Channel Master CM4221 4-Bay (8.5x8.9) Bowtie Antenna (with Reflector Screen) analyzed using 4nec2. Measurements verified against a photo posted by mclapp: http://www.frontiernet.net/~mclapp/Antennas/CM4221/CM4221%20front.jpg
BowLength = 8.5-in, Bowtie Spacing = 8.9-in, Tine Separation = 5.25-in. All AWG8. Feedline Separation = 1.4-in, with 0.8-in at Crossover. All AWG8 wire size (0.125" O.D.). Center of Crossovers 2.75-in from Center of Outer Bowties. Screen Reflector (36"H x 20"W). Separation from Bowties = 4.5-in.
Gain increases from 12.2 to 14.9 dBi at 698 MHz. SWR is mostly under 2.0, climbing as high as 3.3 at 470 MHz, which results in minor Net Gain loss at Low Freqs.
If the Crossovers were moved from their current location (2.75-in away from Outer Bowties), so that they are Centered between the Bowties (as is done in some UTube 4-Bays), the Gain at the highest frequencies is significantly reduced with about a 1 dB loss in F/B Ratio. The SWR at Low Freqs also increases, resulting in an additional 1 dB Net Gain loss at 470 MHz.
Results for Ken Nist's CM4221a.ez file are also included in the comparison Charts, showing minor Raw Gain loss vs the ACTUAL CM4221, higher SWR (reducing Net Gain at low freqs) and a higher F/B Ratio.
Date(s): 27Aug2009. Album by holl_ands. 1 - 17 of 17 Total. 5676 Visits.
enlarge 0KB, 1024x696 1 CM4221 4-Bay Gain Chart: IF Crossovers were Centered between Bowties, Gain is significantly reduced (esp for Ch57-69) and F/B Ratio is reduced by about 1 dB.
enlarge 0KB, 1024x696 2 CM4221 4-Bay SWR Chart: In Actual CM4221, Crossovers are much closer to the Outer Bowties.
enlarge 102KB, 791x1024 3 CM4221 Actual w Reflector - 4nec2 File SYmbols make it easy to vary parameters.
enlarge 0KB, 598x1024 4 CM4221 Actual - 3D View
enlarge 0KB, 629x1024 5 CM4221 Actual - Front View [One small box = 0.5 inch]
enlarge 0KB, 391x1024 6 CM4221 Actual - Top View [One small box = 0.5 inch]
enlarge 0KB, 1024x819 7 CM4221 Actual - UHF Raw Gain
enlarge 0KB, 1024x819 8 CM4221 Actual - UHF SWR (300-ohm)
enlarge 0KB, 1024x819 9 CM4221 Actual - UHF Impedance
enlarge 0KB, 1024x819 10 CM4221 Actual - UHF Raw Gain IF Crossovers were Centered between Bowties
enlarge 0KB, 1024x819 11 CM4221 Actual - UHF SWR (300-ohm) IF Crossovers were Centered between Bowties
enlarge 0KB, 1024x819 12 CM4221 Actual - UHF Impedance IF Crossovers were Centered between Bowties
enlarge 0KB, 1024x819 13 CM4221 Actual - Hi-VHF Raw Gain TOWARDS THE REAR DIRECTION
At Hi-VHF there is MORE GAIN towards the Rear
enlarge 0KB, 1024x819 14 CM4221 Actual - Hi-VHF Raw Gain TOWARDS THE FORWARD DIRECTION
enlarge 0KB, 1024x819 15 CM4221 Actual - Hi-VHF SWR (300-ohm) Excessive SWR, although Ch9-11 are lowest
enlarge 0KB, 1024x819 16 CM4221 Actual - Hi-VHF Impedance
enlarge 0KB, 293x1024 17 CM4221 Actual - Side View [One small box = 0.5 inch]