2 Meter & 70 Centimeter Mobile Halo Antenna Project
by Mike Fedler N6TWW


February 15th, 2009

"Updated February 18th, 2009"


Completed mobile loop antennas mounted on truck

Final version of mobile loop antennas as of 2/15/09
Antenna design as of 2/15/09

Please note that all of the small pictures shown in this article are backed up by larger images so that you can get a better look at the details if you so desire. Just click on any of the small pictures to look at the larger version. When done looking at the larger picture, just hit the "Back" button on your browser to return to the main article.

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 Goal
I have wanted to try 2 meter SSB mobile for some time but did not have a horizontally polarized antenna to try it out. Since I had success building a pair of 2 meter loops for my base station I thought it should be fairly easy to come up with something for mobile use.

Mobile loops prior to final enhancements As long as I was at it, I thought it would be a good idea to build a second loop for 70 centimeters. I set out with the goal to build the antennas out of materials I had laying around or materials that were easy to obtain from the local hardware store.

When starting the project I knew it was going to take some experimentation and some effort to work through the design and eventual problems that would crop up. For that reason I decided to take my time and work though the issues and tuning at a leisurely pace. This would allow time to experiment and try different things to get it right. Even at this point with the antenna system working and in a fairly robust state there are still parts of the design that need improvement. Since taking the picture to the right I have improved a couple of items that may not be noticed at first glance. I will explain in more detail later in this article.

Material Selection
With this project I wanted to try using aluminum rod for the radiating element instead of the copper tubing I used to build my base antennas. The reason for wanting to use aluminum rod were #1 improved strength, #2 less wind load due to a smaller diameter than the copper tubing, and #3 less oxidation.

Gamma tube material (Click for larger view)
Even though I used aluminum rod for the main antenna element on both loops, I ended up using 1/4 inch copper tubing (shown to the left) for the gamma match because I did not have a source for small aluminum tubing. You will notice in the picture that there is a piece of coax cable that is inserted into one of the pieces of copper tubing to show how the gamma match capacitor is constructed.

The next question was what to use for the mast and loop support? I found some pieces of fiberglass tubing that I had from an old camping tent that had since been discarded. The fiberglass tubing was about 5/16" in diameter. I figured the fiberglass tubing would make a pretty good selection for the loop cross brace. It could also be used as a temporary mast for the debug stage of the antenna build. Fiberglass rods & copper tube material (Click for larger view) The picture to the left shows a couple of cut off pieces of the fiberglass tubing from the discarded tent. They are the dark color lengths shown in the picture. Selecting this second-hand material was a major mistake as I found out later while trying to tune the antenna. As it turns out the fiberglass tubing from the tent turned out to be slightly conductive. I was unable to tune the antenna and the tuning results I did get were very confusing. After I realized what the problem was I ended up changing to white fiberglass rod purchased from the local TAP Plastics store. A piece of it is also shown in the picture.

Fiberglass and aluminum rod material (Click for larger view) The picture to the left shows a length of each type of rod used in the project. For the final build I used 1/2" white fiberglass rod for the mast. The cross braces for the loops are 5/16" white fiberglass rod and the antenna loops were made out of 1/4" aluminum rod.

For the 2 meter gamma match connector I used 1/4" aluminum bar stock. For the 70 centimeter gamma tube connector I used .030" aluminum sheet material. The aluminum sheet material was also used for the ground point connectors on both antennas.

Angle bracket purchased at Home Depot (Click for larger view) For the mast connection to the antennas as well as the base mount connection I used steel angle bracket that I purchased at Home Depot. These brackets had to be modified for their individual use. Using this bracket for the mast base connection turned out to be too weak. Original base bracket (Click for larger view) It started to bend due to the wind load when the vehicle was in motion. I strengthened the base mast bracket by adding metal rod from a coat hanger in an effort to correct the problem. The modification kept the bracket from bending back even when under load from the wind. The modified bracket is show in the image to the right. Even after adding the rod for strength I still was not satisfied because the bracket seemed to have too much movement from right to left.

