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Channel Master 4228, old style - seriously modifying it??

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  • Channel Master 4228, old style - seriously modifying it??

    If anyone has ever dxed DTV with an old style Channel Master 4228 8 bay antenna, they know how well it performs.

    I run a CM 4228 old style for UHF DTV dxing. The 4228 also performs well on VHF 7-13. I've researched the different ways to tweak the 4228 so it performs the best possible. I've read that you can wrap 1/2" hardware screen across the metal reflector to improve rear signal rejection, while it supposedly improves the F/B ratio slightly. Here is a link to a picture of one antenna that a dxer did this to...


    http://www.avsforum.com/photopost/da...lectormod.jpeg

    I'm looking into doing this to my antenna and am curious if anyone else has tried this. Also, I have read different comments about the way the balun connects to the *two* antennas. Technically, the 4228 is two 4221s (four bays). Channel Master figured out a way to combine the two to make it a deep fringe UHF antenna. Some have said the way CM connected the separate bays isn't as efficient as it should be and actually loses some dB at the coax connection point.
    I'm curious if anyone has experimented with this type of antenna and modified the 300-75 ohm connection so that the dB loss would be as low as possible.

    Any suggestions greatly appreciated.
    Jim Thomas
    Springfield, MO EM37

    "Let's just plop them in front of the TV. I was raised in front of the TV and I turned out TV." - Homer Simpson


    Fall & Winter 2019 dx equipment
    Antennas - Antennacraft MXU59 UHF antenna & home-brew version of Antennacraft VHF Y-10-7-13 antenna @ 25'. Both antennas fed through a Channel Master 7777 30dB pre-amp.
    Tuners - Zenith DTT901 converter box; AirSpy HF+ sdr; Silicon Dust HD Homerun Dual ATSC tuner, using Rabbitears autologger support.

  • #2
    Jim, I have an old-style 4228 that I use to regularly receive UHF and VHF TV from stations 90 miles away. I too had read disparaging things about the feeder design. I modeled the antenna and got results that weren't so hot, which seemed to confirm what I had read. Before modifying anything, I decided to measure the antenna SWR. Any feeder problems should show up there. I connected the antenna to instruments in my RF lab and measured the return loss in the UHF-TV band. Return loss is equivalent to SWR and it's easy to convert one to the other. SWR was perfectly fine, contradicting both what I had read and my own model.

    I noticed that the elements were supported by a large amount of plastic or phenolic at their high-impedance ends. Unlike dipole centers, these points are highly susceptible to influence. I added very small capacitors to the model at these points in an attempt to account for the plastic. When I did, the model started to agree with my measurements. I could adjust the capacitor values to get good agreement. The final values were tiny, just a few tenths of a pF as I recall. I put the antenna in service without modification and it's worked fine for several years.

    I learned several things from this exercise. First, don't trust what you hear or read without verifying it yourself. Two, take small, second-order effects into account whenever possible. Third, it's easy to construct seemingly good but unrealistic antenna models at UHF.

    Brian

    Edit - I just looked at the photo of your antenna. One thing to be careful about are the balun leads. They are very long and they appear to pass through the rear screen. My 4228 came with the same kind of balun. The impedance of the leads for the spacing shown is greater than 300 ohms. Their length is quite significant at UHF. The leads are acting as a not-so-short transmission line of undesired impedance, which will adversely affect SWR. (I have modeled such leads at 100 MHz and they have an adverse effect even there. They are capable of really misbehaving at 600 MHz.) I did not use this balun when I measured my antenna. I can't quite remember now, but I believe I used a custom-built halfwave coaxial balun with very short leads. I'm not sure what proximity to the screen metal will do, but I wouldn't want to find out.

