Building a 11 Meter DDRR antenna for CB
The DDRR stands for Directional Discontinuity Ring Radiator. These antennas are very compact when compared to other antenna types designed to operate at comparable frequencies. Even though this antenna is used in a horizontal position, it radiates a vertically polarized wave. To build a DDRR antenna for the 11-meter CB band, you will need the following materials, which can be purchased at any hardware store:
1 – 8 1/2′ of 1/2″ copper water pipe (usually comes in 10′ or 12′ standard section)
8 – 45 degree copper elbows to fit 1/2″ copper water pipe
4 – copper Tee joints to fit 1/2″ copper pipe
1 – 2′ section of 1/2″ PVC water pipe
1 – roll of metal chicken wire
1 – 4″ long by 1/4″ bolt, with a standard hex nut to fit the bolt
1 – tube of “PC Metal” putty epoxy
1 – roll of silver solder
1 – alligator clip
Using a pipe cutter, prepare the following lengths of tubing:
1. Cut 5 segments of 13″ lengths of 1/2″ copper water pipe
2. Cut 2 segments of 4 1/8″ lengths of 1/2″ copper water pipe
3. Cut 2 segments of 1 7/8″ lengths of 1/2″ copper water pipe
4. Cut 2 segments of 10 1/2″ length of 1/2″ copper water pipe
5. Cut 1 segment of 4″ lengths of 1/2″ copper water pipe
6. Cut 4 segments of 4 1/4″ lengths of 1/2″ PVC water pipe
Fit the parts together on a flat non-flammable surface as per the illustration. A cement garage floor will do fine. Clean the ends to be soldered with steel wool so the pipe is shiny where it will be soldered. Also, remember to clean the insides of the 45-degree elbows and Tees.
The copper Tee at the bottom as shown in the drawings below will have to be left faced to the side so it will not cause the loop to be crooked when you solder it together. Once the parts have been fitted together, and any last minute adjustments or trimming has taken place, take a plumber’s soldering torch and silver solder and solder the parts together. Before you begin soldering, be sure to remove the 4 1/4″ PVC tubes from the loop so it doesn’t melt. When you solder the Tees, use a square to ensure they are pointing straight up. Insert the 4″ length of copper tube into the copper Tee in the bottom of the illustration labeled “copper ‘T’ facing up” and solder. When you have everything but the last copper Tee soldered together, (marked “facing down” in the illustration) insert two of the 4 1/4″ PVC pipes into the two Tees that are opposite each other and about half way up the drawing.
Turn the loop over so it is standing on its “legs” (2 PVC and 1 copper). Solder the last copper Tee (marked “facing down” in the diagram) using the square to ensure that it is pointing straight up. (See Note below. You may want to solder the last Tee so it is pointing straight out and parallel with the ground.)
When everything is soldered together and has cooled, insert the 4 1/4″ PVC tubes into the loop between to two Tees (see illustration). Drill a small hole through each of the Tees and into the PVC tube. Insert a couple of small screws. This will strengthen the loop so the open ends can’t move toward or away from each other.
Take one of the remaining three 4 1/4″ PVC pipes and use the PC Metal putty epoxy to glue the nut to the end of the PVC pipe. (This is the “Tuning Bolt”) When that has set, insert the PVC pipe into the copper Tee as per the above illustration. Solder about 1 foot of wire to the head of the bolt and thread it in about half way. (The tuning bolt is used to tune the antenna as the name implies.)
The next step is to roll out and flatten the chicken wire to make a ground plane. Depending on how wide your chicken wire is, you may have to roll out two or three rows and then tack solder them together so they will be electrically as a single piece. When finished making the ground plane (you may have to lay some rocks on the chicken wire to keep it fairly flat) you can now place the loop onto the ground plane and solder the one copper leg to the chicken wire and the coax shield as illustrated below. Note here that the tuning bolt wire is not soldered here, it is soldered to an alligator clip and the clip only is connected here so that it can be disconnected while turning the tuning bolt.
