For this posting I will discuss a vertical design. This is the most difficult of the three to build, but it will fit in a very small area. I would not advise it for a new ham without another ham with antenna adjustment experience to help.
Because I had it, I used 27 feet of 2 inched, outside diameter, aluminum irrigation pipe. The large diameter metal pipe makes the antenna easier to tune. The antenna can be built with other sizes or even wire*, but this set of instructions is for 2 inch OD pipe. I would recommend using this size if at all possible.
Please be sure to read all the instructions before starting to build.
Use a 2 inch outside diameter pipe with a coil at the bottom for impedance matching and a tuning coil at the top with three 102 inch CB whips for the top-hat tuning capacitor (see drawing below). Use 2 inch schedule 40 PVC for the top and bottom coil forms. The bottom piece also serves as the antenna's insulator. Two inch ID PVC makes 2.375 coil form. I would not use a small coil form in this application. The irrigation pipe slides right inside PVC. Mine was a snug fit but it still needed help to stay in place.
COIL FORM PREPARATION
Cut off two 36 inch pieces of PVC for the bottom and top of the antenna. You will use 3, 1/2 inch #8 sheet metal screws the hold the PVC in place at both ends of the irrigation pipe; six screws all together. These screws must go through the PVC and the metal pipe. When you mount the PVC, it is best to pre-drill small "pilot" holes for these screws. Make sure the holes are small enough to assure that there is solid electrical contact between the metal pipe and the screws. Slide the PVC down over the pipe 3 inches and drilled the pilot holes about 1 inch up then slid it off again. If the PVC is a tight fit you may want to drill the pilot holes when you attach the the PVC permanently to the metal pipe. What ever you do, be sure to draw a line with a permanent marker from the PVC onto the pipe before you slide the PVC off so you can easily align the holes later. Two marks on the bottom and one on the top PVC so you can tell which end is which later.
MATCHING COIL CONSTRUCTION (Bottom end of the antenna)
The make 14 turns of #12 AWG spaced out over 18 inches on one piece of PVC for the bottom coil. (You will use bare wire here.) If you cannot find #12, #14 or #10 will be fine. (Smaller than #14 is not advisable.) Start the coil by making a half turn around a sheet metal screw the you place 18 inches up from one end. This screw holds the bottom end of the coil. Wind on the wire so that it ends about 1 inch below the other end of the PVC and tape it in place. The remaining 18 inches at the bottom to provides the bottom insulator for the antenna. Later the ground plane and the coax shield are attached to this same screw after the antenna is tuned. For not just attach the bottom end of the coil. Attach the PVC with the matching coil on it to the metal pipe as described above with three sheet metal screws.
TOP HAT AND TUNING COIL CONSTRUCTION (Top end of the antenna)
To mount the CB whips in the top end of the second piece of PVC, use 5 or 6 inches of the same metal pipe to slide inside. Do not slide the top of this metal pipe down past the top of the PVC. The should be even. Hold it in place with one sheet metal screw near the bottom end of this piece of metal pipe. Standard CB whips use 1/4" - 20 mounting studs on the bottom end. Drill three 1/4 inch holes equally spaced around the top end of PVC. These holes need to go through the PVC and metal pipe about 1/2 inches down from the end of the short piece of metal pipe. (I tried it with just large washers to support the whips and they pulled out of the PVC in just a couple of weeks.) Do not drill the holes for the whip more than 1/2 inch down or it will be next to impossible to tighten the nuts on the whips. This short piece of pipe will connect the CB whips and the top end of the the tuning coil together in the finished antenna as well as providing the necessary strength to support the CB whips. Do not attach the whips yet.
The tuning coil at the top of the antenna may the same wire as the matching coil at the bottom, but this wire must be insulated. Regular #12 solid wire that is used in house wiring works well. Now make 100 turns close wound along the PVC. Start the bottom of coil by connecting the wire to one of the 3 sheet metal screws that will hold the top piece of PVC to the metal pipe. Temporarily tape the top end of this coil in place because you will have to adjust the spacing to tune the antenna. Later you will place another sheet metal screw to hold the wire in place but not now because you do not know where it will have to be. Now make 5 more turns to reach the top of the PVC and tape it in place. Leave about 24 extra inches before cutting the wire. This extra wire may be needed during the tuning adjustment and will be trimmed off later after the antenna is tuned. You will have to strip off some insulation to go under the screw that holds the short piece of pipe inside the PVC. Loosen this screw and slide the wire under this screw. Tighten just enough to hold the wire in place and make electrical contact. Do not bend the wire around the screw! Let the extra 24 inches just hang out in the air. Do not wrap it around because it will change the tuning. Putting this wire under the screw connects the top end of the tuning coil to the CB whips.
