Saturday, November 17, 2012

"Balanced Line" Franklin Collinear Arrays


"Balanced Line" Franklin Collinear Arrays

The Franklin Array exhibits good gain (~4.5 Db), and delivers better than average signals, even when hung at only a quarter wavelength (low). This is possible since the array has no vertical component or vertical directivity, while capture area, and horizontal directivity and gain increase substantially with the 4-element collinear configuration. The MFJ-64xx arrays use fully balanced tuners (MFJ-974H, MFJ-974, and MFJ-971 series) to apply power to the array, and match the system to coax input. Figure 1 illustrates a Franklin Array.
Dimension "L1" represents a ½ wave, "L2" represents slightly less than ½ wave, and "F" a ¼ wave, which, in this case is a ¼ wave stub, used for phase-shifting. The antenna is a "broadside array," and is bi-directional perpendicular to the run of the wire.
A pair of these antennas at right angles (NE/SW; NW/SE for example) can provide world-wide coverage when used with a suitable antenna switching system. The added gain means your array presents an effective power gain of "3". This means 100 watts applied results in the effective radiated power of ~300 watts! What if you apply 1500 watts out? Pretty good results for "hanging wire!"
If height is a constraint, this antenna functions better than average at even a bit less than a ¼ wavelength above ground/structure. However, it really performs best if "hung" at a ¼ to ½ wave; up to ¾ wave. Higher than that provides diminishing return. Once fully balanced tuned, you'll find the antenna presents a low SWR across the entire band. However, it is a "one-band antenna" functioning as a Franklin Array; other bands provide varying patterns when tuned and matched.
Especially important is keeping the symmetry of the antenna in reference to its ½ wave radiators. They must not sag more than a foot or so, with pulled-tight being the best installation. The feedline/stub can be "bent" at the bottom, or pulled away at an angle, and some twisting is OK. Practically, it is best to let the stubs/feeder hang straight down and tie them off with rope to avoid lashing about in windy conditions.
Figure 1
Franklin Array (Four Half-Waves Collinear/In-Phase)
To summarize, these antennas are intended for single-band use (your favorite):
  • 15 meters (92 feet long; 12 feet high)
  • 17 meters (108 feet long; 13.5 feet high)
  • 20 meters (136 feet long; 16 feet high)
They require two tie-off points spaced far enough to accomodate their width and length, and a height at least close to ¼ wavelength. Note that these arrays "hang and play" in about same distance taken by an 80 meter doublet. When using two antennas, try to locate the antennas in an "L" or "T" ("X") configuration (at right angles) as illustrated in Figure 2.
(Looking Down From Above)
Figure 2
Two Antennas @ Right Angles (Suggested Optimum)
We hope you will benefit from their ease of installation, efficiency, and quality construction.

10 comments:

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