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Hang 'em High: Options for antennas, masts and towers
First, we should note the difference between antennas, masts and towers. A mast or tower is simply the thing that holds the antenna up off the ground. Masts are usually metal, but under unusual circumstances they might be made of wood or other substances. In FM broadcast, the antenna is usually mounted to the mast near the top. A mast is generally a single piece of pipe, while a tower is a set of interlocking pipes (typically in a triangular configuration) which can be much taller.
One thing that may be confusing is that in AM broadcasting, the antenna and the tower are the same thing - this is because AM wavelengths are so big that they require an antenna the size of an entire tower - so the solution is just to use a metal tower as the radiator. For FM broadcast, the wavelenths are only about ten feet, meaning the antenna can be much smaller. In FM broadcast the only reason to get so tall is so that the antenna radiates from a high up point that has line of sight to more of the terrain around the radio transmitter.
Caution: Starting Small Is Better Than Not Starting!
I know many stations that founder because they wait to build the tallest thing possible to start. In many cases, I encourage groups to go ahead and put up a short pole now, and wait for the big tall tower until later. A very tall tower built from scratch can cost many tens of thousands of dollars and cause many zoning delays.
At Prometheus, we approach the task of getting the antenna high up like community organizers. If, like many of these stations, you have a small core group and not much money, a tower can be a very daunting task. We think it is best to put something quick up with the small resources that the few of you can muster, gain the support of the community and get everyone started doing their show, and then you will have a much larger group to work together to tackle the task of building the tower.
If you are clearing the rooftops of most other buildings around you by at least 20 feet, you will go farther than you expect. That may be enough to get you going and to pull your community in the doors, and you will be amazed by what they might come up with that can save you a lot of headaches!
A few words about limitations on antennas:
Antennas are deceptively simple. On the one hand, an antenna is just a hunk of metal. They are usually somewhere between two and fifteen feet long and two to 6 feet wide on the FM band. On the other hand, they are sort of complicated. They are picky about where they are placed, and what is near them. In radio, everything is different from how regular electricity works. For instance, if you wanted to plug in a lightbulb at the top of a pole, it would not matter if you had a hundred foot extension coiled up for fifty feet and laying in a pile at the basis of the pole. But with radio, that could totally ruin your signal. The elements of an antenna and the transmission line that feeds it need to have a very particular geometry in order to prevent reflections and radiate properly.
An FM antenna should be at least one wavelength (about 12 feet) away from any other obstruction. FCC rules require that a transmitter radiating a hundred watts ERP be at least 13.5 feet away from where anyone hangs out for more than a few minutes at a time.
Locating at a site that has other radio equipment (TV transmitters, cell phone stations, amateur radio equipment, etcetera) often requires a study by a radio engineer to make sure that your new 100 watt transmitter is not the "straw that broke the camel's back" which puts the site over the legal limit for radio radiation exposure to the public.
Another consideration is whether you will interfere with the other equipment, or the other equipment will interfere with you. The engineer who is the manger of the tower will probably be able to make a good educated guess at the potential for interference between the different equipment on the tower. Interference between equipment that operates on different bands is unlikely, but not impossible, especially if some equipment is at much greater power than others and the antennas or transmitters or feed lines are very close to each other.
Low power stations are allowed 100 watts ERP at 30 meters Height Above Average Terrain (HAAT). If your antennas are taller than 30 meters HAAT, then they will have to reduce their ERP to make up for it. The FCC will tell you on your construction permit if you have to do this. If you are shorter than 30 meters HAAT, you do not get to turn up your power to make up for it. You must build no more than 2 meters higher or 4 meters lower than what was stated in your construction permit. If you change it more than that, you must submit a “minor amendment.
What antenna are you going to use?
Probably 75% of low power radio stations will want to use a 150 watt transmitter and a pair of circularly polarized antennas. The antennas will need to be about 10 feet from each other, one directly above the other. The exact number depends upon the frequency at which you will be transmitting. Two circularly polarized antennas have a gain of just about exactly 1db, meaning that if you put a hundred watts of transmitter power (TPO) into them, a hundred watts of Effective Radiated Power (ERP) will come out of them. Low power stations are allowed 100 watts ERP at 30 meters Height Above Average Terrain (HAAT). If your antennas are taller than 30 meters HAAT, then they will have to reduce their ERP to make up for it. The FCC will tell you on your construction permit if you have to do this. If you are shorter than 30 meters HAAT, you do not get to turn up your power to make up for it. You must build no more than 2 meters higher or 4 meters lower than what was stated in your construction permit. If you change it more than that, you must submit a “minor amendment. “
Another common option is to use a single circularly polarized antenna. A single circularly polarized antenna has a gain of –3db, which means that if you put 100 watts TPO into it, it performs as __ watts ERP. That sounds bad, right? But actually it is good, because the ERP only counts the horizontally polarized part of the signal. The vertical part of the signal is not counted, so you "get it for free." so you are allowed to put about 250 watts TPO into a single circularly polarized antenna, and it is considered to be 100 watts ERP. But of course, the vertical signal is still going out and giving signal to your coverage area. Using one bay with 250 watts or two bays with a hundred watts is almost exactly the same in terms of coverage.
