Masts and Towers|
A means of support for your antenna is required by all but the vertical-user, and is often thought of as being the major expense in an amateur contest station. A glance at the websites of some of the top contest stations will show fields full of huge rotating towers and the like; much money must have been spent here; but for the budget contester there are alternatives to be considered.
How high is enough ?
Given the choice, the sky would be the limit, but for practical purposes the height of your antenna depends on the frequency in use. If you want any kind of efficiency and low angle radiation, you should aim for at least a half wavelength above ground, i.e. 66 feet on 40m, 33 feet on 20m etc. For low angle radiation of 10 degs or less you need to be at least 1½ wavelengths high. Is it worth increasing an existing support tower ? According to K8JP, a general rule on height increases is that 50% increase is worth the increase. Less, don't worry about it doing it.
As can be seen, the lower the frequency, the higher up you need to go, so for those on a budget, a single band entry on the higher bands is a more sensible proposition. A 33 or 40 foot high support is not very difficult to acheive using readily available materials such as scaffold tubing or timber. A tree is often handy to most shacks and can be used at little cost. Some amateurs have mounted their antennas IN the tree - for instance a delta loop works well and seems little affected by summer foliage. If your tree is too small you could consider extending it - I increased the height of a pine tree in my garden by 10 feet using a 17 foot length of timber which brought the total height to 60 feet and made it an excellent support for my full wave 80m loop - a great antenna.
Mast or Tower ?
When, you might wonder, is a tower, i.e. a multi-leg structure, necessary and when will a mast, i.e. a single leg structure be sufficient. I am not an engineer so I can't give a theoretically qualifed response, but just from my own experience its suprising what can be acheived with a minimum of resources. I obtained a mast-base hinge; a device that has two sockets for a 2" pole set 90 deg apart and hinged to a flat base. A 20' scaffold tube went in each; one was the mast and the other the gin-pole. On top of the mast went the rotator, above this another pole to a rotating bearing. A 3el 20m beam on the top, a lot of guy ropes, and there you have it - a beam in the air for very little money. Its as fiddly as hell to erect and takes all morning to do it, but its cheap, strong, and it works. This is a rather extreme example, of course, and for someone requiring a single mast to support a wire antenna, life is a lot easier. Plan the job thoroughly, assemble sufficient helpers and you'll soon be on the air.
There are few resources available on the home construction of masts and towers, probably due to the worry of legal action in the case where someone who has followed your suggestions and since had an accident. Indeed, for legal reasons I must state that the ideas on this website are only meant AS ideas and should not be used as any form of construction guideline. But due to the cost of commercial towers, as well as the shipping charges, many amateurs have had to homebrew their own supports or enlist the services of a nearby engineering company. Here in Ireland, our own EI7BA, of homebrew quad fame, has some information about a simple tower design that he uses on his website, whilst over on the other side of the world, VK4VKD has a website describing his designs, and he even offers a video (PAL format) to help you along.
Value for Money.
The best value for money, when it comes to towers, has got to be the guyed tower, made from tower sections as required. The disadvantages are the real-estate required for layout of the guys, the difficulties of getting an antenna up the tower and past the guys, and when stacking is on the agenda stopping the guys clashing with the beams lower down the tower. However, the cash required and the visual impact of a self supporting tower over, say, 80' may convince many to go the guyed-tower route.
The guidelines when considering any form of home construction must always be based on safety. There have been too many cases of amateurs falling to their deaths from towers, or for poorly construsted towers collapsing for this to be ignored. Anyone making a crank-up lattice tower for themselves would have to be very careful indeed,as there are many areas where disaster could strike, but a simpler guyed or tiltover tower made from suitably strong materials should not be outside the realms of possibility for the competent home mechanic. If in doubt, ask your local engineering company to come up with something - it could still be cheaper than shipping in a ready-made one.
Steel has got to be the material of choice, as its strong, long lasting if protected, cheap to purchase and easy to weld. Secondhand steel is readily available - just visit your local scrapyard and see what they have on offer. Aluminium masts and towers are very popular due to weight and corrosion-resistance; the big drawback here is the price. Many amateurs have made wooden supports - a look through the handbooks should bring forth some useful ideas. OK, so it might not last 100 years, but then neither will you. At least a wooden mast is something that most people could attempt with few specialised tools. Some people suggest other mast material, such as fibre-glass fishing poles or PVC pipe. All of these will do as long as the design is a sensible one - the key point for the budget contester is to use what is available readily and cheaply, and to always be on the lookout for other possibilities coming your way such as a discarded utility pole or a job-lot of scaffolding. Don't forget the military-surplus stores, which often have masts and mast-sections on offer.
Guy Wire and Rope.
A variety of materials are available for guying your mast or tower, and the choice of which one to use, as well as the size of that material, must be made VERY carefully, as life and property could both be at risk.
For a temporary installation of a field-day mast, Polypropylene rope is one of the cheaper options, and can often be purchased for a reasonable sum at a Farmers Cooperative store or Discount Mart. However, as Dr. Barry L. Ornitz WA4VZQ points out, you should bear in mind that "..... as well as its tendency to stretch, Polypropylene rope has virtually no ultra-violet resistance, and in bright sun, you will begin to see degradation within months. It will generally fail within one to two years depending on load and local climatic conditions. For longevity, polyester (such as Dacron) rope should be used. Nylon is also suitable if you live in a area with little industrial activity, which produces acid smog. The best colors for UV resistance are bright white (loaded with titanium dioxide to reflect the light) or black (loaded with carbon black to absorb the ultraviolet)." There is more detail on this subject to be found in the 'HF Contesting>Tips and Techniques' section of this website.