The A35JT DXpedition team is pleased to announce that it has been been accepted as a qualifying expedition for the DXpedition Trophy awards! You can find out more about the awards over on their website (click here).
2019: A35JT OC-049 Tonga
Antenna development work continues for the A35JT DXpedition. Today, after several failed attempts over the past month, and some yacht rigging experience from Matt VK5ZM, we successfully mated a SpiderBeam 12m Fibreglass pole with a CrankIR portable antenna. What does this give us you might ask? In short, the ability to run a folded 1/4 wave mono-pole vertical on 80m on a tune-able antenna rig that will support our expedition across all bands from 160-10m. It is, in effect, a variation of the same antenna I took with me to Vanuatu last year – but in a much more robust form.
While the original CrankIR does have an 80m add on kit, our measurements indicated that it was rather inefficient at that frequency. Converting the antenna using the SpiderBeam pole and moving to a full size folded monopole design for 80m with elevated radials raises the efficiency considerably on 80m, while retaining the rest of the versatility of the CrankIR system.
So, how did we do it? Using a combination of components from both the 80m add on kit for the original CrankIR and all but the top section of the SpiderBeam 12m Fibreglass pole. These were arranged in the form of a box rigged yacht mast. Through careful selection of guying points and pole segments, we were able to achieve sufficient compressible strength to take the head load of the CrankIR spreader system while improving the lateral stability of the top half of the SpiderBeam mast at the same time. Doing so required several pieces of ingenuity and home brew development, as well as a few visits to the local hardware store.
To achieve the box rig guy configuration required two main elements, the 4 way spreader arm and hub assembly plus a base plate at the bottom of the mast. This spreader arm assembly was created from a rubber dolly wheel of about 100mm diameter and 4x 600mm fibreglass tent poles. The wheel was drilled out to the appropriate size to fit the mast at the height we wanted. Then 4 holes were drilled to fit the tent poles at 90deg to the mast.
We then manufactured a base plate from a 150mm square piece of 3mm plate aluminium, some Carabina clips and a door stop to centre the pole.
As noted by the SpiderBeam, the top couple of sections of their 12m fibreglass poles are not designed to take any load more substantial than a piece of wire. Our experiments confirmed this, with problems encountered with the pole bending uncontrollably with the ~300g+ head load of the CrankIR spreader arm unit applied, even before rigging the wire radiating element. So, in addition to the box mast rigging addition, we also had to rearrange how the masting we had from both the Spiderbeam Pole and the 80m CrankIR kit was being used.
The ultimate objective was to get the top of the antenna at least 11.7 metres off the ground. This would allow the 80m folded element to reach the required height of 10m with the base of the antenna at least 1.7m high. This was important as it raised the antenna impedance up to 50 ohms once the 8x25m ground radial lengths were adjusted (note these are longer than needed to raise the impedance).
Our solution was to not use the top section 12 of the SpiderBeam mast at all, and to indeed lower the 11th section (held by a hose clamp) so that only 10cm was visible above the 10th section (for stiffness and diameter matching for the CrankIR spreader). The mast was then guyed from the bottom of the 10th section. The box rig guys use 1mm polyethelene cord while the main guys used 2mm dia “Spectra” rope – very light weight but also high breaking strain. The end result looks like this:
The SpiderBeam mast then fits neatly on top of the connecting aluminium pipes supplied by CrankIR allowing us to extend the mast by the 1.7m we lost at the top. The overall height doesn’t change, but the strength does. With this setup, we had finally created our multi-band tune-able vertical!
The total weight for the entire antenna is predicted to come in under 10kg (without pegs) including an addition that will also give us tune-able access to 160m through mounting an inverted L to the same mast.
80m Antenna Operation
Tuning the antenna for 80m is a combination of adjusting the amount of wire in the driven element plus the length of the earth radials. An important point to note is that the impedance is sensitive to how much of the radial is on contact with the ground. In our tests we arranged the radials to be 25m long and elevated off the ground for approximately 3-4m away from the base of the antenna. Doing so allowed us to achieve some great results! We were able to tune and match the antenna with relative ease from 3.5 – 3.85MHz.
We also want the expedition to be able to have some access to 160m, although it is not a primary band for us (we just cant take enough gear with the number of team members we have to build a full size 160m station as well). To achieve this, based in part on a design by Rick DJ0IP, we wound in the wire on the 80m CrankIR. Then on a second pully and string we had rigged for the purpose, we attached a new radiator wire to the top of the guyed section, brought it down 11m to the 160m feedpoint, and extended the other end out ~34m to form the inverted L. For testing and initial tuning, we then passed it through a 5-500pf 15kV variable vacuum capacitor (VVC). The horizontal part of the L is then varied to tune the antenna across the band.
