The 10/15/20 Meter Trap Vertical Antenna
This antenna project came out very nice. It's a rugged homebrew multi-band trap vertical antenna that works the 10, 15, and 20 meter amateur radio bands. The antenna can be mounted on the ground or on a mast. Mounted on the ground the antenna has a low take off angle for working DX. If mounted on a mast the antenna will acheive both a low angle as well as another radiation lobe that has a much higher take off angle.
I made the bottom section of the antenna using three 4' lengths of telescoping aluminum tubing -- largely due to not wanting to cut the tubing as I may want to reuse it for another project. Of course overlapping that much tubing made the base of the antenna very sturdy. The base of the antenna can be made with 3' lengths instead of the 4' lengths that I used. It is also possible to use one 8' length of aluminum tube but if you do then I would suggest using larger tubing for making the traps. I used 1", 7/8" and 3/4" 6061-T6 aluminum tubing for making the 10 meter section of the antenna. The 15 meter section is a 5" long piece of 3/4" tubing. The 20 meter top section was made with 3/4" and 5/8" tubing, both 1' long but could have been made using one piece of 3/4" aluminum tubing that is approximately 18" long.
Ring connectors are used on the coaxial cable feed line instead of a PL-259 UHF connector. The center conductor of the feed line is fastened directly to bottom of the antenna with a sheet metal screw and an internal tooth lock washer. I drilled a 1/8" hole about a half inch from the end of the 1" tube to connect the center lead of the coax. The shield side of the feed line is connected to the mounting bracket. The mounting bracket is the same one that I used in making the 10 meter vertical antenna.
The traps were made the same way as NU3E's with RG-58/U coaxial cable wound on 3/4" PVC pipe couplers. I added reducers, some short sections of 1/2" PVC pipe, two 7" sections of 5/8" aluminum tubing, and some hardware to adapt the design to aluminum tubing. Later I found that I had to cut off 1-1/2" of tubing from the top of the 10 meter trap and the bottom of the 15 meter trap so that I could get the correct length for the 15 meter portion of the antenna.
In order to get the 5/8" aluminum tubing to fit inside the 1/2" PVC pipe I used a Dremel tool with a small sanding drum to enlarge the inside of the PVC pipe.
A notch is cut out of the 3/4" pipe to allow it to fit over the coaxial cable when assembling the traps. A second notch was needed so that the sheet metal screw used for connecting the tubing to the trap would clear the 3/4" PVC pipe. This second notch is not shown in the above picture but is described in the video.
Since heat dissipation seems to be a concern I used hot glue to hold the coaxial cable in place on the PVC pipe coupler rather than using vinyl electrical tape. After tuning the trap and checking it with a grid dip meter I used a few pieces of masking tape to hold the cable in place on the form. After running one bead of hot glue across the trap I then had to retune the trap. This process was repeated until the cable was finally in place. Hot glue seems to contract when it cools and could cause the spacing in the cable to shrink.
I should also note that the RG-58/U coaxial cable used in making the traps is the type with the plastic center dielectric and is not the foam type.
Video Instructions Below is a four part video detailing how I built this antenna. Part 1: Making the 10 meter trap (23:12). Part 2: Adapting the trap to the aluminum tubing (30:35). Part 3: Making the 15 meter trap (32:54). Part 4: Final assembly and test (18:17).
How To Tune the Coaxial Cable Traps
Tuning coaxial cable traps was easier than I thought it would be but you will need a grid dip meter to tune them with. I used the MFJ grid dip meter adapter for my SWR Analyzer and with a little practice I found tuning traps as actually a very simple task.
To tune the traps place the trap over the adapter and check the meter for the lowest SWR reading. The frequency shown on the meter is were the trap is most resonant at. Move the cable around just slightly and you can see the resonant frequency change. Ajusting the cable on the form is how it is tuned.
The two traps for this antenna made with 3/4" PVC pipe couplers were a tight fit on the grid dip adaptor. At first I was seeing the trap high in frequency but found that when I had slipped the trap over the grid dip adapter it had pushed part of the coaxial cable back out the hole in the coupler. This meant that I needed to bend the wire connections inside the trap aside for a better fit. Once I readjusted the cable on the inside of the trap I then saw a more accurate reading on the meter. The reading become even more resonant with the tip of the adapter placed inside along the edge of the last turn in the trap with the rest of the adapter centered between the turns of coaxial cable.
At what frequency should the trap be tuned to? This seems to be under great debate. Some say tune it low in frequency for slightly better gain (but possibly increased SWR) while other antenna builders tune traps in the middle of the band. And of course, someone else says to tune it high in frequency past the band limit. For this experiment I tuned the traps for the center of the band in an attempt to maximize bandwidth. I did have very good results with the antenna but tuning the traps near the desired operating frequency may actually increase loss. See W8JI's web page for more information.
I was running short of wire and only used two radials per band to test the antenna. You may use two, three or four radials for a ground mounted antenna or two or three radials per band if the antenna is to be mounted on a mast. For a ground mounted antenna the radials may be placed on the ground or buried a couple of inches. For a mast mounted antenna I'd suggest three radials per band brought down at about a 45 degree angle in order to match impedance. Using only one or two radials per band will probably make the antenna somewhat directional. The radials, and the shield side of the feed line, are connected to the bottom U-bolt on the mounting bracket. To hold the radials in place on the ground I used landscaping staples at the base of the antenna and at the ends of the radials.
14 gauge solid copper wire works very well for radials. Cut the radials as follows: 10 meter band, 8' 4" (2.54 m). 15 meter band, 11' 2" (3.404 m). 20 meter band 16' 4" (4.978 m).
Tuning the Antenna
Tuning the antenna is as simple as adjusting the lengths of the aluminum tubing. First tune the 10 meter section, then the 15 meter section and finally the 20 meter section. Changes made to the 10 meter section will affect the other two bands. Tuning the 15 meter section will also affect the 20 meter resonant frequency. Unlike using resonactors changes to the 20 meter band should not change 10 or 15 meters. So just start at the bottom of the antenna and work your way up.
Below are the antenna lengths after taking the antenna down and measuring the 20 meter top section. The SWR reading after tuning is shown in the video. Retuning the antenna will make the antenna just slightly shorter or longer.
About 60" (153 cm) of RG-58/U coaxial cable (non-foam).
14 Gauge AWG solid copper wire for radials (see above for lengths).
Ring connectors for the radials and feed line.
Landscaping staples, if ground mounted.
Safety First! Please use caution and keep common sense safety rules in mind when installing an antenna. Never install antennas near power lines or in any location that would place people or pets within the near field radiation pattern of an antenna. All users understand and agree that the owner of this web site is not responsible for accidents or other mishaps that may have been caused directly or indirectly as a result of the information published on this web site and/or in any of the video presentations.