Try Try Again – New WI-001L Bruce Array in Construction 8 - May - 2008Posted by k9zw in Amateur Radio, K9ZW Learned, US Islands.
Tags: Bruce Array, K9ZW, W9EVT, Washington Island, WI-001L, WI9DX
On Washington Island WI-001L two Bruce Arrays have met their fate. The first was lost to groundskeepers who damaged it beyond reasonable repair with power equipement.
The replacement was lost when one of the support towers suddenly collapsed, as its crank-up cable perished from concealed damage.
A Third is in Modeling – with NEC2 models of various exact measurements suggesting a number of tweaks that should make “Third Time Lucky” a reality.
One issue in Number two was an unexpected difficulty in bringing the antenna into resonance exactly where we expected until a considerable amount of effort was made adjusting tuning stubs.
Models suggest that the feed point impedance was greatly higher than planned for, which likely exacerbated the “twitchiness” in tuning that version (Link to Article on Tuning that Array).
Bob Rumsey, KZ5R’s Balun Designs LLC #6113 6:1 Current Balun has been ordered for the feed point duties, after antenna modeling indicated that the antenna impedance is actually in the 300-325 Ω range, rather than the 200 Ω range assumed.
A Number of Design parameters are being used in doing the redesign and modeling:
- Supports are 70 (max) tall at tie-off point.
- Supports are 125 feet apart.
- Target Frequency is 40m SSB Net & DX frequency range
- Feedline will be Coax to reduce interference & interaction with other feedlines & existing ladder line.
- A 10 foot headroom clearance under the antenna would be desirable.
- The Supports are Crank-Up Towers, so the antenna must allow for their movement.
- Design that would prevent the antenna being in the snow or on the ground when the towers are lowered would be a plus.
- The Antenna will be built in warehouse space in Manitowoc, and must be transportable to Washington Island WI-001L with little fuss. (Link to Build Article)
- An easier method of adjusting resonance & element lengths is highly desirable.
- Once on the Island it must be simple to install.
It seems appropriate that this array is called a “Bruce Array” given the legend about “Try Try & Try Again” surrounding the ancient “Bruce” namesake Robert Bruce of Scotland:
According to legend, at some point while he was on the run during the winter of 1305-06, [Robert] Bruce hid himself in a cave on the east coast of Rathlin Island, where he observed a spider trying to spin a web. Each time the spider failed, it simply started all over again. Inspired by this, Bruce returned to inflict a series of defeats on the English, thus winning him more supporters and eventual victory. The story serves to explain the maxim: “if at first you don’t succeed, try try again.”
There is a lot of differing information published on Bruce Arrays. The Application of Wire Velocity Factors vs. Open Air factors is one discrepancy between various sources that would change the length of a 40m Bruce Array by 4m in length alone! In one case a 5% shortening, specially of the end elements, is suggested, while other sources add 5% to the direct length calculations, calling out an “array factor.”
Given that each of the previous two Bruce Arrays needed wire extensions to come in-band I will be studying this one carefully!
Should have some new models and figures to post for comments right after the Dayton Hamvention.