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My J-Pole in a Tree

My 2-meter J-Pole Mounted in a Tree
Photos courtesy of Photos by Aliesha
  Some time ago a good friend of mine, KI8GM, gave me a couple of antennas and a radio.  This was before I got my license and he was tired of me procrastinating.  It worked, I purchased a study book and got my technicians license. One of the antennas was a 2-meter J-Pole which he had built himself from 1/2" copper pipe.  At the time I was in college and lived in a third story apartment.  I ended up storing the antenna quite a while and only operated mobile.

  J-Pole antennas get their name simply enough because they are in the shape of a J.  The short part or tail of the J (length A in the image to the right) is a 1/4 wave matching stub which feeds the longer part (length B) which is the radiator.  To find 1/4 wave in feet divide 234 by the frequency in MHz. Usually the coax center conductor is connected to the stub while the braid is connected to the radiator somewhere around points X and Y in the image.  Ideally the stub is separated from the radiator by a length of 22 / f(MHz) in feet which is represented by length C in the image.  Some jpole designs such as the rollup ones which are made from twinlead ignore this.  They are trading some efficiency for portability. The radiator (length C) is 705/f(MHz) feet long. Finally there is another section which extends down from the radiator and is shown in dark grey in the image.  Although this area is electrically connected to the rest of the antenna it is a high impedance at radio frequencies so the length of this part isn't critical it is only there to act as something to mount to.  It may even be in direct contact with a metal tower which makes this an easy antenna to mount.

  J-Pole antennas have a reputation of being hard to tune.  Partly this is because J-Poles are very sensitive to nearby metal objects.  They can be detuned easily.  One way to overcome this is to tune it in place where it is going to be used.  Obviously this is not always an easy thing to do, for example if it is to be mounted at the top of a tall tower.  Also, there is how a J-Pole is tuned.  A J-Pole may be tuned by changing the length of the stub or by changing the height at which the coax is connected.  Tuning a traditional wire antenna by changing it's length can be a little tricky. You can always shorten it but if you go to far you have to solder on more wire.  With an antenna made out of copper pipe this is all the more difficult because trimming it means cutting all the way through the pipe and if you go to far with this you probably will end up replacing that whole section of pipe. 

  Some people have worked around this by purposely making their stub a little short.  Then they drill a hole in the cap at the top and screw in a large machine screw.  They can adjust the height by screwing it in and out.  I thought about trying this but I was concerned about the screw moving after it was installed.  I was also concerned about weatherproofing both because this would mean drilling a hole in a top facing surface where rain would hit and because it would mean placing dissimilar metals in contact with one another.  Perhaps if I found a copper machine screw I could solder it in place once it was tuned thus solving both problems but I have never seen such a thing.

  The first time I installed the J-Pole was still while I was in college when I moved into a condo with a couple friends of mine.  I am pretty sure I was not supposed to have any kind of antennas there so I decided to hide the antenna in a tree.  We were on the ground floor and my room had one window facing the parking lot.  fortunately there was a large evergreen bush just outside my window.  I mounted my antenna in the bush.  This was not an ideal location, it had to be low in the bush or it would have been in direct sight of the people above us. It had to be on the condo-facing side of the bush or it would be easily seen from the parking lot. This meant my antenna was in direct contact with the bush in multiple places.  There also were big metal pipes and meters only a few feet away. I didn't know if this would work at all but it was all I had so I tried.

  My friend gave me the antenna without any wire attached so I chose to tune it by placing the wire.  I used a piece of sandpaper to make most of the length of the stub nice and shiny as well as the section of the radiator which was across from it.  First I attached the antenna to a couple of branches in the bush using 2 zip strips.  I used one to tie the bottom, mounting part of the j-pole to a branch.  That was probably ok.  I used the other to tie the radiator to a second branch.  It was probably not good to do this but I was working with what I had.  Next I used hose clamps to temporarily attach the wire to the antenna where I had previously sanded it.  I tried a few positions and checked my SWR meter. Once I was reasonably sure I had the best spots I soldered the wire in place.  I chose to solder my coax directly to the antenna.  This is not as easy as it sounds.  It takes a lot of heat to solder to such a large piece of copper because it acts as a heat sink, at the same time you must be care full not to burn the wire.  It took a couple of tries but finally I got the wire soldered on using a propane torch head connected to a small camping size propane tank which went with a grill I had. Some people solder a chassis mount UHF connector to their antennas.  Now that I am out of college and have more to work with I will probably do this on my next one.  That or an N-connector.  For waterproofing I used a roll of 3M insulating "putty".  It was really more of a really thick tape than a putty but once it's in place you can squeeze it and the layers kind of ooze together a little bit to form a solid mass.  I then covered this in electrical tape for good measure.  

