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Building this project requires the use of tools that are capable of serious injury to you. If you attempt to build this project or something similar be sure you wear safety glasses and use all necessary safety precautions. If you are not familiar with the use of the tools required, obtain assistance from someone who is familiar with their proper use.
Project Background & Goals
I looked around on the web and found a couple of different designs that helped me get started. After building two different versions of the loop I came up with something that worked. It was a copper loop antenna with a capacitive fed gamma match. I had to rebuild the gamma match several times to get something that worked. I liked the way the antenna worked so I built a second one and stacked them for some additional gain using a phasing harness with impedance matching. They seemed to work pretty well but the harness and antennas were pretty flimsy. I had to disassemble the system several times due to a house move and a home construction project. I found that each time I had to touch the set up, something would go wrong. That is why I decided to try to build something that was a little more reliable.
The antenna I show here is my third generation design. The goal for this next design was to build something that I could get a good SWR response. I also wanted a design that allowed for adjustment. And and third goal was easy construction using parts available from the local hardware supply. I wanted good mechanical reliability and a minimum of metal support components that might affect the response of the antenna. As a result of these goals I had to play around with the design until I found something that gave me a good SWR match. It took me a little longer, but now I have a design that I think will work well.
Mast Selection & Mounting Bracket
I decided to mount the antenna to a fiberglass mast that would eventually be mounted on top of a metal push-up mast. The fiberglass mast I selected is a 1 inch rod that is 72 inches long. I was able to find the rod at my local TAP Plastics store. The price was really more than I wanted to pay but I figured it would last a long time. The fiberglass mast I purchased is shown in the image to the left of this text. Click on the image to see a larger picture.
The next thing to consider was how to mount the fiberglass mast to the top of the metal push-up mast. I decided to make a plate with holes drilled in it to allow the bottom of the fiberglass mast to be bolted to one side of the plate and the top of the metal mast to be bolted to the other side of the plate. I used U-bolts to secure the masts to the metal plate, two U-bolts for the fiberglass mast and two for the metal mast. The fist picture below is a picture of the metal plate marked and ready to drill. The second picture is the plate after the holes have been drilled. The plate is aluminum that is 3/16" thick and 3" wide x 10-1/2" long.
You will notice three other items in the second picture besides the metal plate. There is a U-bolt that is one of four that will be used to secure the two masts to the plate. Also there is a PVC coupler section that is normally used for PVC plumbing. I cut one of these couplers into four semi circle sections to act as a spacing shims between the U-bolts and the fiberglass mast. I split the coupler length-wise with a hack saw and then cut each half section in half across the circumference to give me the four separate pieces. One thing to note is that the coupler has a ridge on the interior surface that you will need to avoid or you will end up having to file it off later. Of course you don't have to use the shim at all if you prefer not to. I wanted to use a shim so that the diameter of the fiberglass mast matched more precisely with the diameter of the U-bolts. I also wanted to avoid cutting into the fiberglass mast with the U-bolt when it was tightened down.
In this picture you can see the completed mast plate attached to bottom of the fiberglass mast. Notice the two PVC shims installed between mast and the U-bolts. One of the shims was seen in the previous picture prior to installation. Installation of the shims is not absolutely necessary but it is a nice touch that gives the mast a more secure fit to the U-bolts.
The first picture shows the copper tube that was used for the radiating element of the antenna. It is 3/8" outside diameter soft copper. The second picture shows the copper that was used to make the gamma match tube. The copper tube used for the gamma match is 1/4" outside diameter. Also shown in the picture is a completed cross boom, the completed gamma tube clamp, and two metal plates that came off the U-bolts. These plates were not needed for clamping the mast to the mast plate. However they will be used for clamping the antenna boom to the mast later in the assembly. A piece of the plastic tube that I used for the radiating element separator is also shown.
