Spiderbeam® Yagi Design
The following essay gives an overview about the complete antenna theory behind the Spiderbeam® yagi design. It was written with the 3-band antenna (20-15-10m) in mind, but can be applied to all other configurations as well. The story below can also be seen as a history of all improvements and learning experiences made during the Spiderbeam® evolution. Specifications, manuals, packing lists for kits are at the bottom of this page as clickable links.
The Spiderbeam® yagi is a trapless multiband yagi constructed of 3 interlaced monobanders, making it a highly efficient antenna with simple construction. The driven element is a directly fed multiband dipole, again contributing to a simple, broadband and low-loss construction. The design is very forgiving, provided the wire elements were cut exactly at the beginning and high quality (non-stretch) wire is used. The mechanical design is optimized for quick, lightweight portable installations, using high quality materials for good reproduceability and a long life without performance degradation.
Next, what a Spiderbeam® yagi is not: It's not a hexbeam and has nothing in common with that style of antenna. Hexbeam is a 2 element compromise antenna with elements bent into a curved or "W" shape designed to be used in a small space. Spiderbeam® yagis use full size wire elements on all bands and are 3 el 20m, 3 el 15m, 4 el 10m with gain and F/B ratio typical of a larger, heavier triband yagi antenna.
The basic antenna principle is quite simple. No magic involved. Start with a normal 3 element yagi and bend the director and reflector in a V-Shape:
The resulting antenna can be built using wire elements strung on a supporting cross, which makes it possible to use lightweight materials like fiberglass and wire. Bending the element ends towards each other has the additional benefit of enhanced coupling between the elements ("capacitive/inductive end coupling"), which in turn seems to enhance the F/B ratio and antenna operating bandwidth. Dick Bird, G4ZU was the first one who had the idea of bending the elements like this, and named it the "Bow-and-Arrow Yagi" or "Bird-Yagi".
The Spiderbeam® yagi is a multiband design. Three monoband "bow-and-arrow yagis" are interlaced on the same supporting cross.
With such a design, several challenges must be met:
1. Minimal interaction:
This is the biggest challenge with any multiband antenna: we need to find a design where the interaction between the monobanders is minimal. After endless NEC computer modeling and testing runs the final Spiderbeam® yagi dimensions evolved. They have negligible interaction, resulting in near-monoband performance on each band.
2. Uncritical design:
Special attention has been paid to come up with a forgiving design. The spacing of the wire elements is not critical, which is quite an important point: this antenna will not only have good performance in the ideal world of a computer model. It will also perform equally well in real life (where it may bend and flex in high winds) - even when it is put up "quick and dirty" on a DXpedition (where nobody has time to tune and prune a critical system). The Spiderbeam users only need to pay attention during the very first set-up, to make sure the wire elements are cut exactly to the given dimensions. Once this job is completed carefully, the antenna will always perform well, and the repeatability is very good. Note that on the "assembled" versions sold by Spiderbeam® all the elements are precision pre-cut for you.
3. Feeding system:
Another challenge with most multiband antennas is the feeding system. A very simple and robust solution could be found here. The 3 driven elements are 3 separate dipoles that are all tied together in one common feedpoint. The trick is to space the dipole centers apart and use short pieces of symmetric transmission line to interconnect them. This minimizes the interaction and results in a very forgiving and broadband, low loss multiband dipole. The feed point impedance is 50 Ω, fed directly through a W1JR type current choke balun. No phasing lines or lossy matching devices to worry about. A single coax cable can be used for feeding up to 5 bands without problem.
4. Corrosion protection:
The driven elements and short pieces of symmetric feedline are manufactured in once piece, leaving no electrical joints open for corrosion. The same is true for all reflector and director wire elements. All active parts of the antenna are protected by a very tough PE coating, protecting them against the environment for many years. There are no corrosion problems with such a wire antenna. This is a huge benefit over aluminum antennas (especially if traps are involved), whose performance can suffer greatly once corrosion starts to set in.
5. Mechanical Strength:
The fiberglass tubes will take a lot of beating, since they are very flexible. The antenna will flex in the wind but it will not break. Remember a strong and flexible material will often outlive a brittle and rigid material. This is especially true for the reinforced fiberglass tubes which are used in the heavy duty version - these are very strong tubes but still highly flexible. Another trick is the extensive guying with Kevlar lines. Each spreader is guyed 4 times (up/down/left/right) - a concept very well known from sailboat masts. Of course all other parts used in the kit are UV ray and weather resistant. For the wire elements, we use top quality "CQ-532" wire from the Wireman in our antenna kits and recommend it to all homebrew users as well. This wire will not stretch at all, which is really important. Otherwise the resonant frequencies of the parasitic elements may change and antenna performance is lost.