Final base bracket material (Click for larger view) Completed base bracket attached to magnet mount (Click for larger view) After having problems with the original bracket selected for the base of the mast I decided to use something that was more robust. I found another piece of steel that I happened to have on hand. After some modification it would provide a much stronger base bracket. The raw material for the mast bracket is shown in the image to the left. The stronger completed mast bracket is shown in the image to the right. As you will notice it has been painted and attached to the magnet mount.

Completed 2 meter gamma clamp (Click for larger view) One of the last items to select was the material for the connection point between the antenna element and the gamma tube as well as the ground connection point. I ended up using two different materials. For the 2 meter gamma connection I used 1/4" square aluminum bar stock. This was similar to the material that I had previously used on my loops for my base station. The image on the left shows the completed clamp on the 2 meter loop. Flat aluminum sheet material (Click for larger view) For the 70 centimeter gamma tube connector as well as the ground point connections I use flat aluminum sheet material. See the image on the right. The sheet aluminum is about 0.030" in thickness. It is thin enough to be easy to work but strong enough to work for the intended use. I would not want to use aluminum that was any thinner due to strength issues.

2m loop cross rod bracket {Top view} (Click for larger view) The last material needed was the mast bracket material used to attach the fiberglass cross rods for the 2 meter loop. I ended up using a piece of sheet steel that I cut out of a discarded computer hardware unit. There was nothing special about the material other than it was thin enough to manipulate into shape and strong enough for the intended purpose.

Tools
The tools used for this project were as follows:

  • Hacksaw
  • Wire cutter
  • Knife
  • Flat metal file
  • Round metal file
  • Drill press along with drill bits (a hand drill would also work)
  • Phillips screwdriver
  • Adjustable wrench
  • 7/16" open end wrench
  • Heat gun
  • Tin snips
  • Hammer
  • Bench vice
  • Tape measure

Fabrication

(Bending the loop)
Bolts and plastic sleeves used for bending aluminum rod (Click for larger view) The first thing I wanted to do was to bend the antenna element. I knew that the aluminum rod that I had selected would be more difficult to bend into the correct shape than the soft copper tube I had previously used for my base antennas. Bolts with plastic sleeves installed for bending aluminum rod (Click for larger view) For that reason I needed a method to bend the rod other than just using my hands. I found a couple of lag bolts and cut a couple pieces of plastic tube to use as a bending tool. I used the bolts to bend the aluminum rod once they were clamped into a vice. The two pieces of plastic tube were used to allow the aluminum rod to slide easier past the bolts as I bent it. Once the two lengths of plastic tube were cut, they were installed on the bolts as can be seen in the picture to the right.

Before bending the aluminum rod for the antenna radiating element I cut it to the correct length. I then marked the center point for later connection to the ground clamp. Marking the center point before bending the rod into a circle is much easier than trying to do it after the rod is bent.

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. 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 purposes. First I convert 186000 miles to a number that will mean something to me. So I convert 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 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 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. This value is still a little longer than what will actually be needed. This allows for final antanna adjustment once everything is built.

Bolts and plastic sleeves clamped into vice (Click for larger view) Aluminum rod being bent for radiating element (Click for larger view) To bend the aluminum rod into a circular shape I clamped the two bolts along with the two pieces of plastic tube into my bench vice. I then was able to insert the rod between the bolts and slowly bend the rod into shape. The bending process should be done a little at a time over the entire length of the rod. As the rod is being bent try to keep from creating any sharp bends. Keep making small bends along the length of the rod until you end up with a loop that is almost a complete circle. It may help to use something as a circular pattern to judge your work against such as a 5 gallon bucket.