In this configuration, the antenna itself then presents a feed impedance of ~ 32ohms, which when connected via a 1.5:1 Binocular Core matching transformer (thanks to Neil VK5KA), returned the antenna to 50ohms for transmission back to the shack. While the VVC is too heavy and bulky to take with us, it did allow us to determine the correct fixed value high voltage “door-knob” disc capacitor to place in series to allow operation on both 1823-1843kHz and 1908kHz (for the Japanese FT8 operators).
We were very happy with how the 160m inverted L modification worked out. This combined with the 128m beverage Rx antenna we are taking will hopefully give us at least a reasonable chance of 160m across Asia/Pacific and the Americas – and if we are very lucky perhaps parts of Europe as well!
Now, you might be saying why go to all of this effort? It really came down to using the materials we had available in the best way possible to build as efficient antennas as we could. Given weight, cost and availability of other options to us (remember VK is a long way from the manufacturers of anything) we were happy to put together what we did and achieve the results we did.
With this work done, we have now achieved a working tune-able vertical which is ready to be packed up for use on Tonga. Another major milestone complete in our preparations to go to Tonga in September!
The team today undertook the first set of tests on the elements that will form the basis of our 30m 4-square array that we are planning on taking to Tonga!
The first challenge was to build one of the elements and validate the mechanical and the electrical characteristics. We were looking for a 50 ohm match point so that it would work properly with the 4-square array and were very happy when it matched almost spot on. We have left the radiator a bit long for now as when we add the other three elements we know the impedance will change.
We also make some more progress on adapting the CrankIR to cover 160-10m on a 12m Spiderbeam pole as well. More on that experiment in the coming week!
The A35JT Expedition team wishes to thank the Swiss DX Foundation for their support of our endeavors to activate Tonga in 2019!
The A35JT team is very grateful to the European DX Foundation for their support of our efforts to activate Tonga in September 2019. We will be doing our best to target your part of the world as much as possible on every available opening we can find recognizing that it is in the top 60 most wanted on Clublog for Europe.
The first major new antenna for the DXpedition has been completed and was taken out into the field for its first on air trials this weekend during the CQ WPX SSB contest.
Our 40m 4-square array, designed and built by Oly VK5XDX will take full legal power and then some. We verified that the directionality definitely works, being able to switch and see an immediate change in received signals between NA, EU and Asia across the contest. Unfortunately for now, switching involves going outside and replugging the antennas into different ports of the hybrid and phase shifter. We are looking at ways of building a relay switch box to add to the antenna so we can switch it back in the shack.
The array is supported by one of our equipment partners. The 75ohm phasing lines between the elements used cable sourced via Messi & Paoloni in Italy, who make foam based light weight coax that can still take the power needed on our station. This contributes to the whole array (minus the star droppers) weighing in at less than 14kg. It was designed to break down into lengths less than 1500mm so that it can be carried on commercial airlines as oversize baggage.
The Ultraflexx 7 50ohm main feeder used also came from M&P. This cable has roughly half the weight of traditional RG-8 – highly important for a weight limited expedition.
Results on the Air
During the contest, we were frequently asked what the antenna was and were complimented on our signals. We were happy to be able to often get stations in Europe and North America to reply with only 1-2 calls, and were pleased to be able to work some small pistol stations on 40m in the USA during the early evening (our time). It all bodes well for our 40m signal on Tonga in September!
The A35JT expedition team is grateful for the tremendous support it has received from the German DX Foundation! With the addition of Steve VK5SFA to the team (who is something of a 160m specialist) and support received from the DX associations, we will be placing some increased focus on our planned 160m operations too!
The A35JT team is pleased to have Steve VK5SFA join our expedition. Steve will be with us for the first week of the trip during the busy build period and the very busy operating period when we will be running both EME + HF (EME wont be workable in the second week due to sky noise).
Steve brings a wealth of amateur radio experience to the team. As an avid DXer and contest participant as well as his experience as a broadcast engineer he will be an invaluable member of our team. Welcome Steve!
The DXpedition team is grateful for the support we have received from the CDXC UK Foundation. We will be trying our hardest to make contact with your members from Tonga in September!
Licence Received from Tonga
After several months of communications we have finally been issued with our licence to operate on Tonga!
We had a small surprise however when we looked at the issued callsign. It has now changed and our final assigned callsign is A35JT. Never mind!
We have also confirmed that we are not able to operate on 60m this trip. We have, however, been granted access to 6m to conduct our EME experiments!
So after a quick rebrand of the website, QRZ and Facebook pages our identity is corrected and away we go!
Work on antennas continues
We have also been busy behind the scenes with Oly VK5XDX designing and testing our home brew 40m 4-square array. Weighing in at only 12kg the early tests are promising. We are hoping to run the array for part of the CQ WPX SSB contest at the end of March as a test.