A $20 HF Mobile Antenna

1 April 200033 While returning home from abrief business trip one evening,I was listening to the chatter ona 2-meter repeater. As one hamextolled the virtues of a new generation ofdiminutive HF transceivers, several otherslamented the expense of good antennas forthe HF bands and 2 meters. I operate HFand VHF Mobile every day. On VHF, I usea roof-rack-mounted 5/8- commercialmobile Antenna that retails for $ the HF bands, a bumper-mounted homebrew Antenna that costs about thesame as the VHF Antenna added the bonusof a fun day at the workbench!My HF Antenna is a bug-catcher stylevertical that has netted me CW and phonecontacts worldwide using my ICOMIC-706 MkII. The Antenna consists of littlemore than some PVC pipe topped by aRadioShack replacement whip Antenna and acouple of coils made from a small roll of #14house wire.

2 The beauty of this Antenna liesnot only in its under-$20 price tag, but also inits simplicity and ease of tuning. The antennacan be built for a wide range of frequencies;By Frank W. King, KM4 IEA $20 HFMobile AntennaA few hours of fun with PVC and wire andyou ve got yourself a respectable road radiator!Ready for the road! A length of 1/2-inch-diameter PVC pipe fastened to the vehicle sroof rack and Antenna acts as a a good match and low SWR is no morecomplicated than moving two taps, one on theloading coil and one on the matching current version of this antennaoperates on 20 through 6 meters with anSWR of :1 or less in any segment of eachband. The Antenna is quite broadband evenin a one-tap setup. One real joy of buildingthis Antenna is that because it s fullyadjustable, construction dimensions arecompletely noncritical!

3 How much easiercould an Antenna be?ConstructionA trip to your local hardware store or do-it-yourself outlet and RadioShack shouldequip you with the majority, if not all, ofthe parts required; see the Materials ll need three pieces of schedule 40 PVCpipe. One piece is a three-foot-long sectionof 1/2-inch pipe that forms the Antenna shaft(center). For the loading and matching-section coil forms, I use 11/4-inch-diameterpipe so the Antenna can be mountedreasonably close to the vehicle. The loadingcoil at the top of the Antenna is a piece ofClose-up view of the matching coil. To the left is the 3/8-24mounting bolt. Attached to the bottom turn of the coil is alength of shield braid used as a ground strap. A small area ofthe PVC pipe OD is ground flat to provide room between thecoil turns and the pipe form for an alligator-clip tap to firmlygrasp a wire turn without loading coil resembles the matching coil except it has agreater number of turns.

4 A RadioShack whip is attached to thetop PVC cap. The wire lead connecting the bottom of the whipto the top of the loading coil passes through a hole in the topof the pipe pipe roughly 6 inches long. Thethird piece of PVC is a 4-inch length of11/4-inch pipe used for the matching-coilform at the bottom of the to the accompanying photographsduring the following discussion. Use a beltsander or a file to make a flat about 5/8 inchwide along the length of each of the two11/4-inch coil forms. The flats provide roombeneath the coil windings for attachingclip-lead taps. Each coil form has a PVC end34 April 2000cap on one end and a 21/2-inch-long dualfemale coupling fitted with a standard PVC11/4-inch to 1/2-inch reducer (Genova and DO-IT #30245) on the other end. The reducersand couplers mate each end of the main shaftto the coil forms.

5 You can assemble for fit,but don t glue the pieces together both end caps. Drill the topcap on the loading coil to accept aRadioShack replacement whip assembly(RS 21-952). In the matching-coil s bottomcap, drill a hole to accept a 3/8-24 bolt forthe mounting stud. Also, drill a small holethrough the side of each top and bottom cap,near the top, to pass a length of #14 the bottom of the whip and mountingbolt, attach 12-inch-long pigtails of #14bare wire, passing the wires through theircap holes to the outside. These wires,respectively, connect the bottom of thewhip to the top of the loading coil, and themounting stud (RF feed) to a clip lead forthe matching coil. Fasten the whip and the3/8-24 bolt to their respective PVC caps,securing them tightly. Place a drop or twoof thread-locking compound on the threadsof the whip base in the upper cap and onthe threads of the 3/8-24 bolt in the bottomcap.

