VDA’s are becoming more and more popular with DXpeditions as they are light and perform great at the beach. For DXpeditions mono-band VDA’s are used as they can be used for multi-station deployments and causing less interference. However for one-man DXpeditions, bringing mono-band VDA’s is not practical and therefore I was looking for a multi-band VDA. In the past I had some e-mail exchanges with Diego – F4HAU and he provided me with a design for 17, 15 and 10m. I liked the idea but it had a coax for each band. Moreover, in the current situation, 10m is not a band to expect a lot of activity.
My past one-man DXpeditions were always based on commercial verticals 40-10m and lately I switch to the so called magic antenna, a 12.85m long wire on a 15m fiber pole and a 1-4-9 balun at the base with 6-8 radials. This magic antenna is very simple, light, easy to transport and can be installed quickly and in nearly all circumstances. It is certainly not an ideal antenna but for my purpose and situation suitable.
The new idea is to have a magic antenna for all bands but a 20-15-17m VDA for the best higher bands during this low sunspot period. Based on the experience with the 30m VDA I designed a 20-17-15m VDA with a single coax feeder.
I used the same construction for the cross section and the spreaders. A 12.5m GFK (DX-wire) is used for the mast. Two Lakeside-1 300 fishing rods (just € 5 each) are used for the boom section. The two smallest elements are not used.
For the cross section I used a U-shape aluminum profile and put U-shaped rubber on the rim in order to avoid slipping on the mast. The U-shape profile was tied with two tiewraps onto the mast. To attach the two boom sections, I use a piece of wooden stick available from any DIY shop. I drilled a hole in the aluminum profile so the stick fitted well.
For the spreaders on the boom to keep the antenna wire at the right distance from the mast, I used a large hole saw to cut them from an IKEA breadboard and drilled the holes in the middle with a conical drill. The breadboard material is very easy to process. For the top of the mast, I made a small bracket and the other fixation points along the mast are made of a tiewrap inside a square rubber tube (to avoid shifting). This rubber tube is also used for fixing each mast element and can be purchased at DX-wire.
I started to fix the 20m antenna and tested for the correct SWR. Once satisfied, I added the 17m antenna, did the SWR adjustments and next the 15m antenna with adjustments. For each case, the coax was directly attached to the respective antenna. Obviously, adding a second or third antenna would de-tune the others, so now I had to test and adjust the antennas when all together. But first I had to construct the antenna feeder to allow a single coax to be used. You can best see from the pictures how that was constructed. Once ready, I could test the complete antenna for three bands with a single feeder.
At home I fix the fiber pole to a wooden pole that is well fixed in the ground. This allowed me to easily erect the antenna and attach it to the wooden pole and secure it with a rope. It is now easy to put it up and take it down for measurements and adjustments.
Obviously, the soil at my home is different from the DX location, so adjustments will be needed once at the spot, but I was able to tune the wires to be at least correct at my home location. One complicated issue was the wind. Since the antenna was used without guy wires, the wind was bending the mast and boom quit heavily. This had a big effect on the SWR. With the VNA doing a continuous sweep, you could see the fluctuations. Nevertheless, I was able to get the tuning good enough to complete the works.
For those of you that want to build this antenna, drawings of all the parts are also available in a VDA 20-17-15m – 2020-04-20 PDF. Be aware that the type of wire you use for antenna elements may have an effect on the length since the velocity factor differs per wire type.