Spiderbeam Basics

Für alle die etwas mehr über die Antenne erfahren möchten gibt der folgende Artikel einen Überblick über die gesamte, dem Spiderbeam Design zugrunde liegende Antennentheorie. Er wurde im Hinblick auf die 3-Band Version (20-15-10m) geschrieben, lässt sich aber problemlos auf alle Konfigurationen anwenden. Er kann ebenfalls als eine Geschichte der Verbesserungen und Erfahrungen gelesen werden, die während der Evolution des Spiderbeams gemacht wurden.

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:



Die so gewonnene Antennenform kann mit Drähten auf einem Tragekreuz aufgespannt werden, woraus sich die Leichtbauweise ergibt. Da die Enden der auf diese Weise abgeknickten Elemente recht nahe beieinanderliegen, tritt ein weiterer positiven Effekt auf: Die wegen des kurzen Abstands auftretende kapazitive/induktive Verkopplung wirkt sich positiv auf das Vor/Rück-Verhältnis und die Bandbreite der Antenne aus. Dick Bird, G4ZU kam als erster auf die Idee, die Elemente einer Yagi auf diese Weise zu verbiegen und nannte die entstandene Antenne "Bow-and-Arrow Yagi" oder "Bird-Yagi". Nach meiner Kenntnis entwickelte er nur Monoband Yagis dieser Art

Der Spiderbeam ist eine Multiband Antenne.
Drei Monoband "Bow-and-Arrow Yagis" sind auf dem selben Tragekreuz ineinander verschachtelt aufgebaut:


With such a design, several challenges mut 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 spider beam 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.

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 simply are no corrosion problems with such a wire antenna! This is a huge benefit over aluminium 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. Otherwhise the resonant frequencies of the parasitic elements may change, and the good 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 (only two #10 spanners). 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 clamps with a rubber padding).



CONCLUSION:
The spider beam is a trapless multiband yagi constructed of 3 interlaced monobanders, making it a highly efficient antenna and 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 in 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.

 

Compare the diagrams and tables in the technical data section. You will find that the spider beam is always equivalent or better than a big conventional tribander with about 7-8m (25ft) boom length. Especially the operating bandwidth is much better than with trap designs:

• forward gain is about 5dBd (7dBi) in free space (= 12dBi in 10m height above ground) and stays nearly constant over the whole band(s)
• F/B ratio is about 20dB or better on all bands
• SWR stays below 1:2 on all bands

4. Similar antenna designs

5. Antenna Measurements and Test