Channel Master CM4228a 8-Bay Bowtie with Screen Reflector analyzed using 4nec2. Three versions were considered: a) Ken Nist's CM4228a.ez.nec file, unmodified. [There are subtle differences wrt what I measured.] b) Modified modeling approach, replacing "Balun" with a simple SOURCE wire at the central feedpoint. c) Moved Horizontal Harness from between the 5-Bay stacks from between Screen Reflector and Bowties to IN FRONT of Bowties.
BowLength = 7.87-in, Bowtie Spacing = 8.0-in, Tine Separation = 5.5-in. Approx. AWG8-9 wire size (0.125" O.D.). Feedline Separation = 1.25-in, with 1.25-in Hop at Crossover. Approx. AWG8-9 wire size (0.125" O.D.). Center of Crossovers 4.8-in from Center of Outer Bowties. Left to Right Bay Separation = 20-in. Screen Reflector (36"H x 40"W), AWG12, 1"H x 2"W Grid. Separation from Bowties = 4.5-in.
On the lower UHF channels, Raw Gain falls off and SWR is much higher, resulting in poor Net Gain. This illustrates the need to rescale to a larger size antenna.
Hi-VHF Raw Gain in the FORWARD direction is moderate, except for notches at 180 MHz (between Ch7 & Ch8) and 205 MHz (Ch12). Note that on these channels, Raw Gain is improved in the REVERSE direction. SWR on Ch7 & Ch8 is also excessive, which could cause standing wave problems.
UPDATE (3 Dec 2011): Ken Nist's cm4228a.ez model mimics a Balun coming directly FORWARD from the feedpoint. This results in worst case SWR of 3.9. Since the Balun might be going backwards, or downwards, I eliminated it. SOURCE wire is directly across feedpoint, mimicing Balun wires with minimal length. Worst case SWR improved from 3.9 to 3.3, which is a fairly minor improvement.
UPDATE (3 Dec 2011): Analysis if Horizontal Harness is MOVED from between Screen Reflector and Bowties and to IN FRONT of Bowties. Worst case SWR improved from about 3.3 to about 2.8 and Gain was barely affected, with only few tenths of a dB loss on 470 MHz. Hence this modification is barely worth the effort.
Date(s): 17 Aug2009. Album by holl_ands. 1 - 45 of 45 Total. 6943 Visits.
enlarge 136KB, 663x1024 31 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties 3D View
enlarge 352KB, 1024x867 32 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Front View [1 big box = 5in]
enlarge 82KB, 259x1024 33 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Side View [1 big box = 2.5in]
enlarge 155KB, 1152x584 34 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties UHF Raw Gain Very small loss on 470 MHz
enlarge 174KB, 1152x584 35 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties UHF SWR (300-ohm) Under 2.8 Small improvement on low frequencies.
enlarge 152KB, 1152x584 36 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties UHF Impedance
enlarge 305KB, 1024x1024 37 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Azimuthal Pattern on 470 MHz
enlarge 302KB, 1024x1024 38 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Azimuthal Pattern on 590 MHz
enlarge 307KB, 1024x1024 39 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Azimuthal Pattern on 698 MHz
enlarge 303KB, 1024x1024 40 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Azimuthal Pattern on 758 MHz
enlarge 304KB, 1024x1024 41 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Azimuthal Pattern on 806 MHz
enlarge 75KB, 299x1024 42 Channel Master CM4228 (Nist) - 3D View Horizontal Harness MOVED IN-FRONT of Bowties Top View [1 big box = 5in]
enlarge 134KB, 791x1024 43 Channel Master cm4228a.ez.nec Simulated Balun Wires modeled in FRONT of Feedpoint. 4nec2 file converted from Ken Nist's EZNEC file
