposes. Crank-up towers are also interesting for their portability as all segments excepting the largest and lowest one can collapse each in the others showing an overall retracted size that does not exceed 2 or 3m high. Drawback, it is not secure trying to climb on a crank-up tower. Fixings are sturdy and made to last but Murphy is never very far. Some crank-up towers are equipped with a tilting base too, an hinge allowing the tower to be partly folded over or totaly titled up to the ground. This system applies to tubular towers and telescopic masts. By the same occasion a rotating system turns the antenna in the right direction to prevent it to bend or to break at landing... However, tilting towers have led to some accidents, mainly when some amateurs, not used to work with such mechanisms, tried to climb on the tower while the hinge was not secured. That stay an excellent system if you country regularly undergoes high winds like on top of hills. At last note that some guyed tower are equipped with a lift system at which the antenna system is attached. By high winds the rotator, the supporting mast and the antenna can easily be lowered to the ground, the tower standing alone in the wind. This is also very practice to check and service the antenna system without having to climb on the structure. The towers market Brand new towers are not more expensive than other ham products. But made of a "simple" tubular structure, they worth well their price, all the more that their stability can never be ensured at 100%. To fix your ideas their price is ranging from $400 for a 3m high (9') roof-top tower to more than $1500 for a 16m high (55') crank-up tower, plus handling/shipping and maybe a serious percentage of import and local taxes.
The secondhand market can also provide you reliable models at one third of that price if not less. Only one precaution, if you buy a secondhand tower, check the inside of tubings where rust could accumulate or where small cracks could appear. It shouldn't, as all towers must be either galvanized or at least painted (about 3 layers) or coated against rust. In all cases avoid to purchase a tower older than 20 years showing spots of rust or cracks on the outside, looking suspect inside tubings, showing bended rungs or that were repared after have been damaged; they might look sturdy from the outside, even climbing on some elements of the structure, but you have no idea of the way that it supports the strain on the long run. Do not provoque Murphy a second time ! If you are searching for a lighter but sturdy pylon look also at military or fire department supplies that often provide very practical and sturdy solutions like heavy duty electrical crank-up tripods reaching 10m high (33') or heavy duty pneumatic masts reaching 15m high (55'). Remember only that the top element of these "portable" masts are not always suited to support an antenna over 20 kg (40 lbs), the top being always the weakest point of the structure while the base has to support all the weight and the possible torque. Select thus a model sturdy and large enough to support the weight of your antenna and the force of the wind. This being said, the towers sold by Rohn Industries, US Tower or De Kerf are probably among the most appreciated by the ham community. Then remain the problem of the transport (renting a trailer for example) and assembling on site. Constructing of the tower base
Assembling a tower requests skills and a serious planning. If this is your first construction, that it is to support a 2-element quad or a bigger gun, it should be interesting that you visit some near building sites to see how they start work on the concrete base or how they construct a big pillar for example. Read also the ham magazines that regularly publish the works of the best handymen as well as books devoted to antennas published by ARRL or RSGB for example that usually reserve several pages to masts and supports (ARRL Handbook, RSGB Antenna File, etc). You could also find these publications in large libraries. If you purchase a new tower, the manufacturer will provide you detailed instructions for properly constructing the base and assembling the tower. If you have no plan, you can always "copy" the instructions of any other tower or the ones found in magazines, and if necessary slightly modify the dimensions to your needs, what most amateurs probably do. The foundations Any builder used to work with concrete and steel should tell you that a tower should be erected over a concrete base reinforced with a steel-bar framework on which the feet or the tower base assembly will be boltened. This is the classic way of installing a tower safely. The concrete base is driven in a hole from 0.5-1.5m depth (20-60"), depending the weight, the length and the design of your tower. This hole is 20% larger than your tower in order to insert without problem the steel framework just in the middle of the space. The hole must be large enough so that the anchorage system can be partly sunk in the concrete. Place at the bottom of the hole a few big rocks to support the framework. To keep the liquid concrete in position, a wood form made of fir wood boards 5 cm thick (2") is constructed around the top of the hole. If the soil is poroux, crumbly like sand or too soft, the concrete might flows into the ground. In this case densify your concrete and drive a hole twice as large.
