UHF+Hi-VHF Vertical Stack 2 ea Free-Form 4-Bay (FF4) with Double Angle Reflector (48" H x 36" W using 1x2-in Grid) were analyzed using 4nec2. FF4 dimensions are same as found for Optimized version with Double Angle Reflector.
Many times Multipath results in signal strength varying with height in alternating Peaks and Nulls. Zss = Vertical Stacking Separation between Antennas (Feedpoint-to-Feedpoint) was chosen so that when Nulls form across the Bowties on one Antenna, hopefully Peaks will form across the Bowties on the other.
Two versions are presented here, after using nikiml's Python Optimization Scripts to determine "best" Stacking Distance (coincides with "best" Anti-Multipath) and Interconnection Parameters.
a) Two Equal Length 450-ohm Transmission Lines connect to a common Feedpoint (Balun): Dimensions: Zss=59-in so Zmm=11-in (Reflector Metal-Metal) and TL Length=39-in each. [Open Ladder type presumed. If "Windowed" Ladder with intermittent PLASTIC spaces, then there can be High Loss when WET...which is NOT Included in these Calculations.]
UHF Raw Gain = 17.5 to 19.9 dBi, F/B & F/R Ratio Min = 15.2 dB & SWR (300-ohms) under 2.5. Hi-VHF Raw Gain = 14.1 to 11.3 dBi, F/B & F/R Min=10.8 dB, although SWR (300-ohms) = 42 to 21 is EXCESSIVE, which may or may NOT be an issue in any particular situation.
I also ran Optimization runs for 300-ohm Twinlead, which resulted in nearly the SAME Gain, F/B and F/R Ratio curves. However, SWR increased to as high as 4.1 at 698 MHz and mid-band. So I do NOT recommend this alternative. FYI: Length of each Twinlead would be about 35-in, where different lengths will move the double-humped SWR response curve to somewhat different frequencies.
More info re "Windowed" and "Open Ladder" Balanced Transmission Lines (latter is weather resistant): http://thewireman.com/antennap.html http://www.arrg.us/pages/Loss-Calc.htm [Note Wet Loss for WM-551, 552, 553 & 554] DIY PROJECT: http://www.qsl.net/co8tw/openline.htm
"Windowed" 450-ohm Twinlead is commonly used with Amateur Radio Antennas, but is susceptible to significantly higher Loss in UHF Band when wet. However, "Open Ladder" 440-ohm transmission lines are NOT affected by rain. DIY Ladderline instructions are cited above, but Vertical Harness version would be easier to construct and doesn't require any additional physical supports. Note that although there is a very minor performance difference between 2xTL and VH versions, presumably "better" 2xTL version does NOT take into account degradation from Ladderlines in close proximity to antennas, so in reality they are probably about the SAME.
Hence Vertical Harness version is preferred:
####################################################### b) The second was Optimized using a Vertical Harness between two FF4's, where distance between Harness conductors and Total Length was searched to find "best" values: Dimensions: VHw (FL Width)=1.0-in, VHp=3.25-in (Diagonally = 3.375-in), Feedlines=29.5-in, Zss=60-in so Zmm=12-in (Reflector Metal-Metal).
UHF Raw Gain = 17.1 to 19.7 dBi, F/B & F/R Ratio Min = 14.3 dB and SWR (300-ohms) under 2.5. Hi-VHF Raw Gain = 14.2 to 11.0 dBi, F/B & F/R Ratio Min = 10.0 dB although SWR (300-ohms) = 73 to 19 is EXCESSIVE.
Date(s): 21 Apr 2014. Album by holl_ands. 1 - 48 of 48 Total. 4325 Visits.
enlarge 180KB, 1024x675 7 Vert-Stack FF4 + DoubleAngleRefl 2 Equal Length Transmission Lines UHF Raw Gain = 17.5 to 19.9 dBi UHF F/B & F/R Ratio Min = 15.2 dB
enlarge 187KB, 1024x675 19 Vert-Stack FF4 + DoubleAngleRefl 2 Equal Length Transmission Lines Hi-VHF Raw Gain = 14.1 to 17.3 dBi Hi-VHF F/B & F/R Ratio Min = 10.8 dB
enlarge 172KB, 1024x675 31 Vert-Stack FF4 + DoubleAngleRefl Equal Width Vertical Harness UHF Raw Gain = 17.1 to 19.7 dBi UHF F/B & F/R Ratio Min = 14.3 dB
enlarge 174KB, 1024x675 43 Vert-Stack FF4 + DoubleAngleRefl Equal Width Vertical Harness Hi-VHF Raw Gain = 14.2 to 11.0 dBi Hi-VHF F/B & F/R Ratio Min = 10.0 dB
