What’s the difference between a dipole and a vertical?Maybe not as much as you think. Come along

and try another point of view.

By Rudy Severns, N6LF

PO Box 589Cottage Grove, OR 97424rudys@ordata.com

Another Way to Lookat Vertical Antennas

1Notes appear on page 32.

The grounded vertical is one ofthe earliest radio antennas,well known to Marconi and

widely used today by amateurs, particu-larly for 80 and 160 meters. VHF verti-cals with “ground planes” are also popu-lar. Traditionally, ground has beenviewed as an integral part of the an-tenna—in effect supplying the “missing”part of the antenna, since, atlow frequencies at least, the verticalportion of the antenna is usuallyless than λ/2. Even when the antenna isnot grounded, but raised above ground,we still use the terms “elevatedground system,” “counterpoise ground,”

“ground plane” and so on. In this view,we retain the concept that ground is anintegral part of the antenna and anungrounded vertical must have somestructure that replaces the “real” ground.While this conceptual framework hasserved us well for over 100 years, it tendsto limit our thinking to more traditionalsolutions. A change in viewpoint exposesuseful variations, better suited for par-ticular applications.

The traditional view, stemminglargely from the work of Brown, Lewisand Epstein1 in the 1930s, is that a λ/4vertical, with a ground system of 100 ormore long radials, is the ideal—any-thing else is an inferior compromise.

Recent work,2,3 using primarilyNEC modeling, has indicated that el-

evated ground systems with only 4 to8 λ/4 radials can be very competitivewith the more-traditional 120-buried-radial antenna, although that is thesubject of some controversy, due to thedifficulties experienced with experi-mental verification. There is even theheresy that radials as short as λ/8 maybe only marginally less effective thanfull λ/4 radials and have significantpractical advantages. Elevated-radialsystems have their own drawbacks,such as (1) nonuniform radial cur-rents,4 which lead to asymmetricalpatterns and perhaps increased loss,and (2) the need for an isolation chokeat the feed point. A network of wires,arranged in a circle λ/2 in diameterand suspended above ground, may bemore trouble than simply burying thewires. There has been considerable

discussion—regarding traditional λ/4radials used in elevated ground sys-tems—as to whether these are a poorchoice or not and whether other ar-rangements may be superior.4, 5, 6, 7, 8

Because most amateurs are severelylimited by available space and the costof towers and extensive ground sys-tems, the traditional buried-radial oreven the elevated λ/4-radial systemsare frequently infeasible. What isneeded is a wide range of other choicesfor the antenna structure from whichto choose the best compromise for agiven situation. Obviously, the finaldesign should sacrifice as little perfor-mance as possible.

An alternate way to look at verticalshas been suggested by Moxon (seeNote 5) and others:

1. The antenna is a shortened (lessthan λ/2) vertical dipole with loading.The loading may be symmetrical orasymmetrical, lumped or distributed,inductive or capacitive, or a combina-tion of all of these. Usually, the load-ing contributes little to the radiation,although some loading structures mayradiate.

2. Ground is not part of the antenna.However, the interaction betweenground and the antenna—and the lossin the ground—must certainly betaken into account. This includes bothnear and far fields.

This view can the maintained evenwhen a portion (or all!) of the antennais buried.

At first glance, this seems a trivialconceptual change. Nonetheless, look-ing at a vertical as a short, loaded di-

pole in proximity to ground—ratherthan as a grounded monopole—openspossibilities not usually consideredwith the more traditional point of view.For example, with a full λ/4 vertical,one would not normally consider add-ing a top hat for loading. However, in sodoing, the diameter of an elevatedground system at the base of the an-tenna can be drastically reduced, seem-ingly out of proportion to the size of thetop loading hat. This can be a very realadvantage by reducing the footprint ofthe antenna. A shortened, horizontaldipole antenna with a hat at each end isvery well known; it draws little com-ment. Nevertheless, vertically orient-ing the antenna and manipulating theend-loading devices to suit the applica-tion is not so common—although theantennas are conceptually identical!

