A 20 and 40 Meter Vertical on - K6MHE

1 September 2005 Reprinted with Permission ARRLF igure 1 Using remote relays 0 (K1, K2) for dual-band operation. One DPDT relay couldbe substituted for the two relays. L lVertical on Autopilot An old Hints & Kinks item results in an excellentField Day its size, ease of installa-tion, and reliable perfor-mance, the 40 Meter quarterwavelength Vertical antenna isa very popular HF antenna. What s more,with proper matching, it also performs verywell on the 20 Meter band.

2 Being a half-wavelength on that band, the antenna pro-vides an efficient low-angle radiationpattern, with roughly a dB of gain over theshorter standard quarter-wave article describes a dual-band verti-cal antenna system for 20 and 40 meters thatI built for portable Field Day use. It uses asomewhat innovative tuning method that,after the initial setup adjustments are made,permits operation on ether band with no fur-ther tuning required and it works great!I discovered this scheme while perusingsome past issues of QST. I had a very nicetelescoping 35 foot marine Vertical whip thatI wanted to use for an upcoming Field DayA 20 and 40 MeterDan Richardson, K6 MHEWF igure 2 The modified L-network for dual-band operation without 3 Inside the dual-band modified series capacitor (C2) is at the top left,immediately behind the inductor (L1).

3 The shuntcapacitor (C1) is toward the , while looking for some ideas, I cameacross this novel method for dual band op-eration. It was described by Wes Hayward,W7 ZOI, in an old Hints & Kinks , a quarter-wave Vertical is fedby directly connecting the coaxial feed lineto the base of the antenna. As the base im-pedance is low (typically 40-70 , depend-ing upon the ground/radial system utilized)this technique provides an acceptableSWR and works well. However, the baseimpedance of a half-wave (20 meters , inthis case) Vertical is much higher (around1100 ); therefore a matching network isrequired to successfully feed the antennawith coaxial transmission line.

4 Prior to dis-covering this method, I had only seen thistype of dual-band operation performed us-ing remote relays (K1, K2) in conjunctionwith an L network (L1, C1), as shown inFigure innovative approach involves plac-ing an additional capacitor in series with theinductor of an L network so that, when prop-erly adjusted, a conventional 40 Meter ,1/4 wavelength, Vertical will operate on boththe 40 and 20 Meter bands with an accept-able SWR with no need for switchingschemes or any additional it WorksW7 ZOI s modifiednetwork, shown in Figure2, is easily understood whencompared side-by-side withthe simple L network of Fig-ure 1.

5 L1 and C2 form a seriesresonant circuit that behaves on40 meters as a short circuit. The resultis the same as having the relay contacts inthe 40 Meter position, as shown in Figure1. The shunt capacitor (C1) reactance on40 meters is roughly ten times that of theantenna s base (feed-point) impedance andtherefore has a negligible effect on match-ing. The net result is the antenna s lowbase impedance essentially remains un-changed to the coaxial transmission lineon 40 Meter operation is accomplishedby adjusting L1 and C2 so that the net in-ductive reactance is the same as it would bein the basic L network.

6 The value of C1 re-mains the same in both , we end up with a matching systemthat does double duty. The modified L net-work is adjusted so that L1 and C2 pro-vide the proper amount of inductivereactance such that, when used with C1,matches the high base (feed point) imped-ance of the antenna on 20 meters . And,simultaneously, it maintains the seriesresonance that is required for matching theantenna s low base impedance on 40meters. A pretty neat idea!ConstructionI built the entire network solely fromjunk-box parts. As is usually the case whenbuilding a project like this, my treasure trovedid not contain capacitors of the exact val-ues recommended by W7 ZOI.

7 I substituteda 100 pF capacitor for C1 and a500 pF capacitor for C2. Originally, I wasconcerned that, by using larger values thanthose recommended, particularly for C2, theminimum capacitance values might be toolarge for proper tuning; however, this turnedout not to be the case. The inductor wasmade from a section of coil stock(2 inch diameter; 14 gauge wire; 10 t at8 tpi). All components were mounted on a1/8 inch thick plastic panel, which, in turn,was installed in a watertight housing that hadseen previous life as part of a shipboardantenna system.

8 The completed network isshown in Figures 3 and you choose to build this project takecare to use capacitors that have a sufficientlyhigh voltage rating. Mine are rated at and should easily handle 500 W or more.[Vacuum capacitors would be ideal for thisapplication, but they are expensive. Ed.]TuningAs I stated earlier, my antenna s heightis about 35 feet; therefore, its resonant fre-quency falls below the 40 Meter band. Thiscaused me no problems when tuning; how-ever, I did have problems trying to adjustthe network with an antenna heightthat was slightly less than 1/4 wave-length.

9 I will discuss this in was done empiri-cally. Using an MFJ-259 Bantenna analyzer, the tap onthe inductor was set nearmaximum inductance and ca-pacitor C2 was adjusted for thebest indicated SWR on 40meters. Next, the analyzer s fre-quency was changed to the 20 meterband and C1 was adjusted for the bestSWR, but the match obtained was notgood. I next moved the tap on the coil inthe direction of less inductance and C1 andC2 were adjusted again for their respec-1W. Hayward, Dual-Band Operation with a 33-Foot Vertical , QST, Jun 1977, p 2005 Reprinted with Permission ARRLF igure 4 Tuning the network with an SWR analyzer.

10 See text for tuning 5 The SWR at 20 meters (A) and 40 meters (B).tive bands. This process was repeated un-til satisfactory SWR readings were ob-tained. (My requirement was to have theSWR fall below 2:1 on both bands. Manysolid-state rigs start reducing their poweror, worse yet, shut down at an SWR of2:1.) The resulting SWR curves are shownin Figures 5(A) and (B). I should point outthat this system was tuned using a mar-ginal radial system (only twelve radials)and the SWR curves are therefore ratherbroad. Those curves would be more pro-nounced had the antenna been adjustedutilizing a more efficient ground telescoping mast comprises six sec-tions that, when extended, reaches about35 feet.