6 If either of those mounting nuts comeloose once you have glued the whole thingtogether and wound the coils, you may shall I say utter a few words of disap-pointment! Once the connections are tight,align the flat sides of the two coil forms atthe opposite ends of the main shaft and gluethe entire shaft assembly. At this point, yourcreation starts to look like a real Antenna !Now, wind the coils. Strip a 25-foot rollof #14/2 (with ground) house wire. Wrap thewires on their respective forms, holding theturns temporarily in place with electrician stape. The matching coil on my 20- through6-meter Antenna consists of 11 turns spaced1/8-inch apart (a length of about 17/8 inches).The loading coil has 25 turns spaced 1/8-inchapart for a length of approximately 37/8inches. Wind the third wire on the antennashaft, spacing the turns about 1 inch to 11/4inches apart.

7 Don t wind the turns of the helixany closer than an inch apart, otherwisetuning the Antenna on 10 and 12 meters willbe a real the coils are wound to your liking,mix a couple of inches of epoxy putty andcut it into six strips. (Duro Epoxy PuttySealant works well. I use the 30-minuteTwo of these reducers (Genova and DO-IT#30245) are needed to make the transitionfrom 11/4-inch pipe to 1/2-inch 1 Schematic (A) and pictorialdiagram (B) of the $20 homebrewedmobile Antenna . From bottom to top, thematching coil, main section, loading coiland the adjustable metal taps on the matching andloading coils are made with smallalligator clips. See the Materials Listbefore you go 200035 Materials ListLength of 1/2-inch-wide copper braid3/8-24 mounting stud25-ft #14/2 w/ground house wireTwo 11/4-inch PVC pipe capsTwo 11/4-inch to 1/2-inch PVC reducers(Genova and DO-IT #30245)36-inch length of 1/2-inch PVC pipePVC cementEpoxy putty sealantRadioShack collapsible whip (RS 21-952)Two alligator clipsPlastic electrical tapevariety and find that I have enough puttyin one package for two antennas.)

8 Roll eachpiece of putty into a bead long enough toextend the length of the matching andloading coils. Place three beads of epoxyon each coil, spacing them equally aroundthe forms (but not over the flat areas). Layeach bead on its respective coil and pressthe bead until it flattens and contacts thecoil form. When cured, the epoxy puttyholds the coil turns securely in leads and cosmetics are next. Wrapthe three-foot shaft and its winding withelectrical tape. Spray the coils flat blacktaking care to mask the coil-tap areas abovethe flats. Solder a 6-inch-long clip lead tothe 3/8-24 mounting-stud pigtail and a 12-inch clip lead to the top of the helicallywound shaft cutting off any excess the bottom turn of the matching coilto a length of 1/2-inch-wide copper braid toserve as a ground connection.

9 Solder the topof the loading coil to the whip s the whip and you re ready to installthe Antenna , tune it and get on the air!Installation and OperationI use a standard bumper mount with anextra ground lug placed a few inches awayto which the matching coil s braid isattached. Install the Antenna and guy it Iuse a length of 1/2-inch PVC pipe attachedto the vehicle s roof rack as a helps to mark the tap points of theloading and matching coils for laterreference. I use a gold-paint pen and placethe marks on one of the flat beads of epoxyputty that runs the length of each coil. Onmy current Antenna , I have dots placedevery five turns and keep a small chart atthe operating position. The reference pointsmake band changes quick and consistentevery time. The numbers in the coil-tapchart of Table 1 refer to the coil-tap pointsas counted from the bottom of each Antenna tunes 20, 15 and 17 meterseasily with the whip fully extended.

10 I findthat I have to drop the whip into the shaftabout one foot to tune 12 meters, and abouttwo feet to tune 10 meters. Get out a notepad and a pencil to make a tap-point chartfor the various bands. Connect an SWRmeter in the feed line between yourtransceiver and the Antenna . (If available,you can use your rig s built-in SWR meter;my IC-706 is so equipped.)Key your rig and adjust the power outputof your radio for just a few watts. The SWRis most affected by the movement of theloading coil tap (the upper coil) and fine-tunedby the placement of the matching-coil (lowercoil) tap. Table 1 provides some starting pointsfor finding the lowest SWR on each installation is different, so don t expectthe number of turns given in Table 1 to matchyour installation exactly. Key the rig, checkthe SWR, move each tap up or down one turnand repeat the process until you ve obtainedthe lowest SWR on each 20 through 10 meters, the Antenna is1/4- long.