  I used this setup for around a year or so before moving.  Given the poor installation job and location it actually worked surprisingly well!  My main goal of this project was to reach the Jackson repeater on the W8HVG/R link system which covers most of Michigan's lower peninsula. I was able to reach the repeater with a usable signal from the condo which was in Ypsilanti about 1/2 the time. Usually if I couldn't reach it I could at least receive it.  My radio was a 25Watt mobile which was limited to about 10Watts when I used it indoors due to the under-rated power supply which I had. After moving out of the condo I lived in a string of apartments and rental houses so the antenna went back into storage for quite a while.

 
Antenna mast mounted in tree using nylon straps
  My next installation of the antenna is what I really started this article to write about.  A couple of years ago I moved into my wife's old house from before we were married.  Here I decided to set up my radio equipment in the livingroom.  This was a mistake as it seemed like the TV was always on when I wanted to use my radio but that is another story.  The house was on a corner lot and my desk was on the ground floor against the wall facing the side-street.  I was considering getting an old TV tower for mounting my antenna on but the only convenient place to put one would have been on the opposite side of the house.  Also, I had never erected a TV tower, had other things to spend the money on, etc...  I decided to mount my antenna in a tree again.  This time however I was determined to do things a bit better than the last time and attempt to get a better signal.

  There was a nice tree outside almost directly lined up with my desk which I decided would be the best place for it.   I probably wasn't supposed to be doing this in the public right-of-way between the yard and the road but a quick glance at the abandoned houses to the north and to the east told me this was not a place anyone was likely to care.  I was also careful to place the antenna in such a way that it was unlikely to fall towards the street or the sidewalk, unless the whole tree went.  In which case my antenna was unlikely to be the problem.  There were some challenges involved however, first off it was across a sidewalk.  Had I been on the upper floor I would have probably opted to go straight across to the tree running my wire above the sidewalk about 25 feet in the air.  This unfortunately would have placed my ham shack in a bathroom or necessitated running a wire between the upper and lower floors.  Instead I decided to go under the sidewalk. 

  Getting my wire under the sidewalk sounded like an easy task to me.  Lifting a slab was not an option as this sidewalk was very old and made of natural slate much thicker than the cement ones which are usually poured today.  Instead, I decided to dig two holes directly across from each other on both sides of the sidewalk.  The side facing the house was on a hill so I chose to start at the side facing the road.  I dug a trench from that hole towards the road and placed a piece of PVC pipe in it which I found in our garage.  Then I drove the pipe through the dirt under the sidewalk and towards the other hole with a sledge hammer. The pipe was barely any longer than the sidewalk but it was late in the day and the hardware store was already closed.  I didn't want to wait for my next opportunity because it was getting late in the fall and I was afraid the ground might freeze. 

  I had thought this through a bit beforehand and I did realize that some dirt would pack into the pipe as I drove it through.  I thought about capping the end of the pipe to keep dirt out but thought that would be too blunt.  I looked for something pointy to cap the pipe with but came up with nothing.  The best I could think of was a well-head which would have been about perfect but I was not willing to spend the price of a well-head on this project.  I decided to just go ahead and drive the plain pipe through and hope that either I could get the dirt out or just pull out the pipe itself and run the wire through the hole it would leave behind. 


  That was far too much to hope for.  When I thought I was done the dirt was packed into the end tighter than if it had been dried cement.  The pipe was also lodged into place way to tight for me to even think about getting out.  My next idea was to take my grounding rod which I had not yet planted and try to use it to clean out the pipe.  I put the rod in the pipe and hit it a couple of times with the sledge hammer.  All that happened was that the end of the pipe where I was working on disappeared under the sidewalk.  At this point I new I was in danger of losing the tunnel I had started to make and stopped to think about what to do. 

  I decided to try cutting some of the pipe end which was clogged with dirt to see if I could lose the clogged part.  I knew that if I cut through dirt it would be very hard on the saw blade so I used an old one I didn't care much about.  I cut it as close to the sidewalk as I could but sure enough all I saw was more dirt inside the pipe.  Next I went back to the garage.  There was a second pipe about the same as the first.  I pushed it into the tunnel and butted it up against the first one.  Then I ran my grounding rod in again so that it went into both pipes. This way the new one couldn't slip around the side of the first.  I kept alternating between pounding the pipes further under the sidewalk and cutting a bit more off the end where they came out.  It took me about 5 tries but finally I got through the clogged part.  I even ended up putting a nick in the end of my grounding rod but I don't think that hurt anything.  Next I ran two wires through, one for the J-Pole and an extra so I could eventually add a second antenna.  The second pipe must have shifted off center from the first a bit or maybe some dirt got in.  It was a tricky fit but I did eventually get both wires through.  It was getting late so I put some plywood over the holes so no one would trip and called it a night.

