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!