Several years ago, I happened upon an article in the pages of QST about a $20 HF Mobile Antenna. I was intrigued by the article and scanned a copy of it to put in my files for future reference. Like most of these files, it was dimly remembered, if at all, for 12 years. The other day I was casting about for my next project when I re-examined the article by Frank W. King, KM4IE about his $20 antenna. After looking through my “junque box” and my store of scrap PVC I began to think that this might make a very good next project.
Frank’s antenna was designed to cover the bands from 6 meters to 20 meters with coils wound using 1 ¼” PVC pipe for support. The PVC pipe I had on hand was 2” diameter but as Frank noted back in 2000, the dimensions are completely flexible and the antenna can be built in almost any size. The circuit itself is similar to the famous “Texas Bugcatcher” which was out of production for a few years, but is once again available. The antenna consists of a matching coil at the base, a short mast up to a loading coil and the requisite whip or stinger at the top. The RF is tapped onto the matching coil which is grounded at the base to inductively load the antenna at the proper impedance. From the matching coil a wire winds its way up the PVC mast (about 36” long) to a tap lead which clips onto the appropriate spot on the loading coil for the band of interest. The top of the loading coil is connected to the whip or stinger at the top of the antenna. By varying the tap points for both the matching coil and the loading coil, resonance and a suitable SWR can be obtained on any band that falls within the limits imposed by the overall size of the antenna.
With the concept understood, I started adapting the design to the materials I had on hand and making a shopping list of materials to be purchased. While digging around in the shed I found an old “Hamstick” style antenna that apparently I sort of semi-discarded a few years ago. It occurred to me that if I stripped it of most of its neatly wound wire, I would have a good fiberglass “spine” on which to assemble the new antenna. It also answered the one worry I had about Frank’s original design. I have, in the past, had PVC joints fail due to various stresses. With the entire antenna assembled around a central and one piece “spine”, even if stuff came loose, it could not fall on the road and cause problems. After a quick trip to the local home improvement store I had 2” PVC caps to go with the 2” PVC pipe and ½” PVC pipe and fittings to make a suitable brace to keep the antenna vertical. The brace attaches to the luggage rack on my SUV and prevents the antenna from twisting around the ball mount at the base.
The next order of business was to wind the coils. I started with the matching coil since it was only 11 turns. I even thought those five little words that I know are guaranteed to make any project nearly impossible. You know the ones I am referring to- “How hard can it be?” Boy did I ever find out the answer to that question. I am sure that if I had access to the proper jigs and winding apparatus it would have been a snap. Without those however, it turned into a near disaster. First a combination of drilling two holes with a step drill and connecting the holes with a saber saw made a reasonably neat slot in the form to allow the fly lead to clamp onto the turns of the coil to come. That was the easy part. After much fussing and false starts I managed to wind 10 turns onto the PVC form. The turns were not smooth, each turn had to be tweaked to keep it tight on the form, etc. I finally got it into some sort of shape so that I could secure the turns by laying straight lines of hot glue from top to bottom of the coil across the wire turns. I even managed to get the loose ends of the coil out through the PVC caps at each end of the matching coil. The lower end of the coil was soldered to a length of copper braid for the ground connection. The upper end was soldered to the wire wrapped around the fiberglass core of the old antenna. At that point I looked at the problems I had winding the short matching coil and decided that I should look for an alternative to winding the loading coil by myself.
As luck would have it, the latest issue of the MFJ catalog was on my desk, and a perusal of the contents quickly found the coil I wanted. 10” long, 3” in diameter, wound at 10 turns per inch and well supported by four Lexan rods with each turn embedded in the rods. I called MFJ and they had just made a batch of that very size. One was soon on its way to my house.
