I like to operate portable, and there's a couple of National Parks nearby
that I frequently visit. But I don't always want to lug around a full HF
station, whereas a handheld VHF/UHF radio has very limited range. So I
built this travel Yagi to increase the range. It has 5 elements on
2 m and 8 elements on 70 cm, and a single 50 ohm feed.
It should offer some 10 dBi gain on either band. When packed away for
transport, it looks and tastes like a fishing rod, because it is.
The antenna seems to work ok on 2 m, but I don't know exactly how good or bad it is on 70 cm. I ended up building a 6-element 2 m monobander to complement it, using the same fishing rod concept. Although not designed as a dualband antenna, it also works on 70 cm, though with less gain according to simulation.
|I had an 8-meter long fiberglass fishing rod waiting to be put to use. I discarded the top two sections, used the next two for the boom, and the rest for a mast. The telescoping mast is thus 4 m tall and easily handled. The two-section telescoping boom is long enough to accommodate five elements on 2 m and eight on 70 cm on the dualbander, or six elements on the 2 m monobander.|
The antenna elements are made of 4 mm thick aluminum rod that is easily
evailable from hardware or hobby stores. (I could not easily find aluminum
TIG-welding wire, which also seems a popular material for Yagi elements.)
The antenna elements could maybe be packed inside the fishing rod for
transport, but it's a tight fit with the large number of 70 cm elements.
I bunch up the elements with rubber bands, and I'm looking for a suitable
fishing rod bag to transport them neatly. The whole thing weighs
xxx g, including guy lines and ground pegs.
The single driven element is in two pieces, isolated from each other with a short piece of fiberglass tube, taken from the top section of the fishing rod. A short coax pigtail with an SMA connector is soldered to the elements with Alu-Sol 45D aluminum solder (Sn/Pb/Ag solder with special flux for soldering aluminum, stainless steel and some other strange materials; nothing smells worse than the fumes from this solder, but it works!). The anodized surface of the elements was first filed off to expose the bare metal. Then the elements are epoxied into the fiberglass insulator, and the connection is gooped up with hot glue and covered with heat-shrink tube.
|To accurately drill the holes in the conical telescoping sections, I wrapped electrical tape around them at several locations, so their diameters became equal. I then clamped the sections onto a piece of board, and carefully drilled through at the marked locations with a self-centering 4 mm bit in a drill press. (Note that without the electrical tape the conical shape of the telescoping section would have left the elements at an angle.) I gooped the edges of the drilled holes with cyanoacrylate (super glue) and filed them to a snug fit.|
|A plastic plumbing T-junction part serves to attach the Yagi boom to the mast. The horizontal part of the "T" was lined with open-cell foam inside, so the boom can just be inserted before the elements are attached, and it is held in place by friction. It can freely be twisted between horizontal and vertical polarization. The mast joins the "T" with a through-pin to keep the antenna pointing where I want it (this picture still has duct tape doing the job). The elements have stoppers (cable ties) on them, and are inserted through the boom from one side. A tight-fitting rubber grommet is placed on each element to keep it in place (not strictly necessary in vertical orientation, and thus missing from this picture). The RG-58 feedline with a choke balun is attached to the mast to relieve stress and connected to the coax pigtail of the radiator element.|
Here's the T-junction with through-pins securing it both to the mast and
the Yagi boom. Due to the radiator element's feedpoint construction, all the
elements are offset slightly from the boom, placing the whole thing a bit off
balance. Vertically this hardly matters, but horizontally a through-pin is
needed to keep it from twisting.
Note also the gray rubber grommets that hold the elements and pins in place.
|Three guy lines hold the mast up. These are tied or pegged at ground level or in nearby trees. Since this is a telescoping mast, locked only by friction, the guy lines should come at quite a shallow angle. Otherwise any wind would cause the guy lines to pull down hard on the mast, possibly causing it to collapse. If this were a problem, through-pins could be fitted at each of the three joints in the mast, locking it up securely. The guy lines are fitted loosely about halfway up the mast using a helpful furry cow. It is at a safe distance from the antenna, and unaffected by RF radiation. The loose fit allows the mast and antenna to be rotated by hand.|
DimensionsHere are the dimensions of the Yagi, and the NEC2 model. The dimensions are shamelessly taken from other peoples' designs, hastily adapted to the 4 mm element thickness, and briefly tweaked with a NEC2 simulation or two. Speaking of which, simulation suggests this antenna should work FB as a monoband Yagi on either band, if you feel too lazy to install elements you won't use. There should be no effect at all on 2 m, and only slightly elevated SWR on 70 cm. I have no measurements of this, however. (And if you're building a monoband Yagi, you can surely find a better design, such as the 2 m monobander below. And feeding a 70 cm beam with a 2 m radiator is especially stupid, and decreases gain quite a bit.)
Here's the dimensions of my 6-element 2 m Yagi, and its NEC2 model. The dimensions are taken from this Yagi. The resonant frequency seems to be just a tad low, so maybe I'll file down all the elements by a couple of millimeters.
The 6-element 2 m Yagi is as compact as the dualband, and has less elements to install when setting up. It also has slightly more gain on 2 m, and can also work on 70 cm in an emergency. I think it will become my primary portable VHF antenna, but I'm keeping the dualband also. Only the boom and elements are new for this antenna; all other parts are shared between it and the dualbander.