Then construct the steel framework and prepare the anchors (hence the interest to visit a building site). The framework is made of four No.6 RE-bar as long as your hole but shorter, that will be placed vertically near the corners of the hole. All ends should be bent to the inside. This frame is consolidated and bond to horizontal loops made of No.4 RE-bars wired to the vertical rungs each 50 cm (20"). So you need only 3 steel hoops on all the lenght of the framework. After building check all dimensions and that all segments are well perpendicular to each another. It is not mandatory to level the steel framework as it will be sunk in concrete but try to prevent any potential problem. Although we will review this issue when we 'll speak about lightning protection, in order to minimize the potential of a spark jumping from the bonded RE-bar directly to the earth, there needs to be a minimum of 10 cm (4") of concrete between any metal element and the earth (like the floor and the walls of your hole). The anchors are specially built to fit your tower mounting base. Either the feet of your tower provide the necessary holes on all their length to screw bolts and nuts or the manufacturer provided a base assembly. This base shows four or more holes used to bolt the anchorage system, hence the necessity that this last is thread. If you must made the anchorage system yourself, anchors have to be made of stainless steel, 40 cm long (16"), sunk at approximatively 3/4th of their length in the concrete. They must stand out of the concrete to bolt the base assembly. The optional thread will be adapted to your nuts. But do not work for nothing as, as we just told, the anchorage system, bolds and nuts are usually provided by the tower manufacturer. When all tubular elements are welded and assembled, insert the steel framework in the hole, well vertically, and just after put the anchorage system in the middle. Bind it to the RE-bar to create a ground system, called in this case an Ufer ground. Check immediately that all is properly set with the level and the ruler. In case of mistake do immediately the adjustment. The concrete base From the proper installation of the anchorage system will depend the sturdiness and plumb (verticallity) of your tower. However it is very hard to see a misalignment of the anchorage system with the tower feet or with the mouting base or, worst, a lack of plumb at ground level. To prevent this type of problem, if your tower base is not too high I suggest you to bolt on the anchorage system the first section of the tower mounting base. That will ensure you that all is properly set once sunk in the concrete. Indeed, in this way it will be much easier to check the plumb on a long panel or a rung of the mounting base, and make if necessary the small adjustments before the concrete hardens. In anticipation of these small adjustments, prepare aside a wood board, a wood saw, a hammer, nails, a tape, a level, a ruler and a pencil. In case of trouble these accessories will help you to quicky made a spacer for example, to adjust the alignment of the anchorage system. Then, as your concrete base will have to emerge at bit over the ground and that the concrete will stay liquid for sometimes after pouring, construct a wood square form around the hole of approximatively 10 cm high (4"). Place a transversal board on top that will be put down on the wood form. It will serve to temporary attach the anchorage system to prevent it to sink in the concrete during pouring and until it hardens.
Now it is time to prepare your concrete. Don't work too fast, the concrete is very heavy, and it splashes ! Now go quickly but conscientiously. When all is ready, that the steel RE-bars and the anchorage are in position, each well bond to the other, that your wood form is firmly attached around the hole and the anchorage well attached to the wood board, pour the concrete over the foundations up to the top of your wood form (thus 10 cm or 4" over the ground). Level the top and check it is well horizontal. This is mandatory if the first tower section is welded to an horizontal plate like in the picture displayed above (below left corner). If you cannot drive in the ground over 30 cm depth, what is common in hill areas and ranges, or if you want lighter foundations to support a small antenna (< 20 kg, 40 lbs.), you can prepare a small hole 50x50 cm aside and 30 cm depth only (20x20x12"), place the feet of the tower inside and pouring the hole with concrete. Of course in this case your antenna must be guy-wired. Such small foundations are perfectly able to support a small beam (up to 20 kg) for years under wind gusts over 100 km/h (60 mph). It cannot resist long times to winds blowing over 120 km/h (75mph) on heavy or very long Yagis. In this case, to increase the security of this small installation I suggest you to sink in the foundation a small wire netting, without to forget the maintain the tower with guy wires, but we will see that later. In all cases check with accuracy the level and plumb of your base and anchrorage system. If you don't do it now, when the concrete will be hard you will need of a pneumatic drill... Your concrete base is now achieved and tomorrow you might assembly your tower and the antenna, lucky you are !
A word of caution about Ufer ground To prevent you creating a specific external grounding system to protect your hamshack against lightning strikes, and if you have not started yet construction on your tower, you should take advantage of the tower's connection with the earth as part of your external ground or Ufer ground. But be aware that if the Ufer ground is improperly constructed you do run the risk of exploding portions of the concrete ! This is to prevent such an accident that it is required that the pieces of RE-bars that constitute the framework be electrically connected to each other, whatever can write some radio amateur handbook on this subject. In your example this was accomplished by welding them to the horizontal steel loops. In addition, the anchor bolts must also be electrically connected to the RE-bars. This is a bit more difficult to accomplish since many manufacturers do not permit them to be welded due to the possibility of changing their mechanical strength. A simple mechanical clamp will work. As we told, in addition there must be at least 10 cm (4") of concrete left between the framework and the surrounding earth.
But for what reason the concrete could explode ? Tower base concrete explosion is caused by an arc that takes place within the concrete between two pieces of metal that are not electrically connected to each other. Since the concrete almost always has moisture, the heat of the spark caused by a lightning strike expands the moisture and weakens or destroys the concrete. By bonding the metallic elements of the tower base together, you achieve two things : no potential for a spark within the concrete and you create a Ufer ground. A strap emerging off the concrete can then be use to ground your hamshack as we will see in the pages dealing with the lightning protection. Next chapter
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