Antenna Construction Details
The first item that I will describe is the boom. This is used to secure the antenna to the mast. It also holds the two ends of the antenna loop apart. The boom is made out of schedule 40 PVC electrical conduit. This material is good because it is an insulator and is also made to be UV light resistant. It is also easy to work with. I used 1/2" conduit for the boom and 3/4" to beef up the center section of the boom where it will be bolted to the mast. 1/2" conduit will not normally fit into 3/4" conduit so I split the 3/4" conduit down one side by cutting it with a hack saw. This allowed me to force the 1/2" conduit into the split 3/4" conduit. You can see the widened split in the 3/4" section in the enlarged picture. The boom is 14-1/2" long and the center 3/4" section is 4" long. To install the split 3/4" conduit over the 1/2" conduit I drove the 1/2" boom into the 3/4" section with a rubber mallet until the 3/4" section was on the 1/2" section. Then I used a deep socket wrench socket that was lager enough to fit over the 1/2" section but not large enough to fit over the 3/4" section to further drive the 3/4" section toward the center of the boom. I finished driving the 3/4" section to the center of the boom by placing the 1/2" section in a vice that was not tightened down on 1/2" section but was closed enough so that it would not allow the 3/4" section to pass by the vice jaws. That way I was able to complete driving the 3/4" section to the middle of the boom by tapping on the end of the 1/2" boom with the rubber mallet. Once that was done I drilled the necessary holes. Two in the center for the U-bolt to go through and one at each end of the boom that are larger. One hole for the radiating element and one slightly larger hole on the other end for the spreader. The spreader is a piece of PVC that I happened to have on hand. It had an inner diameter that was large enough so that the 3/8" copper radiating element could be inserted into it. The spreader is 2" long and the radiating element is inserted 1/4" into it so that the two ends of the copper loop are separated by 1-1/2".
The next item is the gamma clamp which is also shown in the picture. This is used to clamp the gamma tube to the radiating element as well as have the ability to be moved. The ability to move the clamp allows you to to move the feed point on the radiating element as well as vary the capacitance of the gamma tube. The clamp was made out of 5/16" aluminum square stock that I just happened to already have. A hole is drilled through the center of each piece to allow a screw to be used to clamp the two bars together. I also drilled two different diameter arcs near the end of each section. I used two different drill sizes to match the radius of the two different copper tube diameters. The arcs are not complete semi-circles because I wanted the two bars to be able to clamp to the copper tubes without touching each other. That way I could ensure a tight fit once the clamp is tightened down. The way I accomplished this was to put a spacer between the two bars of metal and clamp them together prior to drilling. I then drilled a hole in the center of the spacer which left an arc cut into each metal clamp bar.
The picture on the left shows the copper tube for the radiating element being measured in preparation for cutting. The radiating element is a single piece of copper tube. I calculated the length of the element by calculating what the length should be at 1/2 wave length at 144.250MHz. I don't use the formulas normally found in books because I tend to forget the exact number used. I just calculate it using the speed of light as a starting point. The speed of light is approximately 186000 miles per second in free space. Yes I know that 186000 miles per second is not exact but it is close enough for our purposes. I want to convert 186000 miles to a number that will mean something to me. So I converted the distance to feet. There are 5280 feet per mile so (186000 miles X 5280 feet = 982080000 feet). We now need to divide the number of feet traveled in one second by the target frequency (982080000/144250000 = 6.8 feet). Then to convert 6.8 feet to inches (6.8 feet X 12 inches - 81.6 inches). 81.6 inches is how long one wave length is at 144.250MHz. Since the propagation velocity of the signal traveling through the copper element is somewhat less than the speed of light through free space I used 98% as a multiplying factor. So (81.6" X .98 = 79.97"). I just rounded 79.97" off to 80". The loop is 1/2 wave length so I divided 80"/ 2 to arrive at 40" for the final length of the copper radiating element. I measured the tube buy first carefully straightening out the tube then measuring 40" as shown in the picture. Please note that you should mark the center point for the ground connection point at this step of the process. That is what is shown in the second picture on the right.
After measuring the center point and marking it, I cut the copper to size. You will notice that the copper is being held in a vice, but please note that this has to be done carefully! The vise shown in the picture has rubber covers over the metal vise jaws to help protect the surface of the soft copper. Also the vise is clamped on to the copper very lightly, just enough to hold the copper in place while the cut is being made with the hack saw. If you clamp down too tightly the copper tube will be crushed. The second picture shows the end of the copper tube after it was cut. Notice how rough the end of the tube looks. Before proceeding I smoothed off the ends of the tube with a file. I also used a knife to take the burred edges off the inside edge of the tube.
