Social distancing project #1: Lightweight Linked Dipole

The coronavirus has stopped just about everything, and is undoubtedly changing our lives forever. With almost every group or event cancelled, there are predictions of mass boredom and social isolation. Many people apparently have no idea what they’ll find to do with their time. How odd. That’s never been a problem for makers like me. So in the interests of the bored and isolated, I offer this story of something useful that can be made in an afternoon — a linked dipole.

My old linked dipole

My linked dipole from 2016.

I first made up a linked 40, 30 and 20m dipole sometime around 2016. I modified an old 40m dipole I’d acquired years earlier. I cut a light MDF back-board to wrap it around, and even started penning the summits I’d visited in diary fashion. Sun, rain and even ice and snow buckled it into a gentle banana curve. It worked perfectly, but was larger and heavier than it needed to be.

Log book on backboard.

Objective

I am perfectly happy with a linked dipole as a SOTA antenna. The half wave dipole is a benchmark. In my limited experience of HF antennas both at home and in portable situations, a resonant dipole has never failed to perform.

I use homebrew transceivers for my portable work, none of which have inbuilt tuners. And I dont see the value of carrying another box (tuner) and connectors around at this stage. So I need a naturally resonant 50 ohm radiator on each band. I resolved to reproduce my existing 40/30/20m dipole without substantive change, other than making it smaller and lighter.

Ingredients

I decided that the 1.2mm multistrand insulated wire, available from Jaycar on 25m spools, was satisfactory. While in the shop I picked up a 25m roll of RG174. And two pairs of alligator clips.

Construction

I made the centre insulator from a scrap of single sided PCB stock, in a T shape. A hacksaw blade cut electrically isolates the two sides. Wherever a wire or the coax needed physical tension support, I drilled a pair of holes close together to allow a U-shaped threading. Then, I used the copper side to solder ends together. No 1:1 balun, although one could be easily added at QRP power levels. Another hole at the top allows a small cable tie to loop over the top segment of the squid pole.

I used the same approach for the inter-dipole link pieces, with a clip lead to close or open connections.

My old linked dipole had an SO239 socket on a perspex plate at the centre, allowing the coax feedline to be disconnected. The PL259 and SO239 pair added unnecessary weight right where it matters, at the very top of the springy squid pole. I dispensed with this arrangement by permanently attaching the RG174 at the centre bracket. Much lighter, much better.

Tuning

Each of the three dipole were made and checked for resonance, first the 20m one, then 30, finally 40m. A small amount of shortening achieved a 1.3:1 SWR or better on each band. Tuning was fairly broad.

Dimensions I ended up with: your mileage may vary.

I tested each dipole with my 5 watt CW signal and saw it Q5 on the WebSDRs at VK2OB and Northland Maritime Radio, New Zealand (around 4pm local time on both 30m and 40m). No trace on the nearby VK3KHZ SDR presumably due to the usual absence of NVIS.

Backboard

I designed and cut a compact backboard from the same thin MDF board as I’d used 4 years earlier. The size of the winder is a compromise — a longer span makes winding on the antenna and coax length quicker and easier but is bigger to carry.

On a summit

Under current semi lock down rules unessential travel is prohibited. So I cannot get to a summit! A SOTA activation with this antenna will have to wait. But I will travel lighter when that day comes… the new dipole weighs in at just 300 grams; the old one… 1Kg.

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