20m/40m linked dipole

Not having had the time or weather to go too far recently, I decided to have a wee go at modifying a 20m wire dipole. What I was after was an antenna to do both 20 and 40m.  The idea is simple, a linked dipole is basically a dipole cut for the lowest frequency you want to use, but with connectors on it so you can unplug some of the length and have a tuned dipole for a higher frequency.  These can have 4 or 5 connectors on them depending on how many bands you want to use, but for this antenna all I wanted was 20m and 40m.

I had in the past read up about these antennas and the connector of choice seemed to be the Anderson Powerpole.  I duly ordered a quantity of said items and left them lying around for the best part of a year.  I always meant to get round to it, but as you know, it’s always easier said than done.    It was now time to put my plan into action. 

The day was Remembrance Sunday, and after attending at the local cenotaph for the Service of Remembrance with one of the dogs, we headed up Tower hill to let him stretch his legs after sitting so well during the service.  On the way I spoke to Iain WJZ with my trusty vx7 who was on Beinn Bhuidhe doing a SOTA activation.  Once up on Tower hill itself I spoke to Graeme GIL who was also out with his dog, can’t remember the hill, but it is just above the Whangie.  The WX was glorious, so I thought it was time to set about the linked dipole job.

  A ten minute walk and I was home.  I then set off out in the car to the hill just behind Greenock.  Here, there would be plenty of space to set up the dipole and the remote location ensured minimal QRM from power lines and transformers etc.  I normally use a ‘drive-on’ antenna support which holds the fibreglass fishing pole solidly whether it has a wire attached, or 2m beam for chasing distant stations.  On this occasion I chose instead to strap the pole to the rear of the vehicle using just bungee cords as it was just a temporary set up. 

The dipole I was going to modify was homebrew.  Very simply made, it only consists of a suitable length of co-ax, 1.5mm csa wire ( from the electrical factor ) for the elements and a plastic cable or ‘stuffing’ gland.  The co-ax is stripped back to reveal the inner and outer cores, two lengths of the 1.5mm wire are then soldered to the suitably tinned and prepared wires of the co-ax, and then the cable gland is slid over the co-ax up to the point that the newly made connections are inside the gland.  while ensuring that the cores remain separated from each other, fill the cable gland with Araldite or similar two pack glue.  Once set, this makes a very good, waterproof and robust dipole centre.  Other ‘containers’ may be pressed into service, a 35mm film canister perhaps or whatever you can think on to make the connections in.  I chose the stuffing gland as I had it lying around. 

Remembering that the dipole is half wave, then for 20m operation, the total length should be about 10m, so each element is roughly 5m long.  This is a good starting point and you can tune the antenna for exact SWR from there.  I also have wound 6 turns of co-ax at the centre of the dipole to provide a balun.  This works very well and I have had no problems when using this antenna.  To attach the centre to the top of the fishing pole, I have attached a piece of bent aluminium bar that was lying handy at the time.  I could have looked for something prettier, but this works.  On the ends of the 20m dipole I have fitted Anderson powerpole connections ( suitable ‘how-to’ videos can be found on YouTube ) and a plastic insulator which the 40m extensions connect to.  Again, powerpole connectors are fitted to the 40m extensions so they can be plugged into the 20m section.  Once I had trimmed the antenna so that I had a great SWR on 40m, I fitted keyring split-rings to the ends of the wires so I could easily attach some guy wires to pull the elements into tension.  For this, I use the orange line found on an old hand-held fishing set  –  not what you would get on a rod/reel.  I either use handy rocks to tie the ends off to, or a couple of tent pegs. 

So, that is basically that.  A simple way to have 20 and 40 metre operation on a cheap antenna.

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