The next step is to carefully bend the tube back into a circular shape as seen in the left picture. It is easier to measure the tube when it is straight. If you try to measure it while it is still in a circular shape right from the roll it will be very difficult to measure because the inner and outer diameter will measure two different values. I found it was more reliable to straighten out the tube, make the measurement, cut it and then bend it back into a circle. Please note that you need to be careful while bending the tube because you could put a kink in it very easily. When bending the tube, do it in small steps and move your bend locations along the length. To get it close to the right shape and size use something that is about the right size as a form to bend the tube around. I found a five gallon plastic bucket worked pretty well as a guide. The picture on the right shows the boom being installed on the radiating element. Also note that I have slipped two pieces of plastic yellow shrink tube on each end of the copper tube. This is a refinement of my assembly process. The shrink tube will be installed in one of the next steps.
The next step is to attach the split end of the copper radiating loop to the spreader tube. Before doing this I measured about 1/4" in on each end of the radiating loop and marked that location. That way I would know where to position the copper element relative to the spreader tube. What you can't see in this first picture is the yellow heat shrink tube that has been slid around to the far side of the loop, just beyond view. After both sides of the loop were attached using electrical tape the heat shrink tube was slide back around and over the spreader tube. This is shown in the center picture. I used 1/2" heat shrink tube that was cut to 1-1/2" long. Once the heat shrink was positioned correctly I heated it up with a heat gun to shrink it into place. This process is shown in the third picture. The heat shrink will now cover the tape joint which should keep out any moisture when it rains. The heat shrink should also protect the tape from coming off due to exposure to the weather.
Now you can see the antenna loop after the spreader assembly has been completed. Also shown in this picture are the gamma tube clamp and the shim that will be used between the U-bolt and the mast. Notice how the heat shrink tube is now formed over the spreader and conforms to the smaller diameter of the copper tube as well. The second picture shows the loop after the gamma clamp and tube have been attached. Also the U-bolt was reversed to match the other loop that will eventually become part of the stacked array.
The next step is to prepare the ground connection point. In the first picture you can see the tip of my Weller soldering gun positioned through the end of the boom. In this step I heat up the ground point of the loop and deposit some solder on the copper tube at that point. In order for the solder to attach to the copper the soldering point must be hot enough for the solder to flow on to the copper. Again, this point is at the exact center of the loop, opposite of the split end. The second step to this process is to solder the ground wire to the loop. I also prepare the wire by first tinning it with a little solder so that all I would have to do is to heat up the connection to reflow the solder. This process can be seen in the second picture. You will notice that I am holding the copper wire with a pair of pliers since the wire will become too hot to hold.
This picture shows the completed antenna from the top. The main reason to show you this is so that you can view the entire radiating element. This picture was taken before I refined the assembly process by adding the heat shrink tube over the electrical tape. In the final version I have also removed the tape on either side of the ground point on the copper loop. You can see the tape in the picture on the left side of the loop.
This picture shows the gamma tube attached to the radiating element through the clamping bars. The gamma tube is nothing more than a capacitor formed from a piece of 1/4" copper tube and an insulated wire. The capacitor is made in such a way that you can clamp on to one plate of the capacitor (which is the copper tube) and attach it to radiating element of the antenna. The length of the gamma tube for my construction method is 2-1/2" long. The wire going into the tube is a piece of coax from a car radio antenna. The only reason I chose this type of wire is that it was the right diameter to fit in the gamma tube and I happened to have a piece laying around my shop. The shield in the antenna wire acts as one plate of the capacitor while the copper gamma tube acts as the other plate. The plastic jacket on the coax acts as the capacitor dielectric. The fit of the coax cable that I chose was fairly tight in the gamma tube. Just to make sure that the wire didn't move I put a couple of wraps of electrical tape at the end of the tube to hold the wire in place. The final version will have a piece of heat shrink tube over the intersection of the copper tube and wire. The length of your gamma tube will vary depending on how it is constructed. I have read that the average capacitance for gamma match is about 7pf or less per meter of wave length. In the case of two meters that would mean that the capacitance should be about 14pf or less. Mine turned out to be a little over 11pf when adjusted to resonance.
