In Ref. 1 it was shown that large-area substrates can be covered uniformly with dense plasma by using a plurality of short helicon sources.@footnote 1@ In this source use was made of the "low-field peak" (LFP), a density peak occurring near B = 50G which had been observed in several helicon devices.@footnote 2@ A very quiet, stable discharge could be obtained in the neighborhood of this peak.@footnote 3@ This feature was not predicted by standard helicon theory and was unexplained. In the latest version of the code HELIC developed by Arnush,@footnote 4@ it is possible to model a short helicon source bounded at one end. The LFP is produced by constructive interference by the reflected wave from an end plate near the antenna and not, for instance, by a resonance of the Trivelpiece-Gould mode. This mechanism can also explain previous observations@footnote 5@ of density enhancement by flaring magnetic fields or inserted blocks. This knowledge permits design of more compact helicon reactors.@FootnoteText@@footnote 1@F.F. Chen, J.D. Evans, and G.R. Tynan, Plasma Sources Sci. Technol. 10, 236 (2001).@footnote 2@ F.F. Chen, X. Jiang, J.D. Evans, G. Tynan, and D. Arnush, Plasma Phys. Control. Fusion 39, A411 (1997).@footnote 3@F.F. Chen, J. Vac. Sci. Technol. A 10, 1389 (1992).@footnote 4@D. Arnush, Phys. Plasmas 7, 3042 (2000).@footnote 5@G. Chevalier and F.F. Chen, J. Vac. Sci. Technol. A 11, 1165 (1993).