M2 Driven Element Rehab

Features and Benefits of this Project

  • Soldered copper construction
  • All soldered parts mechanically engaged
  • Ultra violet resistant Teflon insulators on DE
  • N connector cannot be torqued loose
  • Improved waterproofing

Rebuilding the Driven Elements on 432 M2 Yagis

After having a pair of 13 WL M2 432 MHz Yagis in the air for about ten years, I noticed that the return loss had degraded to 6 dB (about 3:1 VSWR) at the feed end of 120 feet of 7/8 inch foam coax. I decided to take the antennas down and find out the reason for the bad return loss.

The antennas basically looked in good condition mechanically without bent or broken elements. The black element insulators were also in good serviceable condition. That left the driven elements, which were carefully removed for analysis

Click on any image for a larger view.

Failure Analysis

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Looking at the driven elements one could see a fair degree of ultraviolet sunlight damage to the milky plastic insulators on the feed block. There were also significant radial cracks in the plastic bushings. These could hold water and possibly make the VSWR performance worse in wet weather. The type N connector on one of the center feed blocks was loose and could be easily turned by hand. Carefully holding the female type N connector with a wrench and removing the mating male connector let a few drops of water out of the connector. I allowed the coax pigtail to hang up overnight with the connectors pointing down and found another teaspoon of water on the tile floor. Clearly the connector seal points and been breached. More about this later.

The second driven element, while also suffering from checked plastic bushings, was dry at the connector interfaces.

The Decision to Rebuild

Before deciding to rebuild, I checked the availability of new driven element sections from M2. As of this writing, the folks at M2 informed me that replacement DEs are still available. Thus I had a worst case back up plan. I costed the materials required to rebuild the driven elements from McMaster and they came to less than $15.00 per antenna for all copper materials. I decided to use Teflon for the bushings as I have had very good experience with this material on antennas. I have had antennas outdoors in the sun and weather for decades and the Teflon parts look and work as good as when they were first erected. I also decided to eliminate the thread-in female type N connector in favor of a flanged UG58A/U female N connector with pressed-in center pin (this is important). This style of N connector is not susceptible to being torqued loose while working with the antennas. Since I decided to fabricate the entire DE out of copper, the aluminum to copper interface buried in the feed block was eliminated.

About the Rebuild

Before rebuilding, a few words are in order. This is NOT a step by step detailed description of the rebuild. Rather it consists of ways of proceeding that I found helpful and useful in completing the task. There are also a number of photos depicting the progress. You may elect to use a different method or approach.

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Undertaking this project requires a fair level of mechanical skills and experience, and access to machine shop tools. This project is not a good place for the uninitiated to go unless one is willing to accept the possibility of failure in the hope of gaining experience. Access to a small lathe and milling machine or combination tool will prove valuable. You will also need some useful hand tools and taps and bits. This project took two days to complete. The first block went slower due to the learning curve and set up times. Finally, patience and care are really part of success here. There is no reason to rush things in the shop. That leads to mistakes and the possibility of injury. 

Taking the Feed Block Apart and getting the Compound out

Disassembly is the first step. The machined screw-in aluminum plug on the bottom of the element was removed to expose the silicone compound inside the block. I tried to remove the compound by picking at it without success. Ultimately, a set of extended diagonal wire cutters (purple handled tool shown in the second photo) was used to sever the copper driven element conductors from the N and F connectors. The pair of 8/32 hex set screws that held the bushings in place were then removed.  One can now extract the plastic bushings. The N and F connectors are also removed and put on the side. Examine the bushing holes for burrs protruding out of the 8/32 set screw holes and remove them with a knife if necessary. Now take a 3 or 4 inch length of 1/2 inch diameter Teflon rod and press it through the bushing holes and right through the entire block. This will force most of the compound right through the block leaving some sheared off in the connector holes. This can now be easily removed. The compound is easily wiped off of the Teflon drive rod with a rag. The first center block had quite a few machining burrs and bits of tooling flash in it. Carefully remove these as it is not a good idea to have errant metallic stuff inside the feed block.

