MODIFYING CHANNEL MASTER'S CM3022/4308 UHF SUBURBAN YAGI
For Improved Reception
Channel Master's 4308 is a small suburban UHF yagi antenna. While this antenna does an adequate job of analog and digital UHF reception, it doesn't have enough gain above channel 40 for my needs. In the Philadelphia area, we have four DTV stations operating above channel 50, and this antenna wasn't cutting the mustard for reliable DTV reception of any of the four.
So, after some tinkering - and lots of paper calculations - I came up with a few modifications that turn this antenna into a much better performer. These modifications can be done by anyone with an electric drill, a vise, or pliers, tin snips or metal shears, a tape measure or ruler, and a standard tubing or pipe cutter.
ARTS AND CRAFTS TIME
Got your tools out? Here are the steps to follow:
(1) The CM 4308 is shipped assembled in one piece. The bowtie-shaped driven element needs to be swung into place. Do not attach the loose ends yet.
(2) Carefully swing open the reflector assembly, but do not swivel the reflector elements into place yet. Measure and mark the four reflector elements to a maximum length of 12" (6 " each side of the center of the swivel mount). Use the tubing cutter to remove the excess tubing. You can flatten/crimp the ends of each tube as before, if you are concerned about moisture build-up..
(3) Drill out the two rivets holding the reflector assembly to the 1" boom. Use your ruler or tape measure to find a spot 4" to the rear of the antenna terminal connections (This measurement is not super critical; between 4" to 4.5" is okay). Mark this spot on the boom.
(4) You will wind up sliding the reflector assembly forward towards the antenna terminal block. Position the reflector assembly so that the two rivet holes lie on either side of the 4" mark - one in front, and one behind. Mark and drill two new holes on the same centerline as the two old rivet holes. Use a pair of 1 2" 6-32 machine bolts, lockwashers, and nuts to secure the reflector assembly in the new holes. Figure A shows this modified reflector assembly and its final position.
(5) Carefully straighten out the metal straps that attach to the antenna terminal block. You can flatten them with a vise, or pliers. Wrap a piece of cloth around each strap to avoid dimpling or otherwise marring it as you bend/flatten it.
(6) There are two straps connected to the other side of the antenna terminals. These two straps run parallel to the antenna boom for a few inches. They are supported at the far end by a plastic insulator. Use the tin snips to cut both straps away from the antenna terminal block. Drill out the rivet holding the plastic insulator, and remove it. SAVE the two straps!
(7) You will need to short together the two threaded machine bolts on the opposite side of the antenna terminal block. Cut one of the strips and drill out two small holes to fit over the bolts. You'll need to find an extra set of nuts (wing nuts are fine) to secure this shorting strap. Figure B shows a close-up view of the shorting strap after modification.
(8) Go back to the long straps you straightened out previously. Using a ruler, measure out from the center of the boom 8" along each strap. Mark this point on each strap. The distance between the two marks should be 16". Cut the excess strap with tin snips. (If the UHF DTV stations in your are primarily on channels below 50, make the total length of the strap 17", or 8.5" on either side.)
(9) Drill two holes to fit over the machine bolts used to attach the antenna leads. These will be the same diameter as the holes used to attach the shorting bar in (7). Using a broomstick or other round object about 1" in diameter, CAREFULLY bend the 8" straps back on themselves, so that the holes you just drilled line up with the antenna lead terminals.
DO NOT crease or bend the straps too much - they may break. You should be able to curl and attach the straps in one motion. A slight kink in the strap isn't cause for alarm, so if you don't get a perfectly-rounded bend, that's okay.)
(10) Once you have the modified straps attached to the antenna terminals, use the supplied washers and wing nuts to attach the supplied balun (or 300 ohm twin lead). The finished antenna should look like Figure C.
Results? I was able to increase the signal strength from local channel 42 by 1.8 dB, channel 46 by 2 dB, channel 55 by 4 dB, and from local channel 64 by 5 dB. The received waveforms were cleaner, too - I saw less multipath notching on the 8VSB waveforms.
As I write this, the modified 4308 yagi is atop my roof, used for reception of all five New York City DTV stations - three of which aren't at full power yet - in combination with the previously-mentioned Channel Master 7775 UHF preamp. I've also picked up DTV channel 41 from Wilkes-Barre, PA; a distance of almost 70 air miles.
At least one reader of a HDTV forum where I posted this modification reported 80 - 90% reliable reception of a DTV station nearly 60 miles distant in South Carolina, using an RCA DTC-100 set-top box. Previously, he was unable to lock up a signal at all.
** - Note that this modification is not endorsed
or recommended by Channel Master LLC. ROAM Consulting, Inc. assumes no
responsibility for any damage or malfunction that may happen to an end-user's
television or receiver from improper modification or use of this antenna,
nor does ROAM Consulting, Inc. make any guarantees about reception of
analog or digital TV signals with this antenna.
Figure A
Figure B
Figure C