VK7TW – Linked Dipole Experiments

Greetings All,

I started SOTA activations back in October 2014 using the VK7JJ vertical (http://www.perite.com/vk7jj/squidpole_notes.html) along with a simple MFJ-16010 L-C antenna tuner that I modified to add a one LED SWR meter (http://www.qrpkits.com/files/LED%20SWR%20v3.pdf). This antenna kit weights 1.4Kg.

MFJ 16010 ATU with One LED SWR meter

MFJ 16010 ATU with One LED SWR meter

VK7JJ Centre Loaded Vertical Kit including QRP ATU.

VK7JJ Centre Loaded Vertical Kit including QRP ATU.

This was a great start for SOTA activations and got me some good signal reports from around Australia. Then following some reading and discussion with Peter VK3PF, Reuben VK7FREU and I made a linked dipole using the SOTA design calculator (http://reflector.sota.org.uk/t/linked-dipole-designer/9234) for the HF Foundation Licence frequencies of 80/40/15/10m.

Reuben VK7FREU linked dipole

Reuben VK7FREU linked dipole

Linked dipole links with alligator clips

Linked dipole links with alligator clips

VK7FREU linked dipole centre - squid pole through hole

VK7FREU linked dipole centre – squid pole through hole

Made from extremely light and strong plastic advertising display board which can be cut with tin snips. This first version we made with standard RG-58CU coax. The pretuning means no ATU is required and this antenna kit weighed 1Kg.

On a rare calm day on Mt Wellington (VK7/SC-001) we setup both the VK7JJ vertical and the linked dipole and did a test thanks to Robbie VK3EK. The report was confirmed as being 1-2 S-points down using the vertical versus the dipole on 40m. This was around 4pm on a December afternoon. From what I have read and the modelling this is about what would be expected.

The author VK7TW then built his own version with 80/40/30/20m and added some refinements. He used RG-174 coax reducing the weight even further. He decided not to put a Guanella 1:1 balun in the mix as we have had no appreciable issue with RF coming back down the coax, this further reduced to weight. He put cable ties on the winders to hold the tent pegs for the dipole ends. This keeps them with the winder and you don’t have to go looking for them when unwinding the dipole. This antenna kit weighed in at 0.7Kg.

80/40/30/20m Linked dipole for VK7TW

80/40/30/20m Linked dipole for VK7TW

VK7TW Linked Dipole Centre

VK7TW Linked Dipole Centre

After testing it was found the following segments gave acceptable SWR in the main operating part of each band:

Wire lengths for each side of the dipole were found to be:

  • 20m (highest frequency 14.3MHz) = 4.84m
  • 30m (centre frequency 10.12MHz) = 1.89m
  • 40m (centre frequency 7.07MHz) = 2.92m
  • 80m (lowest frequency 3.56MHz) = 9.2m
  • Total length one side of the dipole = 18.85m

I use both the vertical and the inverted V linked dipoles with 9m squid poles that weigh 1.12Kg. I have placed two pieces of velcro around the squid pole at the same places as the ties on my back pack and these allow me to velcro the pole to the pack.

Author with squid pole velcro'd to pack.

Author with squid pole velcro’d to pack.

Happy experimenting.

73, Justin, VK7TW

PS. 20150308 – I have copied the SOTA Beams 2-4 band linked dipole spreadsheet and expanded to cover 5-8 band linked dipoles. The updated spreadsheet can be found here:

5-8 Band Linked Dipole Spreadsheet (Microsoft Excel 97-2003 spreadsheet)

This enables you to calculate between 5 and 8 band/segment lengths.

Enjoy!

VK7TW

PPS – March 2015 – Converted the four band to an eight band for the following frequencies:

  • 52.2MHz – 1.225m
  • 28.510MHz – 1.08m
  • 24.980MHz – 0.37m
  • 21.250MHz – 0.35m
  • 18.130MHz – 0.595m
  • 14.310MHz – 0.97m
  • 10.130MHz – 1.88m
  • 7.090MHz – 2.96m

It has been taken on four SOTA activations and works really well.

73, Justin, VK7TW

PPPS – July 2016 – Unfortunately the plating on the Alligator Clips rusts when exposed to the weather. I replaced the Alligator Clips with gold plated Radio Control Power Connectors (thanks to Warren VK7WN for the idea). These connectors (2mm diametre) are simple tube and sleeve arrangement and you just solder into the tube and I placed a short section of heat shrink on the wire end to provide some strain relief.

linkeddipoleconnectors

Linked Dipole RC Connectors Upgrade

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One Response to VK7TW – Linked Dipole Experiments

  1. Pingback: Building A linked Dipole – CQ DE VK7BEN

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