    I use a few feet of 300-ohm twin-lead that I drop vertically from the feedpoint parallel to and away from the screen. I have carefully dressed it in a symmetrical way to avoid coupling to anything. The twin-lead split at the feedpoint is as short as I could make it. Below the screen the twin-lead connects to a 300-ohm preamp. This avoids the large-geometry balun issues, screen-coupling issues, and balun loss. The latter can be a couple dB or more at UHF for a ferrite balun. I forget exactly what the preamp input circuit uses, but when I examined it I thought it would have lower loss than an external balun.

    P0008068.jpg

    Comment


    • #3
      The picture I linked to is another person's antenna, not mine. The extra screen goes on the back side of the factory screen and is cable tied to it. I have read good things about doing this, as mentioned above. I already have a CM 7777 preamp in the mix, so I'm not sure if I should just leave it the way it is or if I should be concerned about any type of signal loss at the balun.

      Here's something I have wondered about and am just about ready to take a chainsaw to some trees. I have some trees directly to the east of the 4228 that are actually in front of the antenna about 10' removed. I know what I've heard about tree leaves affecting DTV signals. I suppose trees being that close probably seriously hamper signals during the growing season. Those trees are on my property line, so I have a right to take a chainsaw to them and bring their tops down so the antenna has a clear view from the NE to the SE. My tower is 25' HAAT.

      Extra research pages I have consulted...

      http://www.hdtvprimer.com/ANTENNAS/cm4228.html

      http://www.hdtvprimer.com/antennas/temporarypage.html

      http://www.highdefforum.com/local-hd...available.html

      Edit - I posted this picture in another thread and am posting it here for reference. The 4228 is the top antenna, a custom made VHF 2-6 is the middle antenna, a Winegard UHF yagi is the bottom one.
      Attached Files
      Last edited by Jim Thomas; 09-03-2016, 09:16 PM. Reason: Xtra nfo
      Jim Thomas
      Springfield, MO EM37

      "Let's just plop them in front of the TV. I was raised in front of the TV and I turned out TV." - Homer Simpson


      Fall & Winter 2019 dx equipment
      Antennas - Antennacraft MXU59 UHF antenna & home-brew version of Antennacraft VHF Y-10-7-13 antenna @ 25'. Both antennas fed through a Channel Master 7777 30dB pre-amp.
      Tuners - Zenith DTT901 converter box; AirSpy HF+ sdr; Silicon Dust HD Homerun Dual ATSC tuner, using Rabbitears autologger support.

      Comment


      • #4
        I would try to obtain the lowest-loss balun possible, minimize the length of its leads, and arrange them so that they form a 300-ohm transmission line. A halfwave coaxial balun is easy to construct and will work over the entire UHF-TV band. It has the lowest loss of any balun. However, it will preclude using the antenna for VHF-TV.

        Brian

        Comment


        • #5
          The hdtvprimer info is where I first read about the 4228 problems. His antenna model SWR curves do not agree with my measurements of an actual antenna.

          Brian

          Comment


          • #6
            P0019040 (2).jpg

            P0019043 (2).jpg

            Jim, I finally located my halfwave coaxial UHF-TV balun. I constructed it from 93Ω coax (150Ω is optimal). 75Ω coax will work fine. I built this balun for a specific antenna, which determined the lug spacing. It's very important to minimize lead length at UHF. If you look closely you can see green on the copper shield. I had used silicone sealant to keep water from entering the coax. But the stuff I used contained acetic acid, which corroded the copper! So I removed it. If I ever use this balun, I'll find a sealant that won't corrode metal. The most difficult part of construction was carefully sealing it against water entry. It took a lot of fussing around before I was confident I had all entry paths covered.

            The only thing I might change on this balun is to substitute thicker wire for the connection to the center conductor. I can't shorten that wire, but making it thicker will lower its inductance. #14 wire wraps around the F-connector. That would also be a good choice for the center conductor.