Note: The tuning bolt on the antenna I built was facing straight up. However, it would be easier to keep the tuning bolt wire from coming close to the loop if the tuning bolt’s copper Tee was soldered so it is pointing straight out, parallel with the ground plane. When the tuning bolt’s wire comes close to the loop, it de-tunes the antenna. Also, this end of the antenna where the tuning bolt is mounted is where the highest RF voltage is found. Be careful!
Before you solder the center conductor of the coax feedline onto the antenna you will probably need to do some experimenting to find the right place to “Tap” onto the loop. My antenna ended up working well with the wire “tapped” at point “A” in Figure 1. (Remember that the first illustration has the antenna on its back, so point A will be on the opposite side of where it is illustrated when the antenna is on its legs.) It is right on the corner of the 45-degree elbow. This is the 50-ohm feedpoint to match the 50-ohm impedance of the coax and transmitter. Before finding the feedpoint, make sure the coax shield and chicken wire are soldered in place as illustrated. Solder the alligator clip to the tuning bolt wire and clip the wire close to the chicken wire and coax shield. Again, the alligator clip is used here so you can disconnect the wire when turning the tuning bolt. This prevents the wire from getting twisted out of shape.
To find the proper feed point, connect the full-length antenna coax to the SWR meter, also connect the SWR meter to the radio. Tune the radio to your favorite channel and key up while holding the bare center conductor touching the loop. Try point “A” first. If the SWR is too high, move it either way on the loop until you find the point where it has the lowest SWR When you find this point, solder the center conductor of the coax feedline to that point. The farther around the loop you go, the higher the impedance. The closer you go to the copper leg, the lower the impedance. It can be a handful to move the coax up and down on the loop and watch the SWR meter at the same time. I took my CB, SWR meter, full length of coax to be used, and my power supply outside. I set up at arm’s length from the antenna and laid the coax out on the lawn so it wasn’t curled up and did my tuning.
This antenna is quite narrow-banded. I only got about 3 or 4 channels out of it with reasonably low SWR without re-tuning the antenna. Also, when you are using a DDRR, you must ALWAYS have an SWR meter hooked up to your rig to make sure the antenna is tuned. If you don’t, you are taking a risk at blowing your finals. This antenna is very sensitive and tuning changes even with temperature. I have been told by fellow amateurs if larger diameter pipe is used, the bandwidth is widened. I wonder what would happen if one were to build this antenna using 2″ exhaust pipe?
To tune, select your favorite CB channel and key up just long enough to get a reading on your SWR meter. If it is 1.1:1 then you are lucky and need not do anything more. Most likely, the SWR will be very high. To tune, try threading the bolt in or out a ways and then key up again and see what happens. If you can’t get a low SWR (1.1:1), then try either raising or lowering the left or right side of the loop. This can be done by using a shorter or longer piece of PVC for the leg or by shimming the legs with something thin (1/4″ at a time will suffice), then use the tuning bolt once again to trim to 1:1 SWR. The distance between the loop and the ground plane seems to affect the tuning considerably. It may take quite a bit of tinkering to get the antenna to tune up properly, but once you do, you’ll know it! When you reach resonance the SWR will suddenly dip WAY down. I could get almost 1:1 (no needle movement) with my DDRR antenna when it was tuned properly!
This DDRR antenna worked very well just sitting on the ground! However, once I figured out the characteristics of the antenna as far as tuning, I put it up on the 25′ high flat top asphalt roof of my home. I do not believe this antenna is very good for DX’ing, but it worked great for local coverage and it kept the landlord and neighbours from complaining about a big ugly antenna.
Note: Information used to design and build this antenna was taken from The ARRL Antenna Book, Thirteenth Edition.
Originally posted on the AntennaX Online Magazine by Gregg Messenger, VE7WGM
Last Updated : 1st May 2024