May 2000 QST

By Frank Ingle, KG4CQKBuild a Flagpole AntennaBoosterThis 10-minute project can help you elevatethe status of a simple VHF ground-planeantenna. It’s perfect for public service.ot long ago, the Duval County,NFlorida, ARES team was conductinga drill with the local firedepartment. One of our operatorswho had been on duty at an outlying stationhad been unable to raise the repeater withhis H-T. During a critique he commented,“Next time I’ll bring a ‘spider’ antenna andhoist it up the flagpole!” That was the seedfor an idea that blossomed into a solutionfor a common problem facing emergencyserviceoperators all over the country:getting an antenna as high as possible abovesurrounding objects and doing it simply.Post-Storm OperationsAfter a hurricane or other disaster, phonelines are knocked down and the need foremergency communications is immediate.In all probability, however, the same stormthat took down the phone lines likely tookdown most “permanent” antenna installationsas well, rendering manypoint-to-point radio links inoperative. Ifrepeaters are also out of commissionbecause of power failures and damage tofixed antennas, all that’s left is simplexoperation. For an ARES operator, theneed to quickly erect a simple but tallantenna is one of our most pressingchallenges.In Duval County, many of our servedagencies have flagpoles. Red Crossshelters located in public schools, firestations, neighborhood police headquartersand the weather bureau are afew such locations. Most of these flagpolesare taller than any surroundingbuildings, so if there werean effective way to usea flagpole to support atemporary antenna,it would be anideal solution to the antenna-height problem.A Fly in the OintmentThe “spider antenna” referred to earlieris a nickname for the well-known groundplaneantenna that consists of a 1 /4-λ verticalelement and two or more counterpoiseelements (also known as radials or theground plane). My favorite example of thisantenna is Zack (W1VT) Lau’s “homebrew”antenna shown in Figure 1. 1 It’s incrediblysimple and costs little, if anything, to make.At first glance, you might think thatgetting the antenna to the top of the flagpoleis a simple matter of attaching the top of theantenna to the flagpole’s halyard and hoistingthe antenna up to the pulley. But this approachis deceiving. As you can see in Figure 2, whenthe antenna is raised to pulley level, thehalyard snap link keeps the top of the antennalower than the top of the flagpole. That meansthat the vertical element is adjacent to theflagpole and parallel with it. Hence we havethe antenna’s vertical element (oneconductor) situated parallel to the flagpole(another conductor, if it’s metallic) andseparated from it by air (a dielectric)—acapacitor. Therein lies the problem. At RF,this capacitor looks like a short circuit tothe flagpole electrically connecting it to yourantenna system. This will adversely affectthe antenna’s performance and likely causeyour transmitter to see a high SWR.What we need is a simple way to getthe antenna above the top of the flagpole(see Figure 3). Then the antenna has aclear shot to the horizon. The flagpole issituated below the ground plane andis no longer seen by the antenna.The antenna’s counterpoiseacts like an RF mirror forthe vertical element,causing it to thinkit’s the top half of a vertically orientedhalf-wave dipole. Because the flagpole lieson the other side of the mirror, it is now invisibleto the antenna, just as the feed line is.Build a Flagpole Antenna Booster in10 MinutesThis simple gadget, easily used withvirtually any flagpole, can be built for about$3. See the Bill of Materials.Figure 3—Theflagpole antennabooster in action.1Notes appear on page 38.Figure 1—Zack (W1VT [ex-KH6CP/1]) Lau’sinexpensive 2-meter ground-plane antenna.Figure 2—In this position, your antennabecomes part of a capacitor, causingyour transmitter to see high SWR.May 2000 37