40 – 15 meter Spring Loaded Linked Dipole

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Thought I would make a linked dipole antenna for my WWFF activities. After researching around on the web, I found a few designs to try out. All were pretty much the same, the only difference being the way the elements were connected. Some used alligator clips, others used electrical joiners.

After a few days thought on the subject I came up the idea to use automotive fuse holders for the connectors. These are spring loaded, a little tension on them and the connection is broken. I replaced the glass fuse with a piece of aluminium rod the same length as the fuse. Any conductive piece of material will do, even the original fuse.

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The wire for the antenna was a 40 meter dipole that I had previously made. The first step was to cut the wire at the desired length for the 15 meter band. I was looking for a centre frequency of 21.300 MHz, the length for this frequency is 3.25 meters, so adding the fuse holder to this gave me a length of 3.35 meters. The element lengths were determined using the Linked Dipole Calculator

15-40

I had drilled a small hole at the end of the fuse holder so that when tension is applied to the wire a small piece of insulated wire could be slid in to hold the join open. The remaining length from the original dipole was connected to the other end of the fuse holder. Both elements were connected to the centre feedpoint an hoisted up on a 7 meter squid pole, the ends tied to 5 meters of light rope and pegged to the ground in an inverted V configuration.

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Connecting the MJF-269 antenna analyzer, the first reading was 1.3:1 on 21.300 MHz, after some trimming I was able to get the SWR down to 1.2:1. The insulating pins were removed and the procedure was repeated for the 40 meter band. The final SWR reading for this band was 1.1:1 on the chosen centre frequency of 7.130 MHz.

The final step was to increase the tension on the guy ropes to break the 40 meter connection. After checking the MJF, the 15 meter section held the SWR at 1.3:1. Loosening the ropes and checking the SWR on 40 meters, the results were good, no change in the reading.

The idea to use the fuse holders seems to work ok, but is only good for a dual band dipole, a multiband antenna with more than one connection will not work under tension, all the connections will be broken. The ability to be able to change bands without having to lower the antenna is the main reason for using this design. Being able to change bands by increasing the tension on one of the guy ropes is the main benefit, instead of lowering the squid pole each time.

I will test out the antenna over the coming weekend to see how it performs in the field, the wind will be the determining factor, to see if it will affect the connections.It does not take a great deal of tension to break the contact, I might have to play around with different springs. It will be interesting see how it goes.

73`s & 44`s

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3 thoughts on “40 – 15 meter Spring Loaded Linked Dipole

  1. Pingback: Long Weekend National Park Trip – Part One | VK2FMIA.COM

  2. Pingback: Long Weekend National Park Trip – Part Two | VK2FMIA.COM

  3. Pingback: 15-40 Meter Spring Loaded Linked Dipole V 2.0 | VK2FMIA.COM

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