My original modification of the antenna in the article was to have used 2” PVC pipe and caps. Until the MFJ coil arrived I had no assurance that it would fit in place of the 2” PVC pipe and indeed it would not. I fretted for a moment or two and then dug into my stash of ABS plastic sheeting. This stuff is 1/8” thick and comes in a 4 x 8 foot panel. I usually split the cost with another ham operator and get to keep a 4 x 4 foot panel for about $30. I then cut it down to about 2 x 4 pieces for storage. For mounting the coil, I cut 2 pieces about 3” square and then drilled a hole in the middle of both just big enough for the fiberglass rod to slip through. I then adjusted the dimensions of the ABS squares so that the coil fit neatly onto them. After hot gluing the ABS squares to the ends of the coil it was an easy matter to first slip the bottom cap onto the core and secure it in place. I then added the coil and finally the top cap again secured to the fiberglass core. Securing the caps to the fiberglass core was accomplished by drilling the top of each cap to fit a ½” NM/SE connector.
The connectors (usually used to clamp Romex electrical cable where it enters an electrical box) were secured to the top of each cap. To do this required sanding the top of each cap with a belt sander to make the material around the drilled hole thin enough so that the clamp could be secured with its included nut. After tightening the nut on the back of each connector, a dab of thread locker was used to ensure that they will stay in place. Before the final assembly I painted everything except the loading coil with gloss black paint. I of course masked off the parts of the matching coil that need to be bare copper for the matching lead to clip to.
The antenna was mounted on the pre-existing ball mount and the ground braid was screwed to the body of the car in a spot under the tail light housing for neatness. The ½” PVC brace, also painted gloss black was secured to the luggage rack with a couple of stainless steel sheet metal screws. (See Figure 5).Mechanical testing was completed over several days, driving around town and on nearby Interstate routes at speeds up to 70 MPH. At least that is what I will admit to, and I am sticking to that story. The antenna and brace held up fine with no untoward vibration or loosening of any mechanical connections. RF Testing was completed a few days later after I borrowed an antenna analyzer from one of the members of our local club. In short order we determined what bands the antenna would resonate on and what taps I would require to match the antenna as close to 50 Ohms impedance as possible. The results were as follows:
BAND Center Frequency Match Turns Load Turns SWR
75Meters 3.800MHz 5 40 1.6/1
40Meters 7.225MHz 5 61 1.2/1
20Meters 14.250MHz 6 86 1.6/1
17Meters 18.140MHz 4 107 1.8/1
15Meters 21.325MHz 7 109 2.2/1
12 Meters NA
10 Meters NA
6 Meters NA
The last three bands, 12, 10 and 6 might be tunable by shortening the whip length above the loading coil. Until I can find a replacement for the tiny grub screws that hold the whip in place, I chose not to adjust the whip from it’s current length. If I can find a thumbscrew of the appropriate thread size I may revisit the tuning at some future date. The coil was not quite long enough to resonate on 160 Meters, but with a little more whip length than was available, or some form of “top hat” it might be possible.For now, I printed up a business card sized “cheat sheet” to attach to the sun visor of the car so I can change bands as necessary.
After a few months of living with this antenna, I realized that there was a significant problem. Not with the antenna itself, but the restriction that the original design placed on being able to wash the car. To remove the antenna the way I had built it required about 30 minutes and required the use of an electric drill to remove the screws holding the brace to the luggage rack and then to remove the right tail light so that I could unscrew the ground strap.I decided some modifications were in order. The ground strap was modified with a short length of wire permanently attached under the tail light and a Power Pole connector set was installed to connect to the ground strap. As for the brace and its attatchment to the luggage rack, a pair of threaded studs were installed into the luggage rack and the holes in the brace where the screws used to be were opened up. A fender washer, lock washer and wing nut then clamp the brace to the rack once the brace slides over the studs. This reduced the time to remove the antenna to about five minutes. Now my SUV can go through the carwash easily.The younger hams reading this may say why not just wash the car by hand. Well, I am over 70 and my strength and endurance is not what it used to be. A trip to the local car wash is much more feasible for me than the alternative. Problem solved and the car is once again shiny and clean, just the way I like it.
N1GY- The simple Approach to Ham Radio