05 September 2016

Constructing a Linked Dipole

One project I'm currently working on is to build a "Linked Dipole".  This antenna is just a simple wire dipole, but it has breaks along the wire that can be either connected or disconnected, allowing the antenna to be tuned for one of several different bands.  I expect this will be a useful antenna for portable operations; in fact we used my friend NU7Y's linked dipole very successfully on my first SOTA activation.

The SOTA website has an excellent calculator app that I used to determine approximate lengths of wire for each side.  My goal was to build a dipole that will cover the 15, 17, 20, 30, and 40 meter bands.  After putting in my parameters, here is what the app recommended for my design (note this is only one side, the dipole requires two for each side).

I used some 14-gauge "THHN" style wire that I obtained from the local Home Depot.  The coating on THHN wire does have some effect, but I was willing to sacrifice a little perfection in exchange for the low cost of this wire and for my first attempt at this particular antenna design.  I cut each length approximately 10cm longer than suggested, with the plan to trim them down to obtain optimal SWR (it's much easier to trim shorter than extend longer!)

For easy connect/disconnect, I decided to try using Anderson Powerpoles.  One side uses all red, and the other side uses all black - that helped me keep things straight during the building process.  I used a label maker to indicate which band each wire section was cut for.  I began by placing a powerpole on only one side of the wire; the other side was left bare until I had trimmed it to the correct length, after which the second powerpole was crimped on.  The powerpoles do work, but if I build another linked dipole, I do not think I'll use them again, because they come apart a little too easily - it's likely I will need to reinforce the connection when the antenna is hanging for use.  I've seen other designs use alligator clips and even bolts to hold things together.

I also used a short piece of parachute cord to keep the dipole together when the powerpoles are disconnected.  Tie-wraps hold the cord and wire together, with a few drops of super glue to create a pretty solid bond so they don't slip.  Like the second powerpole, the cord was not connected until that section had been trimmed down to the correct length.  Here is a close-up of one of the links (note the whole antenna has a total of eight links):

I set the antenna up in my back yard for testing and trimming.  I also thought this would be a good chance to practice setup of the entire antenna system, including an extendable mast.  I had acquired a 10 meter pole from SOTABeams, a store in the UK which caters to SOTA enthusiasts.  10 meters is quite high and to make a portable mast, the poles become extremely thin at the top.  So thin, in fact, that most photos you see of them show considerable bending at the top.

So, I thought bending was normal and proceeded to hang my antenna on the top section and hoist it up.  I had each side of the dipole connected to a balun, and a run of RG58 coax connected to the balun.  As the mast went up, it began to bend.  Unfortunately, the weight of the balun and antenna were too much for the tiny top sections, and I only had it extended about 10 feet up when I heard a loud cracking sound and the mast bent in an ugly downward v-shape.  My brand new mast broke on my first try at setting it up!

It was quite a disappointment but I suppose it's better to have this happen in my backyard than after I have traveled to some remote site.

I still wanted to complete the antenna construction, so I set my frustrations aside and went to plan B.  I have a fairly sturdy tripod mast from MFJ, one that is most definitely NOT light enough to carry on a hike but is good for home use (I usually have a magnetic loop antenna hanging on it inside the house).  So I brought it out and set it up, and proceeded with the somewhat tedious process of fine-tuning the link element lengths.

I began with 15m, and once it was reasonably tuned I connected the 15m/17m link and tuned for 17.  Next up was 20m, which much to my surprise turned out to be too short!  This was not a huge problem, but I did need to re-cut a longer length of the wire and replace this section.  Then I moved on to 30m, and finally 40.  For the 40m section, I have left it intentionally long until I get out in the field with the antenna.  Once I'm in the field, I will either trim it down or determine a fold-back method to effectively shorten it to the best SWR.  I actually would have preferred to do this at home, but my back yard is just a bit too short, and it was getting too hot (105F) to continue the work.

Here are some SWR measurements of the various bands:

Certainly this is not the perfect antenna, but it does look like it will be a useful piece of gear for some of my ham radio portable operations.  I wound each side around kite winders for simple transport and deployment.  Now I just need to find a better mast solution.  It could be hung up in trees as well, but many of the peaks in Arizona that I may hike to won't have any trees available.  I realize that the real test will be how well it can perform, but I know what to expect from dipoles and this does appear to be a great way to get 5-band capability from a single antenna, at a very low cost.  I'll probably build another one to cover the 10 and 12 meter bands.

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