Now attach the PVC to the metal pipe. Put a piece of tape around the bottom of the coil next to but not over the screw holding the wire. You will have to back off the screw holding the bottom of the tuning coil while you attach the PVC. Install all three screws at this time as described above.
TUNING IT UP
Now raise the antenna as near the center of the ground mat as you can. Please watch out for overhead electrical wires. If the antenna come in contact with them, it is deadly to anyone in contact with any part of the antenna or the coax or even your transceiver. Use guy ropes to hold the antenna vertical, but do it so you can easily lower and raise the antenna. Do not use metal guys for this antenna.
Prepare a wire to go from the bottom of the matching coil to the nearest point on the ground mat. Do not connect this wire to the coil at this time.
If it is not possible to place the antenna at the center of the mat, arrange things so the antenna can be easily connected to the ground mat with as short a connecting wire as possible. If it can only be connected at one corner of the mat so be it. It is best, however, to connect to the wire going along the center of the gap as near to the center of the mat as practical. Do not run the connecting wire over the mat. This will cause you problems. Connect it to the nearest point on the mat.
Connect the shield of your feed coax to the wire that connects to the ground mat. DO NOT connect the bottom coil right now. Now connect the center conductor of the coax to one of the sheet metal screws that hold the metal pipe and the PVC in place on the bottom end of the antenna. There must not be any connections to the bottom coil at this time. You are finally ready to adjust the tuning coil at the top of the antenna.
Connect your transceiver to the coax back in your shack. Be sure to use the same coax you plan to use when the antenna installation is completed and you are on the air. This should be 50 Ohm coax. Set your transmitter to its lowest power setting. If your transmitter does not have a SWR meter built in, put one in the line at the transmitter end with a short (2 to 5 foot) coaxial jumper cable.
Set the transmitter up 10 kHz from the bottom of the band and transmit long enough to read the SWR. In all likelihood it will be very high. Record the SWR. Move up 10 kHz, transmit, read and record the SWR again. Continue this until your last measurement is 10 kHz below the top of the band. The difference in the reading may be very small, but there should be some difference.
If you are lucky, the lowest reading will between the bottom and top of the band. If, however, the lowest reading is at the bottom of the band, the turns on the tuning coil, at the top of the antenna, are too close together. Lean the antenna over and try doubling the space between the top 20 turns or so. Re-tape and adjust the wire under the screw at the top end as needed to take up any slack. It is usually necessary to tape the coil in several places to keep the spacing in place. Remember to keep bare wire under the screw at the top end of the antenna. It may be necessary to strip off more insulation.
If the lowest SWR is at the top end of the band, loosen the screw and the tape at the top end of the coil and add a couple of turns to the coil. Re-tape and fasten the wire under the screw again keeping bare wire under the screw at the top end of the coil.
Once you make an adjustment to the coil, stand the antenna back up and make the SWR measurements again. Keep measuring and adjusting until the lowest SWR is in the part of the band where you intend to operate the most. You might not get the SWR lower than 10:1 or even higher. If you are very lucky you may get it to 4:1. This high SWR is to be expected. You will adjust the rest of the SWR out when you adjust the matching coil, but you must first get the antenna on frequency by adjusting the tuning coil for the lowest SWR.
NOTE: It may be that you will need to remove or add some turns from the coil but first keep trying by adjusting the spacing on the coil. Spacing the turns farther apart raised the antenna's frequency while adding turns and/or pushing them closer together lowers the frequency.
Once the lowest SWR is where you want it, secure the top of the coil where it transitions to the 5 widely spaced turns that reach the screw that connects to the CB whips with a sheet metal screw. The wire does not have to be stripped. Do this by placing a sheet metal screw right at that transitions from the closer wound lower part of the coil. The wire should only bend part way around the screw to stabilize the upper end of the close wound part of the coil. This has been held in place by the tape up till now. Tighten the screw down against the wire just enough to keep the wire from jumping out from under the screw. A washer may help to hold the wire here. Now make the 5 turns up the PVC to the screw that holds the short metal pipe that holds the CB whips. Take a turn most of the way around this screw and tighten it down. Trim off any excess wire.