(Mast bracket for 2 meter loop)
2m loop cross rod bracket {Top view} (Click for larger view) Once the loop was bent into shape I cut two lengths of fiberglass rod to use as a connector between the radiating element and the mast bracket. I also had to make the mast bracket so that it could connect to the fiberglass rods. I made the attachment bracket out of two different pieces of metal. The first piece was the piece that held the fiberglass cross brace rods. I made this piece out of a discarded computer hardware bracket that I was able to cut up and bend into the shape needed. This piece of the bracket assembly can be seen in the picture on the left.

2m loop cross rod bracket {Bottom view} (Click for larger view) The second piece of the bracket was made so that it could be attached to the mast using U-bolts. This required four holes to be drilled in one leg of the angle bracket. The bracket is one of the brackets purchased from Home Depot. I drilled a hole in the other side of the bracket that would allow the mast to pass through it so that the antenna could be mounted at any point along the length of the mast. I also drilled holes in both bracket pieces so that they could be attached to each other. The picture on the right shows the completed bracket assembly as viewed from below.

(Attaching the loop to the support rods)
Split coax insulation used for attaching loop to fiberglass cross rods (Click for larger view) Loop attachment method prototype using split coax insulation & plastic ties (Click for larger view) Next was how to attach the aluminum loop to the fiberglass cross rods. I ended up using a piece of coax insulation that was split and then wrapped around the aluminum rod and then attached to the fiberglass cross supports using plastic ties. A picture of a piece of the split coax insulation material is shown in the picture to the left. The picture on the right side shows how the coax insulation was used to connect the radiating element to the fiberglass cross rods.

Prototype loop attachment method using split coax insulation & plastic ties on ground side of loop (Click for larger view)

The picture on the left shows how the loop was attached on the closed side of the loop. I used this method to attach the 2 meter loop to the fiberglass rods during the debug and tuning stage and for several weeks thereafter. I was not satisfied with the way it looked. Also the durability of the plastic ties was in question.

Even though the method of attaching the two meter loop was functional in that the materials used were insulators, I thought of a way to improve it. First I decided to use the same basic materials, but I modified how they would be used. The same coax insulation was used but this time it was attached to the loop with electrical tape rather than plastic ties. The tape was wrapped around the split coax insulation and then over the aluminum rod. The tape was wrapped so that is extended a small distance beyond the coax insulation. This would hold the aluminum rod in place. This assembly was then covered with heat shrink tubing to keep the weather from affecting the attachment. Below are pictures showing the steps involved. The image on the right shows the completed job.

Step 1 of final loop attachment method - installing shrink tube (Click for larger view) Step 2 of final loop attachment method - split coax insulation installed (Click for larger view) Step 3 of final loop attachment method - applying electrical tape (Click for larger view) Step 4 of final loop attachment method - heating shrink tube(Click for larger view) Final version of completed attachment of loop to fiberglass cross rod (Click for larger view)

(Loop ground point connection bracket)
Measureing the sheet materal for 1/2 The straps used to connect the ground point of each of the loop antennas was made out of flat sheet aluminum. A strip 1/2 inch wide was cut off of the sheet to fabricate the brackets. Cutting aluminum strip with tin snips (Click for larger view) The material used was fairly thin which made it easy to work with. When cutting the strip it will tend to curl. You will need to flatten the aluminum back out which you may be able to do by hand. If the material is too tough for you to flatten out by hand you may need to hammer it flat on an anvil.

Easing the edge on the metal stip with a file (Click for larger view)

Once you have flattened the strip of metal back out it is a good idea to ease the edges due to the sharp edges caused by cutting the material. I do this by just using a metal file as shown in the picture to the left.


The series of photos below shows how the strip of aluminum is shaped to form the ground connection bracket. First start by bending the strip around the aluminum rod to obtain the correct radius. Note: if the material is thicker, you may need to clamp the strip to the rod before bending. This may be needed to keep the bend radius the same as the rod it is being bent around.

The first picture shows the strip being bent around a piece of extra rod that was in excess of what was needed to make the 2 meter loop. It is a good idea to hold on to the scraps of material as you work your way through the project so that you have pieces for fabrication jigs if needed. The second picture shows a piece of scrap that will be used when crimping the bracket in the vice.

