Saturday, July 23, 2016

QRP and Backpacking Linked Dipole

When I was at Dayton 2015, the Packtenna was introduced and produced a lot of buzz.  It was a lightweight 1:1 balun with multiple points to attach dipoles ends and support cables.  I liked the design, but could not pull the trigger on the system because of the high cost, so I decided to build my own.  My linked dipole cost around 40 dollars to build and was a fun project.

The requirements for the whole system needed to be lightweight, able to cover 20 and 40m without a tuner and easily deploy-able in either a tree or push up pole.

I researched 1:1 baluns and found a small LDG balun that would work for this application.  It's a 1:1 current balun.  I took the Packtenna design for the attachment points and cut that out of a Ikea cutting board.  The balun has small screw holes that can attach to the cutting board.   You could probably make the attachment back this out of 1/8 plywood or something comparable.

The larger holes are for attaching dipole ends with S-binders and the multiple smaller holes are for attaching small bungee cords to wrap the balun around a push up pole or something similar.

When constructing the dipole ends, be sure to leave enough length between the point where the wire loops between the orange S-clips and the top of the dipole bullet connector pegs.  For my setup this was around 10 inches of wire.  Also when measuring out your dipole for the correct length, be sure to count this length as it's part of the overall length of the dipole.

The orange plastic S-clips can be found on Amazon.  The clips I used are called Yueton 18 50mm S-binder clips.

The top of the balun houses two connectors pegs.  They accept bare wire that can be pushed through holes in the pegs and screwed down for connecting ladder line, or you can use bullet connectors to attach at the top.  These bullet connectors are 4mm in size and fit snug.  The bullet connectors can be ordered from Hobby King.  Below is a picture .  Top connectors are separated.  The bottom is together.

I soldered all the bullet ends, but because of the large connectors and the small gauge wire, I first put shrink wrap around the cable to bulk it up and give it more strength.  Then I soldered the wire in the bullet connector and then put a final wrap of heat shrink.  The wire soldered to the bullet connector will be the weakest link, so I wanted to make it as strong as possible.

The ferrules below were used to crimp the loops to hold the dipole to the S-clips.  I purchased these from Home Depot.  The 18awg hook up wire is just small enough to fit through the 1/16in ferrules perfectly.  You don't need a crimper to secure the ferrules, I just used a pair of pliers to crimp.

I wanted to use small gauge wire to keep the weight down, but one of the disadvantages of using small gauge wire is lower bandwidth.  Since I'll use this dipole for QRP operation, 18awg wire is plenty big enough to handle the wattage.  The wire was purchased off e-bay and often called hook up wire.

When linking the first section to the second section, be sure to leave enough length to connect the ends of the bullet connectors as shown in the picture below.  Pull on the dipole ends to put tension on them to ensure the links do not stretch, as you don't want them working loose after hanging the dipole.

At the final end of the dipole, which is the 40m link on my linked dipole, I pulled the wire through but did not crimp it since I need to raise and tune it.  Once tuned, I'll crimp the ferrule to make it permanent.  There is more than enough friction to hold it in place to tune it while it's in the air.

When measuring out the dipole ends, my suggestion is to measure out your first link, whatever band that is, 10M, 12M, 20M and tune that dipole to your liking.  Measure that first dipole again to get a final measurement and subtract that from the 2nd link total.  This is how long your 2nd link will need to be for the longer length.  You can then tune your 2nd link for the band you want by linking them together.

Another suggestion is to limit your linked dipole to a max of 3 links.  2 if you can.  Trying to tune and measure out a 3 linked dipole can be a challenge.

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