Loaded Dipoles in Free SpaceOne of the simplest ways to resonate

a shortened dipole (less than λ/2) is toadd capacitive elements or “hats” atthe ends, as shown in Fig 1. As indi-cated, the feed point may be anywherealong the radiating portion of the an-tenna. Fig 1 shows symmetrical endloading. Fig 2 shows extreme asym-metrical loading, where only one ca-pacitive loading structure is used.This is, of course, the familiar ground-plane antenna being viewed as anasymmetrical dipole. Actual antennascan vary between these two extremes,since they incorporate various sizesand geometries of loading hats to suitparticular applications.

When the vertical portion of the

antenna, h, is less than λ/4, top load-ing is commonly employed. However,top loading is usually not consideredwhen h ≥ λ/4. This may be due to ourpast view that we need an extensiveset of buried radials, or equivalently,an elevated system of λ/4 radials. Fora λ/4 vertical, the diameter of the ra-dial system will be ≈λ/2, changing onlyslowly as the number of radials is var-ied. On the other hand, if we lengthenthe vertical section beyond λ/4, addsome top loading or even some induc-tive loading, the diameter of the bot-tom radial structure drops rapidly.

A simple example illustrating thispoint is given in Figs 3 and 4. Fig 3shows an asymmetrical λ/4 dipole withtwo radials (L1 and L2) at each end. L2is varied from zero to 22.3 feet, and L1is readjusted, as needed, to resonate theantenna at 3.790 MHz.

Clearly, adding even a small amountof top loading (L2) greatly reduces thelength of the bottom radials (L1), andconsequently the land area required

Fig 1—Short loaded dipole.

Fig 2—Asymmetrical dipole.

Fig 3—1Asymmetric two-radial dipole.FR = 3.790 MHz.

Fig 4—Effect of top loading on radial length.

for installation. This is a matter ofconsiderable practical importance tothose with restricted space in which toerect an antenna. With somewhatmore complex loading elements, thefootprint can be reduced even further.

In addition to greatly reducing thelength of the radials, a number ofother things happen during the aboveexercise:

1. With only two radials and no toploading, the radiation pattern varieswith azimuth by about 0.7 dB, makingthe pattern slightly oval. This patternasymmetry essentially disappears asthe radials (L1) are shortened with toploading (L2).

2. When placed over ground, thecurrents in individual λ/4 radials arerarely equal. This can lead to asym-metric patterns and increased loss.The current asymmetry rapidly de-creases as the radials are shortened.

3. The peak gain, and the angle atwhich it occurs, changes relativelylittle as top loading is added and bot-tom radials shortened while keepingthe vertical section the same length.

4. Small amounts of inductive loadingcould also be used to supplement or evenreplace the top loading. As long as thevertical section is close to λ/4, the radiallengths can be reduced to λ/8 withoutseriously increasing losses.

Modeling IssuesThe realization that everything—

from the length of the radiator to thetype and distribution of loading—is apotential variable that may be ad-justed to achieve specific ends, is avery liberating idea. Unfortunately, itbrings its own set of problems. Whichvariations are best for a given applica-tion? A multitude of questions arisewhen judging any particular varia-tion.

The large number of possibilities andquestions cannot be dealt with analyti-cally, at least beyond an elementarylevel. The only practical way to deal withthe many variables is to systematicallyexplore the possibilities with NEC,MININEC or other CAD modeling soft-ware. Yet, even that is not a simplematter. Each modeling program has par-ticular strengths and weaknesses thataffect its use for this problem. The bot-tom portion of a vertical for 80 or 160meters is usually very close to ground(less than 0.05 λ). For these applications,the modeling software should imple-ment the Norton-Sommerfeld groundand properly model the current distribu-tion in the lower part of the antenna asmodified by induced ground currents.

Only NEC 2 and 4 do this. Of course, ifthe lower part of the antenna is buried inthe ground, only NEC 4 is suitable.

