Now I have arrived at what appear to be near-enough dimensions for my 40/30/20 linked dipole after another session at the sports field, I am ready to make the more ‘permanent’ version.
Lengths were determined using the analyser, with lots of trimming, and twisting together of wires when things ended up a bit too short. I lost count of how many times I raised and lowered my 8m pole in the process!
I placed an order to SOTA Beams back in my home country of the UK for two of their highly regarded winders (with a buy two get one free deal running throughout September) along with 25m of their ultralight DX Wire which is a small diameter (1.5mm O/D) wire with a Kevlar inner stranded core for strength. Even though only 1.5mm O/D, it has a breaking strain of 60kg!
I already have another 25m of super-heavy duty Kevlar reinforced wire, but it’d be too heavy for SOTA purposes, so I’m keeping that for a future (home) dipole project – perhaps an OCF variety.
For the link connectors, after a bit of consideration, I am going to go with 3.5mm dia Gold plated bullet connectors, commonly used in RC models. Extremely light weight, very easy to solder, and low contact resistance and impossible to oxidise or corrode. I obtained a pack of 10 cheaply from eBay.
I’ll report back once the dipole is all complete. Since this will be the ‘final’ construction, a bit of dual wall heatshrink here and there will assist in its longevity.
Also, interestingly, the same day I ordered the parts from SOTA Beams in the UK, I also ordered a photography accessory from Sydney. Looking at the tracking information supplied, both packages were posted out the same day. The package from the UK 17,800+ km and an international plane ride or three away arrived today – just 6 days later – but the package from Sydney 40km away, I’m still waiting for! I could have walked there and back to collect it more quickly! Come on Australia Post – pull your finger out!
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.
Google Earth is a remarkable resource. It’s free, for starters! Google Earth is embedded with altitude data. As such, a programmer called Bernard Sterzback in Germany has come up with a clever .kmz extension file that can be installed, and used to access that embedded altitude data to show ‘flood’ maps, in order to show what land projects above a user-input elevation level.
This is useful for determining the ‘within 25m of the summit’ Activation Zone for SOTA, as well as for hillwakers in determining likely paths and ridges, and the widths of such ridges.
When it has downloaded, run it, and it will associate itself with Google Earth automatically.
Next, open Google Earth and head to a point of interest. I’ll use a SOTA summit – VK2/CW-050 – Caloma Trig Point for this example.
Next thing we need to do is determine the elevation of this summit. We can either click on the Google Earth placemarker, or move the mouse cursor to that point and read the elevation off the info bar at the bottom of the screen. Make a note of the summit altitude.
In this case, the reported elevations are pretty close, but I’ll go with the ‘official’ SOTA elevation of 774m.
Next, looking in the ‘Places’ collapsible dropdown on the left hand side of the Google Earth window, you should see your new ‘Flood’ tool you installed. Hover over it with the mouse, click the right mouse button, then left click ‘Properties’
In the window that opens, you can now edit the ‘Link’ data, which is the flood level you want to set to view. In my case, I want to see what area of land is available within 25m of the summit, so I edit the number to become 749 (ie. 25m less than 774). You must leave the ampersand ‘&’ symbol in at the end.
Then, click the [OK] button.
Now, you can enable the Flood Tool by clicking its check box, and the area will be ‘flooded’ to the level you just set, showing only the land remaining from 25m below the summit, to the summit. This is the SOTA Activation Zone. Easy, wasn’t it?! Once you’ve done it a few times, it becomes quite intuitive.
To clear the Flooded area, simply un-check the Flood tool.
See you on the summits, within the calculated Activation Zones!