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Cobalt RaQ3 | Gentoo APRS Ebuilds | Heathkit GR-54 | J-Pole in a tree | Milk Jug Dipole | Power Supply | Workbench

The Milk Jug Dipole

rf choke made from milk carton for dipole antenna
Photos courtesy of Photos by Aliesha
   Recently I built and installed a 20-meter dipole in my attic.  This was a fairly easy project and my next step towards operating on the worldwide HF bands.  I chose 20-meter because I asked my friend Carl, KI8GM which band I should start with and that is what he recommended.  This actually worked out pretty well as it just fits between two far corners of the attic.  I actually did have to slope the ends down a bit so I am not sure yet if it will perform like a dipole or more like an inverted V.  I aimed it SW and NE as I was hoping to reach Carl who is in New Mexico from my location in Ohio first and later to reach Europe off the other side.  So far I have received hams in Hawaii, Spain and Italy which makes me think it probably is performing as a dipole.  I do not have a transmitter yet so I have not really tuned it yet.  I just cut the ends to a bit more than textbook length and folded them back around my end insulators so that they actually are at textbook length.  Because my attic fits the antenna so tightly I didn't really have room for support lines so my insulators are attached directly to the wall.  I suspect this will be ok for now as I intend to start out with fairly low power levels.  Later If I use higher power I may have to come up with another solution for safety.

  A dipole, or inverted V antenna is probably one of the oldest, easiest and most common antennas a ham can build.  A dipole consists of a 1/2 wave (468 / fMhz in feet) wire cut in the middle to make two 1/4 wave wires.  It may be fed with either coax or open wire such as twinlead or ladderline.  The feed wire is connected at the center where it is cut. If it's coax the center goes to one wire and the shield to the other. If the feed is open wire then just connect one side to each wire.  A dipole is bi-directional, it will receive and radiate mostly broadside to the antenna.  It will not receive or radiate much in the directions of the ends. 

   An Inverted V is similar to a dipole however instead of running in a straight line it angles down by about 60 to 120 degrees from the center forming a point facing up where the feedline connects.  Unlike a dipole, an inverted V is omnidirectional.  Also, an inverted V will need to be somewhere around 5% shorter than a dipole for proper tuning.  I intended to build a dipole however due to space limitations my antenna may be more of an inverted V.  I haven't actually measured the angle mine is sloped at so I'm not 100% sure which it will be. I will find out once I have a transmitter as I will have to trim more wire from it when I go to tune it.

dipole vs inverted-v   I have read that if a dipole or inverted V is fed using open wire and an antenna tuner it makes a pretty good multiband antenna.  To get to my shack however, I have to run my antenna wire around several bends and also through a length of pipe which it will eventually have to share with other antenna wires.  Open wire needs to be kept away from other wires or any metal objects.  For this reason I chose to feed mine with coax.  A problem which hams sometimes run into when feeding such an antenna with coax is that energy can reflect back through the coax shield where it can radiate in ways which cancel out the waves from the antenna.  It can also get back into the shack where it can cause equipment to become RF hot, sometimes to the point of causing painful burns to the operator.  To prevent this one can either place a 1:1 isolation transformer at the feedpoint or a simple choke inline with the coax shield near the antenna.

   I chose to go with the choke.  The easiest way to do this is to make the choke a part of the feedline itself by simply wrapping around something.  Where the coax is wound the shield will act as an inductor but the center conductor will not because the shield isolates the center conductor in adjacent windings.  This kind of choke is often called a "Big Ugly Balun" even though technically it is not a balun.  You can find examples of the "Big Ugly Balun" all over the internet.  There are two things which are important when constructing an ugly balun.  First, the length of the coax which is wound should be around 20-25 feet long.  The width at which it is wound isn't critical so long as it's wide enough not to cause undo strain on the coax.  Too tight  a wind can cause the center conductor to migrate out towards the shield.  Second, the windings should be neat and not crossing over one another.  Usually one drills holes in the form around the first and last windings and uses zip strips or something similar to tie the end windings in place so that they cannot cross.

milk jug ugly balun   People have made their ugly baluns using all sorts of non metal objects as forms from old sports bottles to CD-ROM spindles and more.  The most common material is a piece of PVC pipe and then electrical tape is wrapped around the whole thing once it's finished for extra weather protection and to ensure it doesn't come undone.  If I was installing my antenna outdoors I would probably have used PVC pipe myself however since mine will be in the attic where there is no wind, rain or UV light to deal with I chose to use an old 1/2 gallon milk jug.

   Because the ugly balun gets mounted at the antenna's feedpoint it is convenient to make it a part of the antenna support structure with the radials mounted directly to it.  I also wanted mine to be a reusable part I could easily use in different antennas so I decided to use binding posts to mount the radials, another thing I couldn't have gotten away with outdoors.  I also decided to make mine with a separate piece of coax from the feedline and attached a chassis mount UHF connector to the bottom. The jug's handle also makes a convenient place attach a rope. I used it to tie mine to a rafter.

 One other thing to consider is that the connection points for the antenna wires should be at least a few inches apart.  I had some old scrap PC boards which I keep around for things like this. They were etched and cut for a project that a previous employee at the radio station I used to work at had designed.  No one knew the design so they were getting thrown out and I kept them for this kind of thing.  I just cut a piece about 5 inches long and mounted the binding posts near the ends.  I also drilled a hole in the center and bolted that to the milk jug's cap.  I found that the cap is pretty flexible and once everything was in place just the tension from the coax wanting to unwind was enough to deform it. I was concerned it might eventually pull the bolt through.  I solved this by cutting a second, round piece of PC board which just fits inside the cap using a hole saw.  Then I ran my bolt through both PC boards and the cap.  It looks much better now.

   I mounted the UHF connector on the bottom using both pop rivets and a bolt.  I had to cut away half of the bottom in order to get my hand in to hold things in place.  I didn't want to cut on all sides of the connector and weaken it too much but it would have been difficult to hold a nut in place on the far side of my connector from the hole.  I did manage to hold a washer in place so that the pop rivet would have something stronger than the plastic of the milk jug to expand into.  Still, the connector will be holding the weight of about 6 feet of feedline and I wasn't sure if the pop rivets would hold forever so I did use a bolt and nut in the easiest corner to reach.  Now I will be able to compare over time which holds better, the rivet or the bolt.

fan dipole layout   I haven't had the opportunity to test this antenna for transmitting yet but I am quite happy with it's performance for receiving.  I think I might construct a similar antenna for the 10 meter and 6 meter bands.  I will point this one north and south, partially to minimize interaction with the 20 meter antenna and also because south is where I have noticed the most 10 meter activity so far.  This means it will be crossing my attic the short way but I think it will fit as those band's wavelengths are much shorter. To get both of the bands on one antenna I will make it a Fan Dipole.  A Fan Dipole is basically constructed the same way except you add another pair of wires for each band. They all meet at the feed point.  The wires for the lowest frequency band (longest) go on top and the wires for subsequent bands go below it in order.

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