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In removing the connectors, I noticed that the “O” rings on the connectors do not engage the relief bore in the block before the connectors bottom out on the block. This was the case for the N connector as well as both F connectors on the leaking block. The photo on the left with the scale laying across the block depicts the condition where the “O” rings rest below the top of the block. One may drag the scale across the top of the block without ever contacting the “O” rings. The F connectors remained dry, but it was the N connector to block interface that leaked. Since the N connector was going to be replaced with a UG58A/U connector, this was not a problem. On the F connectors, the “O” ring relief was bored out to accept the next larger thickness “O” ring. It is worth noting that the second center feed block did not exhibit any problems of this kind. The “O” ring reliefs were proper and there were minimum examples of flash and burring.

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A 3/8-32 tap was now inserted in the hole formerly occupied by the N female connector. This provides a convenient way of indexing the part in a milling machine or drill press in preparation for the next operations. Remove the 3/8-32 tap and proceed with boring the hole to accept the UG58A/U connector, its”O” ring and 4 mounting screws. The mounting screw holes should be drilled to a depth of .225 inches with a #42 drill. (I drill one size over to make tapping easier). Start the tapping straight using the machine UN-POWERED to start the 4-40plug tap. Finish with a bottom tap. Be careful and use a thread cutting fluid and lubricant. To get the cutting fluids out, the blocks were soaked in Xylol over night. They were then air dried and then heated to remove all remaining traces of Xylol.  Do this outside with really good ventilation. Check fit all the pieces. 

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One can now machine and drill a pair of 3/4 inch long by 1/2 inch diameter Teflon bushings for each driven element. Notice the addition of an “O” ring on the Teflon bushings to assure the integrity of the bushing end seal. The 1/8 inch copper element rod will fit tight in the bushing when drilled with an 1/8 inch drill.

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Cut 2 lengths of 1/8 inch diameter copper rod for each DE being fabricated to the same size as the aluminum half element rod being replaced. Center bore the rod about .25 inch deep to accept #12 copper wire at one end.

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With a #43 (carbide circuit board drills are real handy here) bit, cross drill a piece of #12 wire about 2 inches long at the half way point. Open the cross drilled hole out with a #39 bit. This allows the F connector center pin to engage the DE wire and not just lay on it.

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Cut an appropriate length of 3/16 inch copper rod for the main part of the DE. Also make up a pair of end clamps for the DE. Again, use tapping fluid and finish the holes with a bottom tap.

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If you are rebuilding more than one antenna, mark one side of the block the same on each block to preserve the phasing. This is very important. Clean up the center pins of the F connectors and straighten them if necessary. Dry fit everything together and cut the #12 wire as necessary with a sharp pair of flush cut diagonals to fit. Carefully solder the #12 wire into the center drilled hole in the 1/8 inch DE piece. Make sure all “O” rings are installed and lubricated with Dow Corning #4 dielectric grease. Assemble and fit the F connector center pins through the cross drilled holes in the #12 wire. Also bend the #12 wire carefully to fit into the solder cup on the UG58A/U connector and solder all connections carefully. Check continuity across the half elements and balun coax. If you have access to a 4 terminal ohm meter, you should see a side to side resistance of 10 to 12 milliohms through the balun coax. On a conventional 3 1/2 digit meter, this will be close to zero. Remember true zero is only approached in super conductors!

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The 8/32 set-screws may now be installed in the center block to retain the DE bushings. Place a dab of DC4 in each of the set-screw holes and insert the screws and tighten snugly. Do not over tighten. The cavity in the center block may now be filled with DC4. Allow this to settle overnight, and add more DC4 if necessary. Re-insert the machined aluminum plug that was removed in the beginning and tighten snugly.

Assemble your antenna, tune and enjoy. The picture on the left shows the completed antenna with compound heat shrink used to waterproof all of the connectors

One final thing

The all copper DE is a bit heavier than the aluminum assembly it replaced, making the antenna a bit tail heavy. Not liking this, I took some 3/4 inch diameter stainless steel rod about 3 inches long and turned down about a half inch section at one end. This was then cross drilled and fit with an 8/32 screw at the front of the beam. It now balances, making it easier to handle in the air.