Loading structures may consist of aweb of wires with multiple wires at eachjunction, perhaps of different diam-eters, and with small angles (less than90°) between adjacent wires attached tothe same node. MININEC-based soft-ware can model multiple acute angles ifsegment tapering is used, but if manywires are used in the structure, thenumber of segments becomes quitelarge. MININEC Broadcast Profes-sional, using a different segment-cur-rent distribution, does an even betterjob without the need for tapering. How-ever, both of these programs do notmodel the interaction properly for verylow antennas over real ground. NEC 2can model the ground effects correctly,but may not handle the multiple smallangles properly, especially if differentdiameter conductors are connected.NEC 4 is much better in this respect,but is not widely used by amateurs be-cause of its expense.

Real grounds are frequently strati-fied beginning only a few feet down. On160 meters, the skin depth is of theorder of 15 to 20 feet, and it is commonto have several layers with differentelectrical properties over that dis-tance. Even in homogeneous ground,the effect of rain and subsequent dry-ing creates a varying conductivity pro-file. None of the presently availablesoftware addresses this problem. Thevalidity of NEC 2 or 4 modeling forground has been questioned because ofdifferences between experimentalmeasurements and predictions madeby modeling. This is a critical issue. IfNEC is fundamentally deficient withregard to ground modeling, then thecomparisons to date between buried-radial and elevated-radial systems areinvalid. That includes the work re-ported in this article! On the otherhand, NEC modeling may be fine, butthe problem lies with the highly vari-able nature of real ground. This is par-ticularly so down to depths of 15 to 20feet, which cannot be simulated withNEC, but that could greatly modifyexperimental results. Some supportfor this view comes from experimentalwork at higher frequencies. There theskin depth is much less, and modelingpredictions are in much better agree-ment with experiments.

The presently available software,while a remarkable achievement, isnot totally satisfactory to fully exploitthe possibilities. The suggested pointof view brings this out. A great deal of

care must be used when modeling avertical with a complex loading systemnear ground.

A Design ExampleThe advantages of employing a dif-

ferent conceptual approach can beillustrated using the 160-meter verti-cal used at N6LF, where an effectiveantenna was built on a very difficultsite at low cost.

The site is on a narrow ridge—approximately 60 feet wide at the top—in a forest. There is no possibility ofinstalling an extensive buried radialsystem because of the dense forest,heavy underbrush, steep slopes andvery large old-growth stumps. Even anelevated system of normal size, about260 feet in diameter, is not practical.

A support for the antenna was con-structed from three Douglas fir trees,fastened together to form an A frame(see the sidebar “A Large A-FrameMast, Inexpensively” for details). Thisresulted in a support 135 feet high.Allowing eight feet from the bottom ofthe antenna to ground and a few feetof slack at the top for sway in highwinds, the final vertical length is 120feet—very close to λ/4. Because theantenna is located over 700 feet fromthe shack, 75 Ω Hardline coax (a free-bee from the local CATV company) isused for the transmission line. Theantenna was designed to have a 75-Ωfeed-point impedance to match thetransmission line. The feed-point im-pedance at the junction of the lowerhat and the vertical wire was manipu-lated by adjusting the relative sizes ofthe bottom-hat and top-loading wires.Alternately, I could have used a largerhat on the bottom and moved the feedpoint up into the vertical part of theantenna, but this was not done be-cause of the limited space available forthe bottom hat. I also tried some in-ductive loading at the base and at thejunction of the top-loading wires.Relatively small amounts of inductiveloading—with very little additionalloss—would further reduce the size ofeither or both of the capacitive loadingelements. I did not keep any inductiveloading because sufficient space wasavailable for the arrangement shown.

The final antenna is shown in Fig 5.There are four radials at the bottom,connected by a skirt wire at the ends.The diameter of this bottom-loadingstructure is only 40 feet, comparedwith 260 feet for normal λ/4 radials.Two sloping wires are used for loadingat the top. A sloped top hat may not beoptimal when compared to horizontal

wires: The radiation resistance issomewhat lower. Nevertheless, thisarrangement is very simple and allowsthe antenna to be tuned by changingthe angle of the wires with the verticalportion of the antenna. This can bedone from ground level by shifting theattachment points for the guy linessupporting the sloping wires.