            Brian

            Comment


            • #7
              Channel Master 4228 (old style)

              Originally posted by Jim Thomas View Post
              ... I'm curious if anyone has experimented with this type of antenna and modified the 300-75 ohm connection so that the dB loss would be as low as possible...
              Jim,

              Have experimented with the 4228. There are a few ways of tweaking it to obtain another dB or so on specific channels or groups of channels. Most mods help some channels and either don't help or reduce performance on other channels, however:

              Antennahacks describes a mod that may work fairly well.

              http://www.antennahacks.com/Hacks/NistHarness.htm

              Personally, have not tested this mod.

              Note added a bit later than original post:

              The above mod applies to the newer 4228HD. The objectives of the mod may still be relevant for the older 4228. The main objectives being better impedance match when combining the two sections, and possibly reduced interference between feed lines and driven elements.


              ------------------------------------

              In agreement with much of K6STI's comments about software models and performance. Seems many antenna models do not include factors that are very relevant at UHF and also relevant (but maybe less so) at VHF.

              ------------------------------------

              Have measured performance of many dozens of commercial baluns. They are all quite lossy at UHF (most are more than a dB loss). Probably, the better one is the Channel Master balun (CM 94444), loss at about 1 dB for UHF. The coaxial half-wave loop has lower loss.

              ------------------------------------

              SWR and performance of a 4228 and a few other antennas are compared here:

              http://forum.tvfool.com/showpost.php?p=55093&postcount=100 .

              .
              Last edited by tripelo; 09-04-2016, 04:38 PM. Reason: Added Note about 4228HD

              Comment


              • #8
                cm4228rl.jpg

                I wish I had kept notes when I tested my 4228. This is the only thing I can find. It's a shot of my spectrum analyzer screen when connected to measure return loss. I'm sure the vertical scale is 10 dB/div with 0 dB return loss at the top line. I'm guessing the horizontal scale is 20 MHz/div, but it may be 50 MHz/div. The center is probably 600 MHz. I remembered the SWR as being below 2, but the 8 dB return loss shown is equivalent to an SWR of 2.3. Return loss is mostly better than 10 dB, which is an SWR of 1.9. Peak SWR is greater than I would like, but the average looks pretty good. The problem is that I don't remember what balun I used and that can make quite a difference. I can't remember whether I adjusted the scope trace to account for the balun loss.

                I noticed that the two conductors of the 4228 phasing lines have unequal length due to the way the lines are bent. That may cause some unwanted signal pickup. I decided that it was too much trouble to modify the lines for the small amount of pickup I expected to occur.

                Brian

                Comment


                • #9
                  Question for Brian. Try modeling this - a 12 bay UHF antenna

                  Would it work or would it be an accident waiting to happen? And would it wreck the high VHF properties that come with the CM 4228? The reason I bring this up. I've seen some dxers take two of the 4228's and stack them or put them horizontal...so I happen to have a 4221 lying around (the four bay). Just curious if I were to take the 4228 and the 4221 and make a 42?? by making it a 12 bay antenna. Add new screen behind the factory screen, put some new angle iron for the horizontal support top and bottom, then re-orient the mast to be center. ??? I can see Brian now

                  Does it work or not? Do I get any added dB to the signal or am I wasting my time?
                  Jim Thomas
                  Springfield, MO EM37

                  "Let's just plop them in front of the TV. I was raised in front of the TV and I turned out TV." - Homer Simpson


                  Fall & Winter 2019 dx equipment
                  Antennas - Antennacraft MXU59 UHF antenna & home-brew version of Antennacraft VHF Y-10-7-13 antenna @ 25'. Both antennas fed through a Channel Master 7777 30dB pre-amp.
                  Tuners - Zenith DTT901 converter box; AirSpy HF+ sdr; Silicon Dust HD Homerun Dual ATSC tuner, using Rabbitears autologger support.

                  Comment


                  • #10
                    Gosh, I wish I had equipment like that to test my antenna.

                    Comment


                    • #11
                      Jim, bigger antennas work better, but they are more difficult to properly feed and to aim. As a general rule, I wouldn't bother doing anything less than doubling the size of an array. I also wouldn't bother adding more screen to a 4228. You'll increase weight and windload for uncertain benefit. Even with a solid reflector you'll get signals from the rear due to diffraction around the edges.

