For the sake of simplicity, only three antenna designs are going to be presented here. If you are a beginner, I suggest that you erect the dipole and worry about the other designs later. For the intermediate and advanced, by all means try out the Vertical and Loop! There are many books available on Antennas, which go into much greater depth, detail and variety.
Perhaps the easiest and most popular antenna is a Dipole. The dipole antenna is easily constructed, almost impossible to mess up and works well at almost any height above ground. For the beginner, this is the antenna to use. For your antenna to work well, you need to determine the frequency you are going to operate on. For example, we'll say 7445 Khz. To determine the length of wire our dipole antenna will need, we use the following formula: 468 divided by Frequency in Megahertz = Length in Feet. So, working the math, 468 / 7.445 = 62.86 Feet. Round that off and we come to 62 Feet 10 Inches. This is TOTAL length of the antenna. To make a dipole, cut two wires, each one 31 Feet 5 Inches long. See Figure 1.
Already I see the plethora of questions spinning around your head. You're probably wondering what kind of wire to use for the antenna. Small speaker wire can be quite serviceable and it's easily found. You will also need Three Egg insulators, One for the center and each end of the dipole. These are getting to be difficult to find but I think Radio Shack still has them. If you cannot find insulators anywhere, small, rectangular scraps of phenolic perfboard will work just as well (at low power levels). You are also going to need some string or high test fishing line to support the ends of your dipole. Also, electrical tape or silicon sealer to be used on the center connection of your dipole to keep moisture out of the coax. A soldering Iron is also a must have item for any station; a 100 watt iron should cover just about any job. They say a picture is worth a 1000 words, so I refer you to Figure 2 for a typical dipole installation.
While not absolutely necessary, a Balun is recommended. For Dipole antennas that are fed with Coax line, a 1 to 1 Balun is suggested. A Balun matches a BALanced Line (Our Dipole) to an UNbalanced Line (Our Coax). This makes for an even greater transfer of power from the feedline to the antenna and will also prevent the ground shield of your Coax from becoming a radiator of RF! Baluns are a complex and difficult subject to fathom but there are books out that explain the How To's better than I could. Just remember, A Balun is optional but is worth the trouble and $25 to install one.
Another question you might be asking How high should I try to get my dipole ?. My answer: as high as possible. If 10 feet is all you can manage, then that will have to do. The higher, the better. Dipoles typically have the most favorable radiation patterns when they are 1 wavelength above ground. In the case of our 40 Meter Dipole, that comes to a whopping 125 Feet! I think it's safe to say that 99.99% of all 40 Meter Dipoles erected DO NOT reach these lofty heights!
The last consideration you need to think about is that of antenna orientation. A dipole will radiate the majority of power in lobes that are perpendicular to the axis of the dipole. What this means is, if you run your dipole North to South, then the majority of your RF signal will be radiated in a East to West pattern. So depending on your geographical location of your transmitter and the location of your listeners will depend on how you orient your antenna. You may also find that there is only one or two possible ways to place your dipole on your property, don't sweat it. Just hang it up and see what happens!
That's it! By now you should have a good working knowledge of radio transmitting and all that's left is to put on a good show!
The final antenna presented here is the Full Wave Loop. These are a little more advanced than a dipole and will possibly exhibit some gain over one. They can also perform as verticals if fed properly. The only downside to using full wave loops is that they require more room and supports than a dipole or vertical. I refer you to Figure 4.
A loop will radiate the majority of it's power at right angles to the plane of the loop, similar to the radiation pattern of our dipole, so it's important to keep this in mind when orienting your loop. To figure the length of wire needed for our loop antenna, we must use another formula. Divide 1005 by Frequency in Megahertz will give you total length in feet. For example, if we wanted our loop to operate on 7415 Kilohertz we would 1005 / 7.415 = 135.53 or 135 Feet 6 Inches of wire would be required.
We also must use what is known as a Matching Section in order to feed this antenna with 50 Ohm coax. To make the matching section, a quarter wavelength of 75 Ohm coax is required. To figure out the length of the matching section, use the following formula: 246 Divided by Frequency in Megahertz Equals Length in Feet. Thusly, 246 / 7.415 = 33.17 Feet or 33 Feet 3 Inches. BUT, DON'T START CUTTING COAX JUST YET, We also need multiply our result by the Velocity Factor of the particular 75 Ohm coax we are using! Don't worry about the what's and how's of velocity factors in Coax cable. Right now all you need are the proper numbers to plug into the formula. The insulation used in Coax cable plays the biggest part in determining a cables velocity factor. Solid Polyethylene insulation has a V.F. of 0.66 and Foam Polyethylene has a V.F. of 0.80. Remember to ask when you purchase your Coax, they should be able to tell you this. RG-59/U is the recommended type and it has a V.F. of 0.66. So, to determine the final length of our matching section we Multiply 33.17 by 0.66 and come up with 21.9 Feet or 21 Feet 10 Inches. Now that you have all the math, building your very own loop should be no problem.