Christman’s comparison (see Note2) between a 120-buried-radial verti-cal and an elevated four-radial verti-cal (both with h = λ/4) indicates thatthe gain and radiation-pattern differ-ences between the antennas are quitesmall: 0.35 dB for peak gain, 1° forpeak gain angle. Because the differ-ence is so small, I have chosen to usethe four-radial elevated antenna asthe reference antenna, since it is mucheasier to model than a complete 120-buried-radial antenna.

Using NEC4D for modeling, radia-tion-patterns for a four-radial ground-plane antenna and this antenna werecompared. The result is presented inFig 6. The model assumes ground ofaverage electrical characteristics un-der the antenna (σ = 0.005 S/m; ε = 13).The wire used was #13 copper, and itsloss was included in the modeling. Theprice paid for drastically reducing thediameter of the bottom loading struc-ture is a peak-gain reduction of 0.5 dB.This is a fair trade for dramaticallyeasing the installation of the lowerloading element because 0.5 dB willprobably not be detectable in actualoperation. In the real world, wherefull-size (λ/4) radials very likely havevarying currents (see Notes 4 and 8),the smaller antenna may not, in fact,be inferior at all. In this particularexample, full-size radials would needto zigzag down a steep hillside at vari-ous angles. It is very doubtful they

would have been any better than thesmall hat that was adopted.

Any antenna with an elevated radialsystem needs an isolation choke (com-mon-mode choke, or balun, if you pre-fer) on the transmission line near thefeed point. One effect of moving theloading from the bottom to the top ofthe antenna is to increase the poten-tial between the feed point and ground.This requires more inductance in theisolation choke to properly decouplethe transmission line. For this appli-cation, I happened to have a roll of1/2-inch Hardline. The roll was abouttwo feet in diameter, so I simply ex-panded it into a coil three feet long andtwo feet in diameter with a simplewood framework to hold it in place.Fig 7 is a photo of this king-sizeddecoupling choke.

The result was a choke with 350 µHof inductance (4 kΩ at 1.840 MHz).When this value of inductance wasplaced in the model with a buriedtransmission line, there was still someinteraction; resonance was displaceddownward. This was also found trueon the actual antenna. This illustrates

one of the drawbacks of very small bot-tom-loading structures: A choke withenough inductance to avoid interac-tion may not be practical, at least on160 meters. Since the current in thechoke is relatively small, additionallosses due to ground currents will notbe very large. The Q of the choke, how-ever, must be high to limit losses in thechoke itself.

The monster balun shown here isextreme and not required. A muchsmaller choke could be used. The largestructure was used because it was ac-tually very convenient with the mate-rials on hand.

Fig 5—Antenna configuration.

Fig 7—Rudy and the “small” decouplingchoke.

Fig 6—Comparative radiation pattern.Fig 8—Flat versus drooping loadingwires.

A Large A-Frame Mast, Inexpensively

A λ/4 vertical is about 70 feet tall on80 meters, and 130 feet on 160meters. Getting this height with atower can be expensive. I neededa less-expensive alternative. In thePacific Northwest, fir trees withheights greater than 100 feet arecommon, and can usually be pur-chased locally and inexpensively ifthey are not already growing on yourproperty. In the southeastern US,there are extensive pine forestswhich, while not typically as tall as thefirs, can be used in the same way. Ihave many tall Douglas Fir trees onmy property, so I selected three ofthem, two with 12-inch diameterbases and one of about 8 inches. Itrimmed the top off the two largertrees at a point where they were aboutfive inches thick. This gave me twopoles approximately 80 feet long.Since I was only going to support awire vertical, I topped the smaller treeat a point where it was roughly twoinches thick. This gave me a pole 60feet long. I was trying to have thecross-sectional area at the top of eachlarge pole roughly equal to the area atthe base of the smaller pole whenthey were overlapped.

The next step was to drag the polesto the antenna site and assemble theA-frame shown in Fig 9:

1. I bought a large, used railroad tieand cut it in half at the middle of itslength. I then buried each half verti-cally with about 18 inches above theground to form a pivot post. I placedthe posts about 10 feet apart.

