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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
For single side band it is really best to have a horizontally polarized antenna which this antenna is not. Using an antenna that is crossed polarized from other stations in your area results in about a 20 dB loss in signal strength. If the band is open and you are receiving signals that are bouncing off the ionosphere then polarization is not so important since the polarization tends to get mixed up. Another consideration is that the radiation angle for a quarter wave antenna is pretty high and will not be as good for DX signals as compared to an antenna the has a lower angle of radiation. At some point I will build something else that is better suited for sideband, but for now having an antenna that is vertically polarized is better than nothing on sideband.
Another consideration was the gain of the antenna. I wanted something that was small and relatively easy to build. I was not as concerned about putting out the most potent signal. Since this is a quarter wave antenna it is considered a unity gain antenna as compared to a dipole "0 dBd". Quarter wave antennas are pretty simple and do not require any matching circuitry when fed with standard 50 ohm coax.
A couple more considerations were materials and level of mechanical complexity. I wanted a design that was fairly simple but mechanically strong enough to stand up to the weather. I live in a relatively moderate climate so I did not have to worry about snow or ice loading. The design just had to stand up to rain and wind. I knew that I wanted to make the radials out of aluminum rod but other than that my design was open to whatever material I could find.
Mast Mounting Materials
I knew that I wanted the antenna mount to be fairly strong so I chose my materials with that in mind. At a local metal supply house I found a length of steel tubing and some U-bolts with brackets that would work great for the main mounting components. The steel tube which the antenna will be mounted to is 1-3/8" in diameter and 12" long. The U-bolts and brackets were originally made for some other type of antenna system and were being sold as surplus. Standard U-bolts would also work for this project but these surplus brackets were cheaper and came with additional brackets to center the mast sections. Also shown in this picture is a piece of 1/4" aluminum plate that will be used to bolt the mast to the antenna mounting tube. I already had the 1/4" plate material that I cut off of a larger piece of aluminum sheet.
Additional Mounting Materials
The square piece of metal in the first photo is the plate that the antenna radiating element will be mounted on. This piece is 1/8" thick steel that is 4" by 4" square. As you can see in the picture, I had already drilled a hole in the center of the plate to mount the feed point hardware in. The feed point hardware shown in the photo is standard hardware that can be purchased from any radio supply company. It accepts a 3/8" x 24 thread size antenna element on one side and is fitted with a standard SO-259 connector on the other side to plug in your PL-259 coax connector.
Notice that the feed point hardware shown in the first photo has two plastic insulators (one already on the bolt portion of the SO-239 and another separate one). These are used to keep the radiating element connections isolated from the rest of the grounding structure. The center (threaded brass) portion of the feed point hardware will attach to the radiating element while the body of the SO-239 will be in contact with the ground portion of the hardware.
The second photo shows a close up view of the mounting tube along with four right angle mounting brackets. The right angle brackets will be attached to the mounting tube and will eventually support the top plate. As you can see in the picture I have already drilled the four holes in the mounting tube where the right angle brackets will be attached.
The material used for the actual antenna radiator and ground radials is aluminum rod. I used 1/4" diameter rod for the ground radials and 3/8" aluminum rod for the radiating element. I chose 3/8" rod for the radiating element so that I could thread one end of it and then screw it into the standard feed point hardware that I had already purchased. The only other hardware needed were some clamp brackets to hold the ground radials in place. I decided to make the radial clamps out of 1/8" thick 1-1/2" aluminum that I already had on hand.
Antenna Construction Details
The first item to fabricate was the mast mounting plate. The first photo shows the raw aluminum plate that I used. In the second photo I am cutting the plate to size with a reciprocating saw. The third photo shows the mast plate after is has been cut to size (3-1/2" x 8-1/2"). Before moving on to the next step it is a good idea to use a file on the edges of the mast plate to remove any rough edges that could cut you. I generally just file all edges at a 45 degree angle.
The next step is to layout where the holes will be drilled to attach the U-bolts and brackets. In the first photo I am laying out lines to determine where the holes will be drilled. The second photo shows me center punching where the holes will be drilled. Center punching helps keep the drill bit in place when starting to drill a hole.
Once the hole locations are marked and punched they can be drilled as shown in the first photo. I used a small drill press to drill the holes but a hand drill could also be used. The drill press just makes the task a little easier. The second photo shows the completed mast plate with the mast tube installed to test the fit. There are a second set of holes, two of which can be seen in the photo. The second set of holes will be used for a second set of U-bots to attach to the mast.
There are a few more pieces that need holes drilled. The first is the mast tube. The holes are drilled to accept the right angle brackets that will secure the top plate in place. When marking the tube for the bracket holes I just made the marks at about every 90 degrees. I was not as careful as I should have been with making sure the holes were exactly at 90 degrees from each other. This made it a little more difficult when locating the holes on the top plate since they were not in the exact location I would have liked. The point is that you should take your time and do it right the first time rather than giving yourself additional problems later on. I was able to deal with the issue but it would have been better to be more careful. The second photo here shows the top plate after the holes were drilled where the mast tube brackets will be attached. These same holes will also be used to attach the radial clamps. Since I was not as careful in laying out the hole locations on the mast tube as I should have been there were slight differences. Due to these slight differences I marked the four locations with a center punch so that the appropriate radial clamp would be used in the correct location.
Next I made the clamps that would be used to hold the ground radials in position. They were cut from 1-1/2" wide aluminum and bent into shape using a vise. Bending the pieces into the shape as shown allows the pieces to hold on to the radial rods without the rods slipping out of position. As can be seen in the photo there are a few scuff marks on the pieces due to test fitting the parts prior to taking the picture.
Another thing I did when fabricating the radial clamps was to cut a notch that would keep each radial from rotating in the horizontal plane. How this is accomplished will be easier to see once you see the assembled top plate. As you can see I cut the notch using a file with fairly aggressive teeth. This made cutting the notch quicker. The first photo shows the notch being cut with the file and the second photo shows the completed cut. The square rod seen in the picture was only there to space the clamp away from the vise jaws so that the vise would not flatten the shape of the clamp. Remember the clamp was bent to hold on to the round radial stock.
These two pictures show the notch that has been made in one of the radial clamps and how the radial will fit into the notch to keep it from moving. Again this will be easier to understand once you have seen the entire assembled top plate.
In the first photo you can see the mast tube has been mounted to the mast. Note the four right angle brackets have been attached to the mast tube in preparation for mounting the top plate. Also you can see that I have painted the steel parts to help prevent rusting. The feed line is in place waiting for the feed point hardware to be attached. In the second photo you can see the radials have been attached to the top plate using the radial clamps. Please note how each radial fits into the adjacent clamp notch to keep the radial from rotating in the horizontal plane. There are other ways that this could have been accomplished but this method kept the number of parts to a minimum. The four bolts that hold the radial clamps in place also secure the top plate to the mast tube brackets.
One of the last steps is to cut the radiating element to size and thread one end of it so that it can be attached to the feed point hardware. In this photo the die and handle are still in place after cutting the threads on the rod. I used a 3/8" x 24 threads per inch die which is standard for this type of antenna mount.
As can be seen the radiating element has been installed in the feed point hardware using the 3/8" x 24 threads. All other components are now in place and the antenna is ready to be raised to test. Up to this point I have not talked about the length of the elements or the angle of the radials. I will talk about these two items next. Please notice that the ground radials have been bent so that they will be at a downward angle from horizontal.