New CM4228HD (As Shipped) analyzed using 4nec2. I obtained measurements directly from a recently delivered antenna. Note that Ken Nist (of HDTVPrimer) determined that the Printed Circuit Board (PCB) Balun ATTENUATED Hi-VHF Gain by 3 dB, which is NOT included in below results. This can be avoided if the PCB Balun is replaced by a conventional 4:1 Balun Transformer.
Choice of AutoSegmentation value (typ. 7 or 11) was very critical to finding a "workable" solution and was different for UHF & Hi-VHF.
BowLength = 8.0-in, Bowtie Spacing = 8.0-in, Tine Separation = 4.75-in. Approx. AWG6 wire size (0.157" O.D.). Feedline Separation = 1.3-in, with 0.5-in Hop at Crossover. AWG9 wire size (0.118" O.D.). Center of Crossovers 1.5-in from Center of Outer Bowties. Left to Right Bay Separation = 20-in. 16 each AWG4 (0.2-in O.D.), 40-in long Reflector Rods, at 4-in separation (32-in Total Height). Separation from Bowties = 4.4-in.
UHF Gain is about 14 dBi on most channels, dropping to 10.5 dBi on Ch14. Excellent SWR under 2.0. Hi-VHF Gain is very uneven with Excessive SWR on most channels, but remains pointed in the Forward direction.
Charts are included comparing CM4228HD (As Shipped) to older CM4228, as well as comparisons to mclapp's M4 Super-Sized 4-Bay and CM4228/CM4228HD with & w/o RF Combiner Mod.
New CM4228HD (As Shipped) was much better than old CM4228 on the (problematic) lowest channels, but had 1-2 dB lower Gain above Ch30. New CM4228HD was actually somewhat worst than mclapp M4 Super-4-Bay, esp on lowest channels.
Sweeping the TIPS of the Bowties directly forward by (optimum) 1-inch provided only up to 1 dB of Gain improvement at the expense of up to 2 dB of reduction in F_B and F_R Ratios. So might not be worth it.
Modifying the vertical Tine Separation between the TIPS of the Bowties by (optimum) of 2-inch separation resulted in only up to 1 dB Gain increase at mid and high channels and up to 3 dB on lowest channels. On-Air Tests are needed to verify.
Combiner Mod for New CM4228HD compared to "As Shipped" to illustrate how much is lost in the horizontal interconnect. If a low loss RF Combiner Module is used (not an easy task), more than 2 dB of improvement MIGHT be possible.... Bear in mind that an RF Combiner Module will have AT LEAST 0.5 dB Loss (and likely more on higher freqs) not shown in the charts, see actual RF Splitter/Combiner measurements at www.antennahacks.com (back-to-back tests, so divide by 2).
NOTE: The Autosegment feature of 4nec2 was used to determine how many wire segments to use when modeling each wire element. The choice of Autosegment=11 was very critical in getting the CM-4228HD Gain and SWR calculations to work AT ALL. And small changes to Autosegment resulted in large changes in Hi-VHF Gain & SWR....so Hi-VHF results might not be all that accurate.
EDIT (29Mar2013): Added (Slides 16-21) ANOMALOUS plots versus Bowtie Tine Separation and Bowtie Forward Sweep, which can frustrate finding an "Optimum" set of dimensions. I believe that this is a 4nec2 Modeling anomaly, rather than inherent in CM4228HD design. There's NO WAY Whisker movements of a couple thousandths of an inch result in BIG Gain changes.
Also added (Slide 39) ANOMALOUS Small Perturbation Test Results showing Raw Gain varies as much as +/- 1.0 dB when Bowtie Tine Separation and Bowtie Forward Sweep are Randomly Perturbed by up to +/- 0.25-in, using EXCEL's 0.0001*RANDBETWEEN(-2500,2500) function to generate variables. Indicates that Raw Gain at 470 MHz may actually be closer to 13.0 dBi than 10.5 dBi found in "unperturbed" 4nec2 Model.
Date(s): 28 Aug 2010. Album by holl_ands. 1 - 45 of 45 Total. 14855 Visits.
enlarge 226KB, 1024x696 1 Comparing CM4228HD vs older CM4228 vs mclapp M4 UHF Raw Gain & Net Gain
enlarge 279KB, 1024x696 2 Comparing CM4228HD vs older CM4228 vs mclapp M4 With and without RF Combiner Mod UHF Raw Gain & Net Gain
enlarge 272KB, 1024x696 3 Comparing CM4228HD vs older CM4228 vs mclapp M4 With and without RF Combiner Mod UHF Front/Rear and Front/Back Ratios
enlarge 233KB, 1024x696 4 CM4228HD: If Sweep Bows FORWARD or BACK UHF Raw Gain & Net Gain
Sweeping FORWARD 1-in improves lowest channels and highest channels by about 1 dB.
enlarge 266KB, 1024x696 5 CM4228HD: If Sweep Bows FORWARD or BACK UHF F/R & F/B Ratios
Sweeping FORWARD 1-in results in small degradation to F/R & F/B Ratios.
enlarge 414KB, 1024x768 6 CM4228HD Hanging From Pergola NOTE re Horizontal Interconnect Harness: Rivets drilled out and replaced by 1-in #8 Screws in preparation for connecting RF Combiner Mod.
enlarge 243KB, 1024x696 7 CM4228HD: If Modify Whisker Tine Separation UHF Raw Gain & Net Gain
Closing Tine Separation to 2.0-in improves Gain on mid and high channels by up to 1 dB and up to 3 dB on the lowest channels.
Hello. I note on the page at back-to-back tests, so divide by 2, and on this page I'm commenting on, you've cited my combiner comparison with the comment "back-to-back tests, so divide by 2". Would you please contact me? I'd like to understand what you mean by this. Thanks. - Tom | antennahacks.com, Mon, 8 Feb 2016 1:20PM