September 2005 Reprinted with Permission © ARRL

Figure 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 antenna.

ith its size, ease of installa-tion, and reliable perfor-mance, the 40 meter quarterwavelength vertical antenna is

a very popular HF antenna. What’s more,with proper matching, it also performs verywell on the 20 meter band. 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 vertical.

This 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 Day

A 20 and 40 Meter

Dan Richardson, K6MHE

W

Figure 2—The modified L-network for dual-band operation—without relays.

Figure 3—Inside the dual-band modified L-network.The series capacitor (C2) is at the top left,

immediately behind the inductor (L1). The shuntcapacitor (C1) is toward the right.

and, while looking for some ideas, I cameacross this novel method for dual band op-eration. It was described by Wes Hayward,W7ZOI, in an old Hints & Kinks column.1

Typically, 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 Ω, 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 Ω); therefore a matching network isrequired to successfully feed the antennawith coaxial transmission line. 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 1.

This 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 adjust-ments.

How it WorksW7ZOI’s modified

network, shown in Figure2, is easily understood whencompared side-by-side withthe simple L network of Fig-ure 1. L1 and C2 form a seriesresonant circuit that behaves—on

40 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 meters.

Twenty 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. The value of C1 re-mains the same in both networks.

So, 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 inductive

reactance 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 from

junk-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 W7ZOI. 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. The completed network isshown in Figures 3 and 4.

If you choose to build this project takecare to use capacitors that have a sufficientlyhigh voltage rating. Mine are rated at 4.5kV 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 height

is 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 adjust

the network with an antenna heightthat was slightly less than 1/4 wave-

length. I will discuss this in amoment.

Tuning was done empiri-cally. Using an MFJ-259Bantenna analyzer, the tap onthe inductor was set near

maximum inductance and ca-pacitor C2 was adjusted for the

best indicated SWR on 40meters. Next, the analyzer’s fre-

quency was changed to the 20 meterband and C1 was adjusted for the best

SWR, 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 46.

September 2005 Reprinted with Permission © ARRL

Figure 4—Tuning the network with an SWR analyzer. See text for tuning suggestions.

Figure 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 system.

My telescoping mast comprises six sec-tions that, when extended, reaches about35 feet. As a matter of interest, I tried tun-ing the antenna with one of the sectionsretracted, resulting in an overall antennaheight of about 30 feet, which is, ofcourse, self-resonant above the 40 meterband (about 7.8 MHz). Using the shorterlength antenna, I could achieve a goodmatch on one band or the other but not on

both simultaneously. It turned out that theadditional inductance required to obtain agood match on 40 meters with the shorterantenna height resulted in having too muchnet inductive reactance in the L1/C2 se-ries combination for a satisfactory matchon 20 meters. Conversely, reducing theamount of inductance that provided a goodmatch on 20 meters did not afford enoughinductance for an acceptable match on thelower frequency. Hence, for this systemto work properly, the antenna heightshould be no shorter than 1/4 wavelengthfor the lowest frequency of operation. Ifound no detrimental consequences, how-ever, when it was slightly longer.

ConclusionAfter the initial adjustments, the net-

work performed flawlessly. Alternatingback and forth between 20 and 40 meteroperation required nothing more thanchanging the frequency/band selection onthe radio, with no concern whatsoeverabout the antenna system.

I am surprised I haven’t seen this tech-nique in wider use. Given that more than25 years has passed since it was first pub-lished, I felt it worthwhile to present itagain for those who may not be aware ofthis practical and versatile design.

I wish to thank to Wes Hayward,W7ZOI, for his review and suggestions inthe preparation of this article.

First licensed in 1955, Dan Richardson,K6MHE, enjoys experimenting with antennas.Dan was an electronic technician in the USCoast Guard during the 1950s and ’60s; hethen joined the family foundry business, re-tiring about 10 years ago. After he had movedto rural Northern California, Dan’s devotionto antennas flourished, and he’s publishedseveral articles in leading amateur journals.Dan can be reached at PO Box 2644, FortBragg, CA 95437 or k6mhe@arrl.net.

September 2005 Reprinted with Permission © ARRL