![]() In addition, in the design, I included a DX-Wire ferrite choke made with coaxial cable, inserted just after the center isolator of the driven element. I also had to test each of them in my garden before packing in the container, so you can see on this page the result of the analyser in comparison of the theorical SWR values of EZNEC. I also started to use EZNEC and made new modelling of the VDA. I was assigned the task to rebuilt a complete set of VDA for 20m to 10m with a more robust design, so it can stand strongs winds and will not fall down on protected wildlife of Tromelin. When we initially start planning for Tromelin Island Dxpedition FT4TA, the choise of using VDA quickly become obvious. It means that the following values have to be multiplied by 0.96 to get the real physical length of the wires. For example, the Spiderbeam CQ-532 stranded copper wire has a velocity factor of about 0.96. Do not start to cut your wires from these values without knowing the velocity factor of your wire. The following table summarizes the theorical dimensions of the antennas (in meters). You can also notice the relatively low elevation angle (14°) of this antenna that takes advantage of the low angle of arrival of DX signals. ![]() I was really surprised by this strong attenuation, both during my tests and also in actual operation during my trip (for example, in FK, the 180° was towards ZL and it was extermely difficult to contact them while I had no problem to work EU/JA/US, located 10000km further than ZL !). In multi transmitter environment, it also reduces interferences between stations if the antennas are properly oriented. ) coming from the beach and houses located directly behind the antenna. I found it very useful, especially when you are on an island, because the antenna is directed to the open ocean and the attenuation is on the local noise (qrm/qrn etc. There is a very high attenuation at 180° and it's very sharp. the salt water is playing a major role in the antenna radiation performance.Īs you can see, the forward lobe is broad, and it's an advantage as you do not really need to turn the antenna, especially if you can get US/JA/EU in this main lobe. So again, when I say this antenna is to be located at the sea shore, it's not only because photos are more beautiful with the sea in the background. For sure, the loss in the coaxial was still higher than when the antenna was only 15m away, but the fact that the antenna position was on the sea shore gave me much better results on this band. But after one day and poor results, i decided to move this VDA near the seashore and connect it with 100m of low-loss H155 coaxial cable. My operating shack position was 100m away from the sea shore, and I decided to build the 28MHz VDA near the shack, in order to save coaxial lengh and reduce the loss in the cable. While in South Cook, I made an interesting experiment on the 28MHz band. The VDA is performant ONLY near the sea water, and with the sea in the path to the DX. ![]() In other cases, and to be more clear : If you do not plan to install your antenna near the sea water, then forget it and look for some other antenna designs. VDA are no exception and based on my own experience, I would even say they are more sensitive to the near ground. ![]() ![]() Performances and patterns:Īs with all other vertical antennas, you need a good ground to get the antenna working efficiently. Below you will see some explanation on the performances and some indications on how to build it. At the end, each antenna has a total weight of 1.5kg (including the 10m fiberglass pole). It then took me a few days to get the first prototype assembled, the most challenging part was to find a solution to build the cross arms as light as possible. I got precious information from Cornelius DF4SA, the founder of Spiderbeam, and I was also helped by Jacques F6BEE for the calculations and modelisation part with MMANA-GAL software. I was always impressed by the strong signals VP6DX (Ducie Isl.) got with their VDA and I tried to make similar (but lighter) antennas that can be transported by a single man in his luggage. Everything starts in summer 2011, when i had to prepare antennas for the upcoming 2012 Pacific DXpedition ( VP6T, TX6T, E51BKV, FK/F4BKV). ![]()
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