Step 1 of makeing a ground clamp - bending aluminum strip (Click for larger view) Showing scrap to be used in crimping of the ground strap (Click for larger view) Step 2 of makeing a ground clamp - crimping the stap (Click for larger view) Crimped stap view before shim is removed (Click for larger view) Crimped stap view after shim is removed (Click for larger view)

The third picture shows the bracket loop being formed around the aluminum rod in the vice. Note that the extra scrap of thicker material is being held between the two ends of the strip while it is being crimped. This will keep the loop from completing a circle around the rod thus providing some travel distance between the two ends when clamping it around the antenna rod. The fourth picture shows the completed bend around the rod while the spacer scrap is still in place between the two ends. The fifth picture shows the completed loop around the rod to show the profile of the completed clamp loop.

Ground clamp {edge view} (Click for larger view) Ground clamp prior to drilling with screw to be used in final assembly (Click for larger view) The next picture to the left shows the completed loop prior to drilling and trimming. The last picture in this series to he right shows the bracket with the screw that will be used to clamp it around the loop rod once it is finished.


Partial view of drill press (Click for larger view) Step 3 of makeing a ground clamp - center punching the drill point (Click for larger view) The next step in making the strap is to drill a hole through it so that it can be clamped around the antenna rod. The first picture on the left shows a portion of the drill press used to make the hole in the bracket. Prior to drilling the hole it is a good idea to mark the spot where the hole will be drilled as shown in the next picture.

Step 4 of makeing a ground clamp - drilling the hole in the strap (Click for larger view) Ground strap after the hole is drilled (Click for larger view) This next picture on the left shows the bracket right after the hole was drilled through it. Note that I used a pair of vice grips to hold the piece while drilling. This is needed in case the drill bit binds while drilling the hole. Since the piece is so small, if the drill bit was to bind up it would start to spin and could easily cut your fingers. The picture to the right shows the bracket after the hole has been drilled.

Step 5 of makeing a ground clamp - trimming the strap to size (Click for larger view) Step 6 of makeing a ground clamp - cleaning up the rough edges (Click for larger view) The last steps to complete the strap is to cut off the excess and file the edges. Cutting the extra length off of one of the legs is shown in the picture to the left. Filing down completed bracket is shown in the picture to the right. After this final step, the bracket is ready to install.

Completed ground clamp installed on the 70cm loop (Click for larger view) The picture on the left shows the ground clamp after installation. This picture is of the 70cm loop. Both the 2m and 70cm loops use the same type ground clamp.


(Gamma match connection brackets)
70cm gamma match connector bracket (Click for larger view) I used two different gamma bracket types for the two different loops. There really is no reason for two different types, I just made them that way. Either type could be used for both loops. For the 70cm loop I used a bracket that was fabricated out of sheet aluminum the same way as the ground point brackets were made. The same fabrication techniques can be used. The only difference between the ground point bracket is that the gamma bracket has two connection loops that are opposite one another with the screw point in the middle. See the picture to the left.

The 2m gamma bracket was made out of two pieces of 1/4 inch aluminum bar stock. 2m gamma match connector bracket (Click for larger view) A hole was drilled through both bars at the center to allow a screw to go through both pieces and squeeze them together. I then clamped the two pieces of bar stock togeter with a spacer between them. I then drilled between the two bars with a 1/4 in drill at the points where the gamma tube and antenna element would be located. This resulted in a semicircular cut out in each bar at the two connection points. A picture of how this all works is show to the right.

(Gamma match construction and mounting)
Gamma match materials (Click for larger view) Both the 2m and 70cm gamma match tubes were made using 1/4 inch copper tubing. The goal here is to build a variable capacitor that can be connected at a position on the radiating element to obtain a 50 ohm match. You are also trying to connect to the loop in such a way as to have the least amount of reactance as possible. This is done by adjusting the gamma capacitive reactance until it cancels out the inductive reactance. I have read that the amount of capacitance needed to accomplish this is about 7pf per meter of wavelength but have found that this value varies depending on antenna design.