A 300 watt transmitter ($4500) plus a single antenna ($200) could cost you $4700). A 150 watt transmitter ($2500) plus a two bay antenna system ($700) would cost about $3200. And the monthly power bill will be about half the size for a 150 watt transmitter. But in some cases, if the property owner or the zoning commission will only allow you to put up one discrete antenna that is only a few feet in size, this single circular polarized antenna option might be better for you.
Typical circular polarized antennas weigh about 7 pounds a piece, and present a windload to the tower of about 8 pounds.
Antenna pole on top of tall building:
The best solution is to have a structure that is already tall, since making tall things from scratch is usually an expensive pain in the neck. Look around for church steeples, water towers, old ham radio towers, bridges, billboards…anything tall. The best solution is if you can put a fifteen or twenty foot pole on top of something that is already tall.
Ideally, the antenna should be about one wavelength (eleven or twelve feet) from any other object (except for the pole it is mounted to). There will be reflections even off of a slender pole, and radio energy will be dampened in the direction of the obstruction.
One very simple but sort of expensive solution is what's called a "non-penetrating roof mount". These mounts can support a ten foot pole, and are held down with cinder blocks. They are very heavy, so you might want to find a local supplier. They generally cost $100-$150, not including shipping.
Radio shack also has a nice selection of clamps and poles that can attach to chimneys, that can drill straight into the roof with bolts and roofing tar,
They do not always stock all these things, so it is best to buy or order a week or two before your project.
Cheap Telescoping Masts
The most common solution for radio pirates has been the 36 foot telescoping mast from radio shack. This is a decent solution for many low power stations that want to get on the air quickly.
You can count on spending a few hundred more dollars on turnbuckles, guy wires, hardware for mounting and so on.
If you drive through town, you will see dozens of these on your neighbors roofs holding little satellite dishes. Some towns require a permit for these, though I have built at least ten of them and never gotten a permit and never had a problem because they are so common.
Be sure to do a good job roof at the base, and make sure the roof underneath is solid. You may want to build a little platform of solid 2x stock for a few feet, bridging from rafter to rafter under the base of the mast or tower, and then roof over it before putting the mast on top. For mounting points to attach the guy wires, I have bolted 2 foot 2x 8s to the underlying rafters, screwed in a big screw eye, and then roofed over the whole thing except for the screw eye. I have also used clamps attaching to iron vent stacks, chimneys and other solid roof objects to attach to.
At the base. I usually extend the whole pole horizontally and lock it in place on the ground or roof surface, so you should make sure you have room and think about which way you want to lay it so you can control it. It is also possible to extend the pole one section at a time vertically in place while controlling it with guy wires, though I have found this more difficult and prefer tilting up.
A rule of thumb says that you need twice the height of the pole in distance from the base of the pole to any power lines. Survey the area carefully so that if the pole ever were to fall, it would not hit a power line!
Another rule is that you should have tie points for the guy wires in three equally spaced directions, at 60% of the total height of the pole. For a 36 foot pole, that would be about 20 feet. You can fudge this a little bit to fit on your roof, but not too much!
The instructions say that you are supposed to put a set of guy wires every ten feet. Maybe I am a bad person but I usually don’t really do this. I have generally found two sets of guy wires adequate on a 36 foot mast. Be careful about tightening down super hard, people have actually driven their poles through the roof by clamping down too hard with the turnbuckles! Everything should be tight, but don’t go crazy.
I have never done a 50 foot telescoping mast. Something that tall is more likely to attract attention than a 36 foot pole, which almost everyone has. With 50 feet, I would not cheat on the number of guy wires or the spacing. With a 50 foot pole on the roof, I think it would be much more likely that someone would notice, so you really might not be able to slip by without a building permit at that height.
"The chance of that is less than getting hit by lightning!”
What are the chances that your tower will get hit by lightning? Not bad, actually- remember that the chance of getting hit by lightning is sort of absurdly small when you are on the ground, but as you get to be the tallest thing for a few thousand feet around, your chances get much higher. I know two small stations that have gotten hit by lightning.
