40-30-20m Linked Dipole
I cut and assembled a linked dipole for 40m / 30m / 20m some weeks ago, cut to the dimensions calculated from the Linked Antenna Designer calculator. I only got around to being able to test it today at the local sports fields, since there is nowhere near enough room in our back yard for such a large antenna.
As this was a prototype just to test the theory and dimensions, I used lightweight Jaycar hookup wire, along with 1/4″ blade grip type terminals for the links, fastened over plastics dogbone type insulators, of which I had a few in the various junkboxes. The idea being once the design was proven, I would use single Powerpole connectors for the links.
I didn’t bother with a balun or any form of choke. The coax simply terminates to the dipole elements at a plastics dipole centre. I put a small PL259 pigtail coax onto the centre insulator such that I can use an SO239 female to female adaptor and connect my coax run, making it both easier to handle and to pack.
The pole was extended and laid out. Two of the guys were fastened at estimated positions, and the dipole insulator slid over the top end of the pole, and the coax connected. The dipole legs were laid out at a gentle angle each side of the pole.
The pole was raised, and the final guy put into position, and the dipole elements also guyed out.
Now for some testing. Starting with all links connected (40m), I connected the analyser to find resonance was a little higher than expected – moreso since I’d already factored in a good 6″ of ‘extra’ wire with the idea to trim down later. Not great news since shortening a dipole is always easier than lengthening one! That said, it appeared to have plenty of low SWR bandwidth, even down on 40m.
Next band checked was 30m. To remove the links, there was enough flex in the Spiderbeam pole to simply put a hand on each element in turn, and pull it down to get to the link, leaving the element pegged out. Unfortunately, 30m resonance was also some way off the desired frequency.
Finally, 20m was checked. Again, by gently pulling on the antenna wire, the pole flexed enough to get to the insulator. That said, I am 6ft 2in and with my arms raised, a bit more again! The connectors were undone, and gently let back to their guyed position. The antenna analyser did not bear great news this time either!
Of course, in both cases of 30m and 20m, the antenna for that part of the band effectively needs lengthening. But of course with a linked dipole, this equates to moving the link position, since 40m is just about workable, although it could really do with a little more length.
So, now it’s time to sit down with the calculator, measure the wire again on the antenna, and figure out where the links would be best placed to achieve resonance nearer to the band centres.
I haven’t given up on it just yet, but I certainly missed out on any good luck in initial testing!
UPDATE!… Mk.2 under construction!
I found another reel of wire, so I cut some longer elements. I also took the opportunity to simplify construction. The link insulators are now far simpler – simply strips of lightweight plastics, and the end insulators are short lengths of GRP tent repair pole offcuts hacked off with the angle grinder. Previously, the end insulators were large egg type insulators and Aluminium carabiners to connect the tie-out cords. What was I thinking?!
Next step will be to take it all to the sports field again (when the rain stops) and run another test.
There is no balun. Initial tests on Mk1 dipole suggested I could get away without one. The coax coming from the dipole centre is a 20cm length of RG58. A little heavy, but it’s all I had. However, since it’s just a pigtail, I can easily connect some lighter RG174 for the main 10m or so run back to the radio.