2. I placed the two large poles,side-by-side, midway between the twoposts.

3. I placed the smaller pole on topof the two large poles—overlappingby about five feet—and lashed thethree poles together using #9 galva-nized smooth iron fence wire asindicated in Fig 9C. To begin the lash-ing, I stapled the end of the wire; as Iapplied each turn, I tightened it with aclaw hammer. After 15 turns or so, Istapled the free end.

4. I then spread the butt ends of thelarge poles out to the pivot posts. [Didyou use a team of mules, orjust your burly “pecs”?—Ed.] Thisspreading tightened the lashings verynicely (!) so that the three poles weresolidly connected.

5. I wanted to raise and lower the Aframe at will and keep the pole endsaway from soil contact (rot!). There-fore, I created a pivot at each post bydrilling a 2-inch-diameter hole through

the post and pole butt. I then inserteda length of 1.5-inch galvanized ironwater pipe as the shaft for the pivot.To keep the pipe from slipping out, Iput a pipe cap on each end as aretainer.

6. The next step was to attach twohalyards (one spare, just in case!) tothe top of the mast. I used two smallpulley blocks—the kind typically usedon sailboats—and then rove alength of black, sun-resistant, 3/8-inchDacron line through each block. Thelines were long enough to form a con-tinuous loop reaching the ground, soI could hoist or recover the antenna atwill.

7. Finally I erected the A frame. Inmy case, I used a nearby tree as a ginpole (suitably guyed!!) along withthree steel blocks and a long length ofwire rope. Hoisting power was sup-plied by a small tractor. I took greatcare because of the forces involved.The initial lift required a pull of over1000 pounds and the A frame weighsover a ton. (Green trees are heavy!)If I were more patient, I could have al-lowed the trees to dry out (months!),

which would have greatly reduced theweight.

I choose not to raise the mast to avertical position because I wanted theantenna and the loading structures tostand clear of the mast and any guys.As shown in Fig 9B, I left frame tiltedabout 15° from vertical and bent thetop over like a fishing pole, so it iseven farther out from the base. Thegreen pole bent relatively easily, andthe bend became permanent whenthe wood dried out.

I used two wire-cable back-guys,anchored at the junction of the poles,to hold the mast in place. Althoughthe weight of the mast makes it un-likely it would blow over towards theguys, I use the spare halyard as a guyfrom the top of the mast in the oppo-site direction to the wire guys. This ar-rangement minimizes conductors inthe near field of the antenna.

The cost of the entire exercise wasless than $75, and I expect to getmany years of use from the mast. Ofcourse, I had the trees, the tractor andthe hoisting tackle, which kept thecost very low.

Fig 9

More ModelingIn the process of developing this

antenna, a great deal of additionalmodeling was performed to explore theeffect on performance of differentloading arrangements. One of themore interesting variations was asymmetrically loaded, two-radial an-tenna called a Lazy-H vertical (seeNote 6). This antenna is intended tobe supported between two trees. Theantenna is identical to that shown inFig 3, except that L1 = L2. Table 1gives a comparison between a full λ/2vertical, a λ/4 ground-plane with twoand four radials and the Lazy-H withdifferent values of h (height of thevertical portion) varying from 120down to 30 feet. Note that the λ/4Lazy-H is within 0.3 dB of the four-radial λ/4 vertical and has greaterbandwidth. If two supports are avail-able, the Lazy-H is much easier to fab-ricate than the four-radial version,and has significant size in only twodimensions instead of three. I as-sumed #13 copper wire and averageground for the models. Zend is the im-pedance at the junction of the verticalsection’s lower end and the lower radi-als. The bottom of all the antennas isassumed 10 feet above ground.

