X-Beams

KQ6RH

(C) 1998, 1999, 2000

Ray Jurgens

(Up-Dated 2/25/2000) 

X-Beams

  The X-Beam has been described in QST and the ARRL Antenna Compendium # 1 in enough detail that it should not be difficult to assemble this antenna and obtain the specifications indicated here, though the specifications claimed by the author may be a little over-stated (see reference 1). The claim is that the antenna has 6.0 dBd and 18 dB F/B ratio. The figures below indicate the reality of the situation based on the E&M modeling program. I present both the free space patterns and the ground mounted pattern for 20' height.

Note that the total gain is 6.4 dBi, not 6.0 dBd, and although the F/B ratio is about 13 dB, the side lobes are much worse (about 8 dB). The performance of the antenna looks a bit better when placed about 20 feet above the ground as indicated by the following two figures.

Note that the gain peaks at a 30 degree launch angle with a peak gain of about 10 dBi or 8 dBd. So this is a fairly good antenna, but not quite as good as the Author indicates. The poor side-lobe level is perhaps the greatest deficiency of the X-Beam, however, it is relatively easy to construct, and portable versions based on telescoping sections of aluminum tubing are easy to transport and erect.

In this case, I'll only consider the design of a 15 meter version, because the 20 meter version requires aluminum spreaders 13' 9" long, and this length is larger than I can construct in my back yard, and the tubing size required is considerably larger than what can be considered as light weight and portable. The spreader (arm) length for 15 meters is only 9' 3" . The director tails are 4' 2", and driven-element tails are 4' 10". The sum of the tails is 9', and the distance between the tips is sqrt(2) * 9' 3" = 13' 1", which leaves 4' 1" to be spanned by nylon line or cord. You will also need 13' 1" lengths of Nylon cord to span the tips in the front and rear to prevent them from pulling apart when the tails are added. The spreaders (arms) can be assembled from three standard sizes of aluminum tubing beginning with 1/2" OD #22 which has an ID of 0.444 inches. This 1/2" will accommodate 7/16" OD #18 tubing which has an ID of 0.339 inches. The 7/16" tubing will then accommodate 5/16" #20 tubing. I used 4' of 1/2", 4' of 7/16" and 2' of 5/16" tubing for each spreader arm with the first two overlapped by 6" and the outer one overlapped by 4". The hub adds an extra inch from the center of the mast giving a total distance to the tip of exactly 9' 3". The author never states wire size for the tails in the ST article, but in the Antenna Compendium article, he indicates #19 vinyl-covered wire was used. We used #20 vinyl-covered lamp cord by splitting it apart. Various light weight speaker cables work equally well. The directions for tuning this antenna are given in the pair of articles and should be followed in order to get best results. My experience with this antenna mounted about 18' above the ground is that the driving impedance is roughly 25 Ohms when properly tuned. The author indicates a good match with 50 Ohm coax, which probably means that some compromise in performance was accepted to get the good match. The driving impedance does vary with height above the ground.

Parts Required

 Discussed above.

References

1. Anderson, Brice, W9PNE, "Designing X-Beams," ARRL Antenna Compendium, Vol. 1, pps. 64-66, 1985.

2. Anderson, Brice, W9PNE, "Horizontal X beams for 15 and 20 Meters," ST, pps. 33-35, March, 1993.

3. Note, all modeling of this antenna has been carried out using Brian Beesley's AO program.

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