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 The U-bolts and brackets were originally made for some other type of antenna system and were being sold as surplus.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.

Antenna Materials 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. Element Length Calculation Up to this point I have not talked about the length of the elements. One of the reasons for this is that this antenna design can be used for many different bands depending on the length of the elements. This type of antenna is usually used for VHF and UHF frequencies but sometimes even lower band designs are made. I don't use the formulas normally found in books to calculate the length of the elements 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. 186000 miles per second is not exact but it is close enough for our use. 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 (982080000/52000000 = 18.89 feet). Since our antenna will be a 1/4 wave antenna we need to divide the wave length of 18.89 feet by 4. This gives us a length of 4.72 feet. We then to convert 4.72 feet to inches (4.72 feet X 12 inches = 56.6 inches). Since the propagation velocity of the signal traveling through a metal element is somewhat less than the speed of light through free space the length calculated will be a little longer that what is actually needed. Using this longer length is actually good since it allows some extra length in the radiating element that we can trim to tune the antenna. We will trim it to the exact length when we tune the antenna. The ground radials should be about 5% longer than the radiating element. Radiation Resistance - SWR We want to match the antenna to the impedance of our radio and feed line which happens to be 50 ohms. From previous experience I knew that the radials should be bent down on a 1/4 wave ground plane antenna to better match 50 ohms. I used an MFJ antenna analyzer to obtain my measurements. Just for the heck of it I started off with the ground radials un-bent so that they stuck out straight in the horizontal plane to see what the radiation resistance would be. First I needed to find the resonant frequency of the antenna. The resonant frequency is where the inductance and capacitance cancel each other and all that is left is the radiation resistance and some feed line resistance. I found the resonant frequency by sweeping the frequency until I found were the inductance and capacitance canceled each other. This happened to be about 50MHz. At that point the resistance read 25 ohms. If left in this condition the best SWR I could ever obtain would be 2:1. I knew that by bending the ground radials down I could raise the radiation resistance closer to 50 ohms. After bending the radials down and taking a second reading I found that the resonant frequency was about the same but the radiation resistance was about 47 ohms. This was much better. Since I bent the radials down without checking for an exact angle I did not have them all bent to the same angle. I checked to find which one was bent down the most. I measured the angle and bent the remaining three radials to the same angle (about 45 degrees). After bending them all to the same angle I took another reading and found that the radiation resistance was right where I wanted it at 50 ohms. This would result in a 1:1 SWR, but not on the frequency I wanted. Since the antenna was resonant at about 50MHz the SWR would be 1:1 at that frequency and rise in either direction away from that frequency. Next step would be to tune the antenna for the frequency I wanted. One other thing to note on bending the ground radials. When I bent them at the top plate I bent them in a slow curve (about a 2" radius) rather than a sharp angle. The reason for this was to keep from creating a weak point at the bend point. If I had bent the radials at a sharp angle, the radials would have the potential to break at that bend point in the future after being exposed to the stress of the weather (wind). Tuning For Resonance I have always found that it is better to have your initial antenna a little too long rather than a little too short. It is much easier to cut a little off of your antenna to tune it to the desired frequency rather than to add length to it. Since the resonant frequency of my antenna was low it meant that my antenna was a little too long. The difference in the calculated values of a 1/4 wave length antenna at 50MHz vs. 52Mhz is about 2.3 inches. When trimming an antenna for tuning you should trim less than the calculated difference to be on the safe side. Sometime there are other effects that make the calculated value too large and if you trim off the entire calculated amount you may have made the antenna too short. This will result in your antenna being resonant at a higher frequency than desired. As a rule of thumb I don't cut off anymore than about 1/2 the calculated value at any one time. This may mean that you have to trim more than once to get the frequency you want, but remember it is easier to make an antenna shorter if it is too long than to make it longer if it is too short. I ended up cutting the radiating element twice to get to a resonant frequency of 51.5MHz. This was close enough to my target frequency to feel satisfied. Conclusions This project was fairly easy to build and it was easy to tune to obtain a good SWR match. I was very pleased with how easy it was to obtain a flat SWR match by bending the radial to raise the radiation resistance. It is not a high gain antenna but will provide an antenna that works well locally. The design can be easily adapted to other frequencies. I would recommend this type of antenna project for the beginner as well as the experienced builder. I hope you have enjoyed reading along with me and good luck building your next antenna. 73, Mike - N6TWW Back to Mike's Radio Info page Back to Fedler.com home page © Michael Fedler, 2009