There are several things you should notice in this next picture. First is the gamma match that we were just talking about. Notice that the center conductor of the feed line has been separated from the shield about 2-1/2". This allows the center conductor to reach over and be soldered to the gamma coax. It also allows the ground shield to be directed toward the ground connection point wire. At this point there are a couple of things that I found out through experimentation that you should keep in mind. #1 it is best to keep the distance that you have to strip back the shield to a minimum in order to maintain the characteristic impedance of the feed line as close as possible to the actual connection points. #2 it is best to have the unshielded feed line approach the gamma feed point as close to a perpendicular to the tangent angle of the radiating element as possible. This keeps interactions between the unshielded feed line to the radiating element to a minimum. You will notice that the angle I have used is a compromise between the angle and the minimum unshielded distance. #3 don't try to avoid separating the center conductor from the shield by tying the ground wire further back on the feed line shield to allow the shielded feed line to go all the way to the gamma match coax feed point. If you tie the ground wire to a point further back on the feed line shield buy striping away the feed line jacket and soldering the ground wire to the shield, you will end up with a large impedance miss-match in the feed line at the solder point that will prevent you from ever obtaining a good SWR match.
Other things to notice about the feed line. #1 notice that the shield on the feed line is soldered to a ground wire that connects to the loop. This allows some flexibility provided by the braided shield so that the gamma coax can be moved in and out of the gamma tube for adjustment. It also allows for easy rework of the ground connection if needed. The ground wire only has to be soldered to the loop one time. You can still unsolder the feed line from the ground wire if needed. Soldering anything to the loop requires a lot of heat, so if you can minimize the number of times needed to do this you are better off. This method requires that you only have to solder to the radiating element once. #2 notice that I used several toroid cores on the feed line. You can see them resting against the mast. This was done to enhance the operation of the antenna system by preventing the signal radiating from the antenna from flowing back down the shield of the feed line.
In the picture you can also see the metal plate on the center section of the boom. This metal plate came with the U-bolt and is there just to enhance the strength of the boom when the U-bolt is tightened down. There is also a metal plate on the other side of the boom. I had extra metal plates from the U-bolts used to attach the fiberglass mast to the mast mounting plate.
This picture shows the ground point on the radiating element. The ground point is at the exact center of the copper loop. I used a Weller soldering gun to make this solder joint (shown in a earlier step). You can see that I have wrapped several layers of electrical tape around the copper on either side of the boom to hold the radiating element in position. Since I have connected the ground wire to the loop by feeding it through a small hole on the end of the boom, the tape is no longer needed and can be removed. You also can see other construction details from a different angle in this picture.
This picture shows the other side of the antenna. This shows the spreader. Again, this picture was taken before I refined the assembly process by adding heat shrink tube over the electrical tape. The copper radiating element only inserts into the spreader by about 1/4" on either side as described earlier. This keeps the two ends of the loop from touching and also secures the open end of the loop to the boom. The space between the two ends of the loop are separated by about 1-1/2". Again I have used electrical tape to secure everything in place. You may want to use another method such as epoxy cement or some other means. I chose electrical tape covered with heat shrink tube. It is a good idea to seal the open ends of the loop to keep water out. When it rains water may collect inside the radiating element and adversely affect the antenna performance.
Here are a couple more views of the antenna before heat shrink tube was added to the assembly process. The first photo shows the mast clamp from the other side. Again notice the metal plate removed from one to the U-bolts used to clamp the mast to the mast plate. The second picture just shows what you have already seen from a different angle. You may notice that the U-bolt attaching the boom to the mast is rather long. I may end up cutting off the extra length before final installation if I feel energetic.
During my experimentation I had a longer gamma tube. Since it was longer I wanted a second non-conductive clamp to secure it at the opposite end from the clamping bars. This picture shows the clamp I made from a piece of the 1/2" PVC. I don't know if I will use this additional clamp or not but I thought I should show it for additional reference information.
These two pictures show the SWR calibration and measurement of the antenna. The first picture on the left shows the meter once it was calibrated to the power level of the transmitter. I used the low power setting on the radio. The power level being transmitted can be seen just below the two meter frequency readout on the left side of the display. I used 144.240 MHz rather than 144.250 MHz as my tune frequency because 144.250 MHz is sometimes used as a net frequency in my area and I did not want to interfere with possible activity. Also note that this radio is an FM only radio while the intended use of the antenna is for SSB. Notice that the needle on the meter is fully deflected to the "CAL" position on the right side of the meter and the center switch on the meter is set to the "CAL" position.
The second picture on the right shows the actual SWR measurement. Notice that the center switch is now set to the "SWR" position. The radio is transmitting as seen on the power level display below the frequency readout. Once the antenna was in it's final adjusted state the SWR measurement barely moved off of the 1:1 measurement position.
My intention is to build a second antenna to be used in a stacked loop configuration. Adding a second antenna will require the building of a phasing harness to split the signal between the two antennas.
© Michael Fedler, 2007