Gamma match construction (Click for larger view) In making the gamma match you are making a capacitor. One plate of the capacitor is the copper tube and the other plate is the wire inside the tube. The dielectric material is the insulation between the wire and the inside of the copper tube. I found that it is best to find a wire that as closely matches the inner diameter of the copper tube as possible. This increases efficiency and also helps in tuning adjustments.

I found the RG58 coax fits nicely inside the copper tube. I connect to the coax shield rather than the center conductor. Gamma match tube with wire {end view} (Click for larger view) The picture to the right shows how tightly the coax fits into the copper tube. One thing to cautious about is shorts. The example in the picture to the right shows how a short could happen. The wire from the shield braid has a few loose wires that could come into contact with the inside of the copper tube and cause a short. This would probably not be catastrophic but would cause the SWR to go up. I addressed this problem by carefully trimming any stay wires and then working the insulation so that it slides out past the braid wire. So far I have not encountered any problems caused by a gamma tube short.

70cm gamma match connector bracket (Click for larger view) Once the gamma match components are assembled and adjusted they should be sealed from the elements to prevent water from entering the assembly. After the adjustments were made I wrapped electrical tape around the end of the copper tube closest to the feed point extending the tape onto the wire. This pretty much seals one end of the tube. The only problem if left like that is that the tape will eventually come loose. The way I prevent that is to install a sleeve of heat-shrink tubing over the tape. The other end of the tube also needs to be sealed from water. On this installation I used small vinyl caps. With this method I have not had any problems with detuning due to moisture.

Completed antennas (Click for larger view) Another thing I found was mounting the gamma tube above or below the loop kept the separation between the shield and the center conductor at the feed point to a minimum. I have mounted the gamma match on both the 2m and 70cm loops below the radiating element as can be seen in the picture to the right.

(Mast bracket for 70 centimeter loop)
70cm mast bracket attachment prototype using plastic ties (Click for larger view) The 70cm mast bracket was made from the same angle brackets purchased at Home Depot as shown at the beginning of this article. The top leg of the bracket was cut down in length since it was longer than desired. Holes were drilled on the long leg of the angle bracket for the U-bolts that attach the bracket to the mast. Four holes were also drilled on the top leg so that the plastic ties could be used to attach to the fiberglass cross rods. A picture of this original set up can be seen to the left.

Final 70cm mast bracket attachment after installation of metal clamp brackets (Click for larger view) To improve the method used to attach the 70cm loop to the mast bracket I made a couple of aluminum brackets as can be seen in the picture to the right. The same methods used to make the ground point bracket were used to make these brackets. I show how these brackets were made below.

In the three images below, the first picture on the left shows a strip of aluminum bent around one of the fiberglass rods. The second image in the middle shows me marking where the edge of the second bend will be made. The third image on the right shows the strip of aluminum after the second bend was made as I was checking it for fit.

Step 1 of makeing a mast cross rod strap - making the first bend (Click for larger view) Step 2 of makeing a mast cross rod bracket - Marking the strap in preparation for second bend (Click for larger view) Checking cross rod strap after step 3 where the second bend was made (Click for larger view)

In this next set of pictures I show how the bent strip of aluminum is crimped to fit as clamps around the fiberglass rods. The first picture on the left shows one of the brackets being crimped. Notice that I have used a piece of scrap material between the bracket so that there is still space left for the bracket to have a clamping action around the fiberglass rod once the crimping is completed. The second picture in the center shows one of the brackets after the crimping has been completed with the scrap piece of aluminum still in place. The third picture on the right shows the bracket that is almost complete. All that is needed is to drill the holes for the mounting screws to go through and file off any rough edges.