Now "paint" the tuning coil with a thin coat Liquid Electrical Tape. Only paint the wire that is not covered with tape. Once it has completely dried, carefully removed this tape taking care not to move the wire and paint over the places that were under the tape. Once this coat has completely dried, paint the entire coil again. The goal here is to weather proof the close wound part of the coil. It is a good idea to paint over the wide spaced wire too to protect the insulation from UV from the sun, but the screws will hold it in place if you carefully take out the slack.
Raise the antenna once more and check that the SWR is still about where it was before you painted the coil, made the final connections, and trimmed the wire. It may change a little, but it should not have changed more than 25 kHz. If it has change more than that, the spacing on the coil has changed and it will need to be fix before you continue, so take great care not to move the wire while finalizing the top coil.
NOW THE ANTENNA IS FINALLY TUNED. You are ready to adjust out the SWR with the matching coil at the bottom.
First, disconnect the center conductor of the coax from the antenna. Second, connect the coax shield and the ground mat to the screw that holds the bottom of the matching coil. Third, make a short flexible insulated wire, 18 or so inches, with an alligator clip attached to one end. Solder the other end of this wire to the center conductor of the coax. Connect the top end of the matching coil to one of the screws holding the metal pipe and the PVC in place. Clip the alligator clip up about half way up the matching coil.
Back in the shack, tune your transmitter to the frequency where you measured the lowest SWR during the tuning coil adjustment. The measure and record the SWR. (Do not change the transmitter's frequency during this process.) Move the alligator clip up or down one turn at a time and measure to find the lowest SWR. Now you can move the alligator clip around that one turn in the coil to see if the SWR will go lower. Be sure not to wind the wire with the alligator clip more than 1/2 a turn around the coil and keep it as far away from the coil as reasonable. If moving the clip in either direction makes SWR worse, leave it in the original position. Don't move the clip more than 1/4 turn at a time. Remember you should not have to go more than 1/2 way around the coil in either direction during this process or you will be on a different turn. It is not likely that moving the clip will make enough change to worry about but in rare cases it will so it should be checked. You should be able to get the SWR down to around 2:1 or less. Yes a tuner will be necessary with this antenna especially if you want to be able to tune the entire band. Even if you get the SWR lower than 2:1 when you change the transmitter frequency very far the SWR will climb rapidly. It is the nature of antennas with tuning coils to behave that way. You may even find that your tuner will not adjust over the entire band.
Once the lowest SWR is found, you may want to remove the alligator clip and solder the wire in place so it does not get moved or knocked off. It is a good idea to paint the bottom coil with liquid tape so the coil will not move around. That would change the tuning of the antenna.
Your antenna is FINALLY ready to use.
By now you are wondering why I do not just give you the correct number of turns for everything. That is because the ground system you build, be it radials, a mat, or clipped onto a chain link fence (not recommended) will cause the coil adjustment to change drastically. You may have to change the number of turns on the tuning coil if adjusting the spacing still does not tune the antenna. I can only hope to get you in the ball park. The rest is up to you.
NOTE: The ground symbol in the illustration is used to indicate your ground plane. It is not a bad idea to also provide a ground stake at the base of the antenna. But a ground stake by itself will not be a sufficient signal ground. The antenna will probable tune up without a ground plane but its efficiency will be very low and the antenna will not perform satisfactorily.
- Liquid Electrical Tape is usually available at WalMart in the automotive section or at an electrical supply house.
- 2 inch aluminum tubing is available online in 8 foot lengths. It would be necessary to "couple" these together some how. A scrap yard may have something too. If you live where agriculture irrigation takes place, you might try ag pipe supplies. Mine came from a farmer who was changing to a different irrigation method. I bought it for scrap price. I do not recommend using smaller than 2 inch because the top hat (CB whips) places a lot of stress on the pipe especially while raising and lowering the antenna.
- Schedule 40, 2 inch PVC is available at most any home improvement store or hardware store. I recommend that you stick to this PVC because it has the strength needed and the coil winding info here is based on the OD of this type of PVC.
* - I would use as large of wire I could manage, not smaller than #10 AWG if you use wire instead of pipe.
** - Some of this type of fencing is galvanized, making soldering difficult. You may want to use split bolt connectors for this job. They are available in the electrical department of home improvement centers.
Next time I'll talk about a Marconi antenna for 160 meters. This requires a little more than an average backyard and has a few special considerations for erecting, but it is much easier to tune up and get on the air. Every antenna has it's compromises.