  The next day I finished the job.  First, I needed to mount the antenna in the tree.  This time I did not want any radiating part of the antenna to come in contact with the tree.  To accomplish this I bought a 10-foot iron pipe which I believe was meant for home gas hookups.  I also got a reducer which screwed into the threads on one side of the pipe and soldered onto the bottom of the antenna.  This was my first time soldering copper pipe myself but it went pretty easy.  I just sanded the copper where it was to be soldered, applied a bit of resin and soldered it on using the same torch head I had used for connecting the antenna wire years earlier.  Still, remembering what pain it was to originally solder on the wire I chose to leave the old coax soldered on where it was.  I knew it was likely to need to be moved to fix the tuning in the new install but I thought I would try it out as it was before bothering with that. The ease with which that went together has inspired me to try constructing my own multi-band j-pole later this year.

  Instead of using zip-strips to attach the antenna itself to the tree like before I got some nylon straps and buckles which I used to strap the iron pipe to the tree.  I placed the bottom of the pipe in the main fork of a tree and leaned it against a second fork a bit higher up.  This way it actually rested in place quite stable even without the straps.  They were just for extra security.  Also, there wasn't any way for it to fall forward or back so cars in the street and pedestrians on the sidewalk would be safe.  The antenna itself stuck into a nice clear area where it was a few feet from any branches or leaves.  You might notice there is a small branch touching the antenna in the photos, this grew in place later.  I would have removed the twig but I was actually about to take the antenna down when my wife took the picture as we had already moved to a new house.

 
Lightning arrestor mounted in a PVC pipe
  The last piece of my antenna was the lightning arrestor and ground.  Even though J-Pole antennas are not supposed to require a ground I had read that they work much better with one.  I had also read that it is best to keep the ground connection close to the antenna.  Since my wire traveled underground to the house from the tree I figured it was unlikely (though possible) lightning would hit it anywhere but in the tree.  The lightning arrestor looked like a good way to introduce ground to the antenna so I decided to put it near the tree.  I didn't want to put it in the tree however because if lightning hit the tree I was afraid it would reach my coax on both sides of the arrestor.  I suppose the arrestor would send lightning to ground from either direction but if the coax broke in the process and lightning was hitting it from both sides... maybe that would be a problem. 

  A local hardware store, Janey's was going out of business.  It was sad to see them go but I went to their closing sale looking for something to mount my lightning arrestor in/on.  I ended up getting a great deal on a small electrical box with pipe openings on one end and the back.  I also picked up an elbow which fit it and a 10 foot piece of PVC pipe which also fit.  I didn't need anything so tall so I cut the PVC pipe in half. On one side I cut a couple angles on to make a point. Then I drove the pipe into the ground next to the tree.  Next I glued to the electrical box to the top of the pipe.  I also glued the elbow to the back so I could run wires in without rain being able to enter. I then dug my trench for the coax right up to the edge of the pipe.  After a couple of failed attempts to drill into the round pipe with a drill I used my torch to make a hole and ran the coax into it and up to the electrical box.  I had thought of drilling the hole beforehand which might have been easier with the drill but I didn' t know exactly how far I would be able to drive the pipe into the ground and didn't want to guess.

  Next I ran the wire which hung down the tree from the antenna into the elbow along with a couple ground wires.  The other end of the ground wires went to a rod I had pounded into the ground at the foot of the tree.  I ran two ground wires for a second lightning arrestor in case I added a second antenna but that never happened.  All I had to do now was solder on a couple connectors, attach the lightning arrestor and close up the box.  I also wrapped the arrestor in electrical tape for good measure just in case moisture did get in there.  Later I painted the PVC pipe and electrical box with some brown Krylon. 

  I was a bit afraid my antenna would be out of tune since I used the old coax and never retuned it in it's new environment.  Actually, this was not a problem.  Adding a ground gives the antenna a broader bandwidth.  I had a better SWR reading than before.  Besides just SWR reading performance was much better, at one point I even reached a repeater in Lansing, MI from our home in Toledo, OH with 10 Watts!  It was about this time that I read that J-Poles cut for 2-meters usually work well for 70cm.  I tried it and it worked great!  I also tried it with an antenna tuner and managed to get a decent SWR reading on 6-meter.  I didn't actually reach anyone to see if it was really getting out though and I didn't try this much as I was afraid such a setup might generate harmonics and cause TVI.  I tried 10-meter and 1.25-meter but I couldn't get the SWR down on those with my tuner.  I think at our new home I might go with a fan dipole for 10, 6m and build a new J-Pole which is actually made for 2, 1.25m and 70cm. Then I will have the old J-Pole for a full time APRS station or I could give it to another friend who is currently procrastinating in getting his license.
 


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