In the 160-meter example givenearlier, the top loading structure wassimply a pair of drooping wires led toanchor points near ground. The ques-tion arises as to the comparison be-tween flat configurations, like thatshown for the Lazy-H and the droop-ing-wire alternative. This questioncan be quickly answered by modelingan end-loaded dipole in free space with

two different configurations as shownin Fig 8. The modeling shows that thedrooping wires must be lengthened toachieve resonance, the radiationresistance is significantly lower withdrooping wires and the far-field pat-tern is essentially the same. From apractical point of view, the use ofdrooping wires greatly simplifies thestructure, and has very little effect onthe far-field pattern. It may reduce theefficiency of the antenna if the radia-tion resistance is lowered too much,however. This is the kind of trade-offinformation critical to a new design.

In general, modeling this class ofantennas shows that peak gain andpeak-gain angle primarily determinedby ground characteristics and theheight of the vertical radiator, h. Theloading means has only a second-ordereffect on the radiation pattern. A vari-ety of loading arrangements can sat-isfy a particular situation with littleloss of performance—as long as wekeep the radiation resistance highenough to control losses.

tenna modeling software should be verycareful when setting up the model andinterpreting results.

AcknowledgementIn addition to the referenced papers,

other workers in this field havepointed out the advantages of thepoint of view presented here. This ideais certainly not the author’s creation,although I wholeheartedly endorse it.Moxon’s work deserves careful read-ing. I am indebted to Dr. L. B. Cebik,W4RNL; Dick Weber, K5IU, andGrant Bingeman, KM5KG, for theircomments and support.

Table 1—Antenna Comparison at 3.510 MHz

Antenna h (ft) L1 = L2 (ft) Zmiddle Ω Zend Ω Peak gain (dBi) Peak angle ° Wire loss (dB) 2:1 BW (kHz)λ/2 137 0 91 >5000 +0.30 16 0.08 270Lazy-H 120 4.4 96 1096 +0.28 17 0.07 280Lazy-H 100 10.4 94 384 +0.12 19 0.07 280Lazy-H 80 17.4 81.3 180 -0.06 20 0.08 260Lazy-H 69.8 21.6 71.2 127 -0.07 21 0.09 240Lazy-H 60 26.3 59.7 90.9 -0.15 22 0.10 200Lazy-H 40 38.3 33.7 40.8 -0.38 24 0.16 140Lazy-H 30 45.6 21.5 23.8 -0.59 25 0.23 100λ/4 (2 radials) 69.8 — — 38.8 +0.11 by-0.39 22 0.15 200λ/4 (4 radials) 69.8 — — 35.7 +0.21 22 0.13 175

Notes1Brown, Lewis and Epstein, “Ground Sys-

tems as a Factor in Antenna Efficiency,”IRE Proceedings, June 1937, Vol 25, No.6, pp 753-787.

2A. Christman, “Elevated Vertical Antennasfor the Low Bands: Varying the Height andNumber of Radials,” ARRL Antenna Com-pendium, Vol 5, 1996, pp 11-18.

3A. Christman, “Elevated Vertical AntennaSystems,” QST, Aug 1988, pp 35-42.

4R. Weber, “Optimal Elevated Radial Verti-cal Antennas,” Communications Quar-terly, Spring 1997, pp 9-27.

5L. Moxon, “Ground Planes, Radial Systemsand Asymmetric Dipoles,” ARRL AntennaCompendium, Vol 3, 1992, pp 19-27.

6R. Severns, “The Lazy-H Vertical Antenna,”Communications Quarterly, Spring 1997,pp 31-40.

7J. Belrose, “Elevated Radial Wire SystemsFor Vertically-Polarized Ground-PlaneType Antennas,” Part 1, CommunicationsQuarterly, Winter 1998, pp 29-40; Part 2,Spring 1998, pp 45-61.

8D. Weber, “Technical Conversations,”Communications Quarterly, Spring 1998,pp 5-7 and 98-100.

ConclusionsThis article has advocated a different

conceptual view of vertical antennas:They can be viewed as loaded dipolesclose to ground. Changing the point ofview makes it easier to recognize thewide range of options available for con-figuring a high-performance vertical tomeet the needs of a particular site andset of limitations. To assess the manyoptions, we need the help of software.Unfortunately, no available softwarepackage provides the desired computa-tional capabilities. Users of any an-