                      Brian

                      Comment


                      • #12
                        CM 4228/4221 kit bash

                        In the tone of Mr. Rogers, "Can you say kit bash? Go ahead, spell it very slowly. K-I-T B-A-S-H. Kit Bash." Yes you guys remember when you were teenagers putting together those model cars and model airplanes, you just couldn't do it the way the factory made the kit. You had to tweak something and make it bigger, better, badder! (ck spelling?) Is there anyone that does reruns of Tim Allen's Tool Tyme aka Home Improvement anymore?

                        Anyway, I told Brian K6STI I was seriously thinking about modifying my CM 4228, by combining it with a 4221, so it would become a 12 bay antenna. It happened today. Spent most of the day working on it. I pulled the cross-member rods from the 4228 (where it joins the two 4-bays for the 75 ohm feed) and made 3 separate 75 ohm feeds (30" length each cable), they feed into a 4x1 combiner (4th spot is my VHF 2-6 feed). Then the combiner feeds to the pre-amp, then into the HDHomerun tuner in the DX shack and a Zenith box in another room. In the mod I tried to pull the whiskers forward some, but I don't know if it really applies to the 4228/4221 scheme. I decided to not add any extra screen, because at 5' wide and 3' tall, this antenna is pretty heavy. It doesn't need any extra wind-load.

                        Conditions are really flat tonight, so can't tell how much it has improved, but I do know the dBi has improved across the UHF band. I wish K6STI could model what I did and tell me what the expected numbers would be.
                        Attached Files
                        Jim Thomas
                        Springfield, MO EM37

                        "Let's just plop them in front of the TV. I was raised in front of the TV and I turned out TV." - Homer Simpson


                        Fall & Winter 2019 dx equipment
                        Antennas - Antennacraft MXU59 UHF antenna & home-brew version of Antennacraft VHF Y-10-7-13 antenna @ 25'. Both antennas fed through a Channel Master 7777 30dB pre-amp.
                        Tuners - Zenith DTT901 converter box; AirSpy HF+ sdr; Silicon Dust HD Homerun Dual ATSC tuner, using Rabbitears autologger support.

                        Comment


                        • #13
                          Originally posted by Jim Thomas View Post
                          In the tone of Mr. Rogers, "Can you say kit bash? Go ahead, spell it very slowly. K-I-T B-A-S-H. Kit Bash."
                          In the tone of Big Bird, "this show brought to you by the letter R"......for RIGHT ON! It's going to be way cool to see the difference that made!

                          Comment


                          • #14
                            Tulsa OK channels in this morning @ 153 miles on an otherwise semi-flat dxing morning. We are on the back side of a cold front, so there is some H pressure enhancement going on anyway. My understanding of this kit bash redesign is that tropo signals become more stable and the antenna pulls out the weaker close in signals. Time will tell....
                            Jim Thomas
                            Springfield, MO EM37

                            "Let's just plop them in front of the TV. I was raised in front of the TV and I turned out TV." - Homer Simpson


                            Fall & Winter 2019 dx equipment
                            Antennas - Antennacraft MXU59 UHF antenna & home-brew version of Antennacraft VHF Y-10-7-13 antenna @ 25'. Both antennas fed through a Channel Master 7777 30dB pre-amp.
                            Tuners - Zenith DTT901 converter box; AirSpy HF+ sdr; Silicon Dust HD Homerun Dual ATSC tuner, using Rabbitears autologger support.

                            Comment


                            • #15
                              Jim,
                              When you combine a 4228 and a 4221, you get a CM 4229 (8+1) ! It looks good. How high is it mounted ? Tulsa can be seen in Arkoma,OK/Ft. Smith, everyday (110 mi) on a CM 3761 with CM 7777 at 32 ft. 153 is very good. Is it on a rotor?
                              You just don't know, IF something will work, until you try........

                              Gary H.
                              Okla.

                              Comment

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