Assembling your antenna system
The light and stealth antennas (II)
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.
Often you will have to fall back on shortened beams, wire beams or even verticals. But even in this case a mast 10m high made of thick aluminium tubing doesn't go unnoticed. If this is of your concern, like Bill Glenn, AA4BQ, whose antenna is displayed at left, you can resort to camouflage and attach a birdhouse on your antenna, or a flag.
If you work portable, you know that these installations use pylons constituted of light material like aluminium tubings or titanium, poles made of wood or carbon fiber. They are not at all suited for a permanent installation, all the less in a perturbed region subject to regular high winds, thunderstorms, and snow or freezing showers in winter months. These installations use small masts, often telescopic attached with low quality accessories, and are usually drive in a few tens centimeters into the ground or even at ground level for a one-day activity at Sun, a Field Day, a JOTA, and sometimes abroad during holidays. In any case they can be used as it for a permanent installation.
During 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 !
For a permanent installation, your mast must resist to the forces generated by the weight of the antenna and the strength of the wind. So you must select a very sturdy mast or a tubular tower made of heavy duty aluminium (light, not too expensive), steel (stronger but heavier) or titanium (light, robust, but more expensive) able to sustain a force over 100 kg hours long.
We do not improvize the assembling of a permanent antenna. Know that by tempesty wind, gusts can reach 180 km/h (110 mph), the wind exceeding often 80 km/h (50 mph) on top of hills. In open fields, your mast or your tower will quite regularly be subject to wind gusts in excess of 110 km/h (70 mph) too.
If you desire to install a light mast in windy areas, know that a galvanized steel mast 3 mm thick (1/8" wall) or a chrome moly mast 8 mm thick (5/16" wall), both telescopic and 50 mm (2") in diameter, cannot support such stresses and will bend (say 20° off vertical) or can even break under winds of 110 km/h. A towerwork, even "light", must replace so light installations.
To resist to high winds, the other solution should be to lower the mast up to 2m high or to tilt it until the Sun shines again. This is also valid for towers.
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 30 cm to 1m wide (10-40"). A 10 m high (33') tower, 50 cm wide at the base constituted of 4 sections displays already a weight of about 40 kg (80 lbs.) for the lightest made of aluminium.
Depending on 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 except 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 towers, guyed or not, are equipped with a lift system (e.g. Hummel) at which the antenna mast 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.
But not be disappointed if you don't find these brands in your country. A good company specialized in the work of high iron or aluminium will build you a good tower as well. The material is easy to handle : made of hundreds of small elements bolt or soldered together, this is a simple Meccano except that it is somewhat...bigger once assembled. Take only care to the way that all segments are fixed together and check the quality of accessories (stainless steel, inox, etc).
Then remain the problem of the transport (renting a trailer for example) and assembling on site.