6. Optimized for portable operation:
The mechanical construction was carefully optimized for portable installation on lightweight push-up masts. The special mounting plate ensures that the mast goes right through the antenna center of gravity, instead of putting the antenna on the side of the mast. Antenna weight and vertical torque momentum are optimally distributed on the mast and rotator, greatly reducing the load on these parts and making extending or retracting the push-up mast much easier. A great variety of mast diameters can be used (30-60mm) and nearly no tools are necessary. The spreaders are made from 20 identical fiberglass tube segments - introducing some redundance and greatly improving repeatability when compared to telescopic tubing. The wire elements and balun are mounted with Velcro straps - a very quick and surprisingly strong method, at the same time maintaining the antenna's flexibility - and even serving as a nice and soft stress relief device. (In the heavy heavy duty permanent version, the Velcro is replaced by V2A stainless steel clamps with rubber padding).
SPECIFICATIONS
Specifications for Spiderbeam® yagis. Includes SWR curves, forward gain, F/B ratio for all covered bands. 7 page .pdf document opens in a separate browser window. Click here.
General specs for all versions of the antenna:
Power handling capability 1500 watts SSB/CW.
Turning radius 16 ft. The antenna is built on a 33 ft x 33 ft fiberglass cross.
Feedpoint impedance 50 ohms. Antenna is fed with a single feedline. The 40m dipole add on kit can be fed with the same feedline or a separate one.
Windload area 3.8 sq ft.
Weight portable version, 13 lbs. HD version, 26 lbs.
Transport size: Can be broken down into a box 47 x 10 x 8 inches.
The difference between the HD and portable yagi models is the HD version uses double wall thickness fiberglass segments and the elements are connected to the cross with rubber padded clamps. The portable version uses fiberglass that is half the thickness of the HD version and the elements are connected to the cross using Velcro straps. The HD version is designed for permanent installations but can also be used portable.
Assembled vs. kit antennas and construction time: Spiderbeam® yagis are offered as a kit or in a factory-assembled version. With the factory-assembled version all of the one-time tasks including the cutting of all wire elements, guy line cutting, knot-tying and the construction of the balun and driven elements is pre-fabricated. Only the final assembly of the antenna is left to do before putting it in the air. Estimated build time for the kit is 15 hours. Build time for the assembled version is about 2 hours.
Spiderbeam® yagi construction manual. 37 page .pdf document opens in a separate browser window. Notes on manual: for the factory-assembled version of the antenna where all one-time tasks like wire cutting and knot-tying are done by Spiderbeam®, the final assembly by the end user is done starting at section 3 on page 18. The manual references the need to cut metal and drill holes for some steps of the kit; these are now done pre-fabricated even for the kit version. Example on page 6 for the center hub - parts like this are are pre-drilled before delivery. This is a carryover from the early days of Spiderbeam® when the antenna was more of a kit project than it is presently. There are small info boxes on various pages indicating no need to cut or form metal parts. Click here.
Illustrated packing kit. 2 page .pdf document opens in separate browser window. Click here.
A 40m dipole add-on kit was developed for the WRTC-2018 held in Germany. This is available in separate versions for portable or HD antennas and is available as a kit or pre-assembled. It may be added on to most existing Spiderbeam® yagi antennas, or ordered together with a new antenna. The dipole is rotated 90 degrees to the other wire elements. Both sides of the boom are extended by one additional fiberglass tube, increasing the turning radius from 5 meters (16 ft.) to 6 meters (19.2 ft.). Each end of the dipole is capacity-loaded using two thin wires tied back to the side spreaders. It can be connected with the existing yagi balun and feedline or with a separate balun (sold as a separate item). When using only one balun, the forward gain and front-to-back ratio for the higher bands are not affected, but the front-to-side ratio is reduced, especially on 15m.
40m add-on dipole kit construction manual. Click here. 8 page .pdf document opens in a separate browser window.
40m add-on dipole packing kit and SWR curve data. Click here. 2 page .pdf document opens in a separate browser window.