All antennas must be grounded. Usually the metal mast or tower acts as the ground, but if it is not planted firmly in the earth (as in it is mounted on top of a roof) you must run a big fat ground wire (at least __guage) down to the earh, and attach to an 8foot antenna grounding rod driven deep into the ground, with just enough sticking up to attach the thick wire to. The griound wire should be as short and direct as possible, and the bends should be as smooth and gradual as possible. Lightning will aways take the most direct path, and you want your groundwire to be that “path of least resistance” (yes, that is where the expression comes from). A further consideration is that the pole itself should be at least 3 feet taller than the top of the antenna that is mounted to it. This is necessary for lightning protection.
A very simple, common option for LPFMs is the radio shack 36 foot telescoping mast, available for about $60 from a local radio shack. With all the appropriate guy wires , turnbuckles and other hardware, installing this mast costs about$200 and can be done in an afternoon with 5 or 6 people helping out. They can be installed on the ground or better, on top of a building. To add stability, it is good to get a tripod or a heavy duty section of mast for the bottom. You will need guy wires, which will need to be anchored to the roof or the ground at a distance of at least 60% of the height of the pole, spread out as equidistantly as possible in usually three but occasionally 4 directions.
Easy for a town. Road or railway right of way Utility poles are made of wood, fiberglass, or steel. Each have their advantages. The general rule for poles of any sort is that they should be buried in a hole in the ground that is ten prevent of their length, plus two feet. Some poles have holes already built in to them for steps. Installing a utility pole generally requires a permit. Utility pole manufacturers are not accustomed to selling single poles, and shipping is a big pain in the neck, so it is best to go local. If you are in a small town, a friendly call to the highway or streets or electric department might find you a used one, or they could tack on one for you on their next order without too much problem.
There are a few approaches to water towers. If possible, the best thing to do is to mount a pole to the top. Often historical and aesthetic concerns preclude this option. The next best option is to buy 4 panel antennas, which would be located at equal intervals around the side of the tower. This is a fairly expensive, but effective solution. Another solution would be to mount circularly polarized antennas on the legs below the water tank. This will cause some distortion of the coverage pattern, but may not be so bad.
Building a tower can be quite a project. Finding a suitable site can be difficult. Your low power FM equipment will probably be fairly light and not subject to a lot of wind loading unless you are installing some sort of solid dish. So unless you plan to rent the tower space to others (which can be quite lucrative, but nay not be the business you thought you would find yourself in), you can use a fairly light duty ham radio tower. Rohn towers has a great catalog to give you a sense of what is out there.
You need to be able to get at the antenna to make occasional adjustments. It is cheaper to put up a fixed tower, but one that you can raise and lower may be a good investment if you have to work on it a lot.
Is it climbable? Towers that can support human weight up at the top are definitely a bit more complicated than your basic antenna pole.
Is the tower site accessible to a crane or a cherry picker?
Many counties across the country have moratoriums on the construction of towers, but there are few places that would prevent the erection of flagpoles.
In Maryland, the owner of the property where we wanted to build set our transmitter was not interested in having a big ugly tower built on his property, but was thrilled at the prospect that we would install an 80 foot marine flagpole with yardarms and a place to hang the American flag and the flags of the local football and baseball teams. Similarly, the county refused to even consider a tower, but let a flagpole of similar size through in just a few weeks.
Trees have been known to work. There are some stations that have installed antennas in the tops of trees. A few issues come into play. One is the proximity to the leaves of the tree. Antennas work best when they clear any other object by at least a wavelength or two, which in FM is 20 to 24 feet. Leaves, particularly, have a lot of water and a lot of reflective surface, and can significantly throw off the tuning of your antenna. If you use a tree, you will have to make an alternate arrangement for grounding, since the tree itself will not conduct electricity to ground like a metal mast would.
Renting tower space
Many wireless towers are owned by companies that would be happy to trade extra space on their tower for monthly rent payments.
Other options for stealth:
Because of the proliferation of cell phone towers and public concerns about radio radiation health effects, companies have in the past few years come up with a number of “stealth” antennas, which fit inside flagpoles, fake palm trees, look like giant cactuses, etcetera.
High gain for low power operation
If you are planning to build at a remote location far from available utility power, you might consider a high gain antenna, which puts out more “effective radiated power “ (ERP) than the actual Transmitter power output (TPO). A smaller amplifier can be used, allowing a relatively small solar and battery installation to power it. A good antenna for this sort of application is “The Comet” costing about $100 from companies like Progressive Concepts.