Step 4 of makeing a cross rod bracket - crimping one of the clamp loops (Click for larger view) View of cross rod bracket after crimping, prior to removing the shim (Click for larger view) View of cross rod bracket {edge view} (Click for larger view)

(Base mast bracket and magnet mount)

The following pictures have been shown previously in this article but I wanted to show them once more to clarify how the mast is connected to the magnet mount. Originally I made a base bracket out of one of the angle brackets purchased from Home Depot. The only modification that was needed was to drill four holes for the U-bolts to fit through and one hole to attach it to the magnet mount.

After a few days of driving I found that the mast was starting to bend back from the wind load. At that point I added the wire rods to keep it from bending. The rods are just a piece of coat hanger. The picture below on the left shows the original bracket once modified with the wire rods. The bracket was still a little weaker than I liked so I looked around for a studier solution. I found a heavier piece of steel that would work but it also needed to be modified. I cut it to shape using a hack saw. Once cut to shape and drilled to accept the U-bolts and magnet mount bolts I painted it to keep it from rusting. A piece of the raw steel used to make the stronger bracket can be seen in the center picture.

The last picture on the right shows the completed stronger bracket attached to the magnet mount ready for use. The magnet mount was purchased from a local radio shop.

Protoype mast base bracket (Click for larger view) Material selected to be used for final mast base bracket (Click for larger view) Completed base bracket attached to magnet mount (Click for larger view)

Tuning for resonance

A couple of things should be noted before we go into how the antenna is tuned. First, any change made in the design will have an effect. Sometimes the effect can be much more than you would expect. I have a saying that emphasizes this thinking. The saying is "Everything affects everything else". So keep this in mind as you go through the tuning process. You may move the location of a component or change how something is made which in turn could change how the antenna operates.

Some of the things I have done in the design of this antenna tend to minimize a couple of effects I have encountered. One is sealing the gamma match tube. This really helps to keep the operations of the antenna consistent in a wet environment. View of ferrite cores (Click for larger view) Another thing that I have found to be very important is to reduce the feed line radiation effect. When feeding the RF signal to the antenna it will also tend to feed back on the outside of the feed line. This will change the radiation pattern and will make the antenna more difficult to tune. The feed line radiation may not have an apparent effect inside the vehicle at the radio but it will have an effect on how the antenna radiates. You may end up with tuning results that don't make sense. In order to minimize this effect I have installed ferrite cores over the feed line near the antenna feed point. This tends to choke off the RF on the outside of the feed line before it has a chance to feed back down the feed line. The picture to the right shows two ferrite cores installed on the 2m feed line to control the feed line radiation effect.

Tuning for resonance is simple but takes time. I built the 2m antenna first and added the 70cm antenna later. Once one of the antennas was fully assembled I used an SWR meter to determine if it was close to resonance at the desired frequency and if it wasn't I determined if it was high or low in frequency. I adjusted the position of the gamma tube connector along the antenna loop for best match. I also adjusted the wire inside the gamma tube for best match. I did this several times and determined that the resonant frequency was too low. This indicated that the loop was too long. I trimmed the loop down a small amount and tried it again. After adjusting back a forth several times I was able to get the SWR down to below 1.1:1. After everything was adjusted and I was satisfied with the SWR readings, I locked the wire in the gamma tube in place by wrapping a few turns of electrical tape across the end of the tube and wire. I then added heat shrink tubing over it to keep it in place.

Conclusions

I have found that the antenna works very well. There is not much activity on 70 centimeters but there is a local group on 2 meters. I have talked to quite a few contacts with good reports. I have not yet driven in all weather conditions to know if there are any unforeseen problems. I have driven in the rain without any detuning problems seen. There are two things that may be issues. First is that the 70cm loop is attached to the fiberglass cross rods with heat shrink tubing and plastic ties. I would like to improve on this method. The other is that the mast which is 1/2 inch fiberglass rod does wobble slightly as you drive over bumps in the road. The movement is minimal and I don't think this will be a real problem, but it is too soon to be absolutely sure.


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Michael Fedler, 2009