1) "BEST" Interconnect between two Identical Antennas would be either the Optimized HVH (Holl_ands Vertical Harness) or HHH (Holl_ands Horizontal Harness) which OPTIMIZES the fol. SIX Independent SYmbol Variables: a) Separation between the Harness Wires (determines Feedline Impedance), b) Distance of Harness from Active Elements (e.g. Bowties) and c) Total Harness Length [actually SUM of 2 Horizontal pieces and 2 Diagonals connecting Harness to Active Elements, which are INDIVIDUALLY Optimized in all except some early versions). NO RF Combiner Loss and Optimizes both Impedance and Phase Matching. Indeed ALL Interconnect Losses are accounted for, incl. SWR Mis-match and (negligible) Ohmic Losses in the Harness Transmission Line:
2) Second "BEST" is HHH or HVH, except with some sort of CONSTRAINT on the Design, such as forcing Horizontal pieces on either side of Common Feedpoint to be EQUAL, resulting in only FOUR independent SYmbol Variables (as I did in some of my early Designs).
3) Third "BEST" is HHH or HVH, except with even MORE CONSTRAINTS on the Design, such as Alternative (b) in the fol. Vertically Stacked CM4221 [old] webpage, where the Vertical Length in the Horizontal Harness was CONSTRAINED to be the SAME as the Vertical Stacking Distance and the Total Harness Length was determined by Optimizing ONLY the Distance of the Harness from the Active Elements. And of course, the Harness Separation determined the Transmission Line Impedance, while the Tapered connections to the Active Elements provided at least SOME Impedance Transformation. [This was one of my earliest KISS Designs with only TWO Independent SYmbol Variables.]
UHF Vert-Stack CM4221 4-Bays - OPT
Additional comparisons for mclapp's M4, incl. Performance vs Vertical Stacking Distance:
4) Fourth "BEST" is Equal-Length Transmission Lines connected to a Common Feedpoint, such as Alternative (a) in the Vertically Stacked CM4221 [old] webpage. Results are for 300-ohm Twin-Lead, although as noted, 440/450-ohm Ladder Line [as well as 600-ohm OPEN-LINE] would provides lower SWR [turns out that Impedance value is NOT critical]. Since there is only ONE SYmbol Variable [Length] for a given choice of Impedance, performance was significantly LESS than the above alternatives. Since Twin-Lead without an overall Shield has high loss when damp/wet/iced-over it it NOT recommended for Outdoor use.....and since Ladder-Line employs Plastic Insulator with perhaps 40-50% Air Cut-Outs, it only provides moderate improvement for the same problem. BTW: Instructions for DIY 300-ohm (or whatever) OPEN-LINE Interconnects can be found here:
Home-Brew Parallel Transmission Lines
[I'm still looking for the link to a particular Cable Stable P/N that was well suited for this purpose.....]
5) WORST is anything with an RF Combiner [typ. 0.5 to 1.5+ Loss, more on Higher Freqs].....which has not only Ohmic Internal Losses [see mrf Spec Sheets], but ALSO significant Amplitude and Phase Mis-Match Losses [which are dissipated by the Internal Resistor on the fourth Port of the RF Combiner's Hybrid Transformer], which can be seen in measurements at www.antennashacks.com
ALSO, bear in mind that these measurements are for PERFECT 75-ohm Input/Output Terminations....whereas in the REAL WORLD, there will be additional, unknown Losses due to Impedance Mismatch to the Antennas and whatever Amplitude/Phase Mis-Match there may be between the two Antennas, esp. if Elements are damaged or Balun and Connections degraded.