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73 Reviewby Jeffrey Sloman N IEWO

Th e AEA IsoLoopTM

1 0·3 0A big antenna in a small package.

Advanced Electron ic Applications, Inc.P,O. Box C2160

2006 196th S1. sv«Lynnwood WA 98036-09 18Telephone: (206) n 4-5554

Price Class: $349

Can a two-inch aluminum band a li ttle overthree feet in diameter worx as anything

more than a dummy load on HF? The text­books say it can, and AEA has proven it withthe new lsol oop 10..30. Loop antennas havebeen in use from the beginning of radio, butpractical loops tor use at HF frequencies faceseveral engineering problems and real worldlimitations that AEA has managed to over­come.

The Isol oop is a 43-inch aluminum loop,with a center portion--made of UV resistant,injection molded high density polyethylene­shaped rooghly like a dumbbell. In the centerof the dumbbell section is a hole designed toaccept a mast up to two inches in diameter,along with stainless steel hardware for clamp­ing the antenna in place. A stainless steelhose clamp is provided for mounting the an­tenna radially, fo r use from, say, a balconyrailing. The stainless U-bolt is also needed,and it is a minor inconvenience that the an­tenna housing must be disassembled-threehex bolts with nylon-retained aircraft nuts-toremove it from its default center position.

In the la rger end 01 the dumbbell is a10 ,OOO-volt scut-stetcr capacito r. The twoends of the irradialed aluminum band thaimakes up the loop are we lded to the twohalves of the capaci to r's stator. This one­piece design is very rugged, its only down­side being the need to detcrm the loop to fit itinto a UPS-shippable box. It takes some workto get the loop rourod again after unpacking it,though it need not be perfectly round to oper­ate perfectly. If you are like me you will wantthe loop to be round for aesthetic reasons. Al·so in this end of the housing is a precisionsteppe r motor and gear train for remote con­trol of the capacitor's tuning.

On the smaller end 01 the dumbbell is aone-turn electrostatically shielded loop madeof coaxial cable. This Shielded coupling loopmatches the extremely low impedance--lessthan 1/10 ohm---of the rad iating loop to the5O-ohm teecuoe. It also acts as a balun whichisolates the feedline from the antenna-theeffect that gives the IsoLoop its name. The in­put to the antenna is through a supplied rightangle PL-2S9 adapter which helps 10 routethe coax at a 90 degree angle to the antenna.The antenna must be mounted with the SO­239 connector facing down, along with the

16 73 Amateur Radio Today · October, 1992

Photo A. The dumbbell shaped housing of thefsoLoop contains a 10,OOO-volt split-statorcapacitor, a s tepper motor, and a one-tumloop for impedance matching.

drain hole also located on this side.Al so in the package are the LC-2 co n­

troller- for tuning the antenna-and its 12­volt wall mount transforme r power supply.Th e LC-2 is a small beige box with twothumbwheel controls, two push-button switch­es, and four LEOs. The left-side co ntrol­ma.rked SEN5-adjusts the sensitivity of theLED audio level indicators; I'~ explain theselater. The control on the right-markedSPEED--adjusts the pulse rate of the signalsent to the stepper motor located in the an­tenna , which adjusts the tuning speed. Thepush-buttons control the direction of the ca­pacitor 's travel. On the back of the LC·2 arejacks for power (standard coaxial), the step­per motor (S-pin DIN), and the audio in/out(1/8 phone).

New and ImprOVed

This IsoLoop is the new and improved ver­sion of the Ofiginal lsoLoop 14-30 antenna in­troduced in 1990. {Ed. Note: See the reviewof the original antenna in the September1990 issue 0173, p. fO.l The original had anoperating range of only 14-30 MHz; AEA hasadded 4 MHz to the low end to cover the 30meter band. The original used aluminum tub­ing and required assembly. This design was

prone to loss from bad connections of thetubing sections to each other and the cecec­tor. The older model used a belt drive for re­duction from the stepper motor to the capaci­tor, while the improved version uses a sear­driven reduction unit

How It Works

The rsoLoop has a wonderfully elegant de­sign. It is a simple tuned LC circuit, with thealuminum band providing the L and the cus­tom designed capacitor providing the C. Theconnection to the antenna is made th roughmutually coupled air core inductors. The one­tum electrostatically shielded loop is induc­tively co upled to the resonating loop. Un­doubtedly, many of you have already recog­nized this as the same design common to an­lennas used by BeB (Broadcast Band) OXenthusiasts. The difference between this an­te nna and the IsoLoop is twofold . TheIsoLoop is designed for much higher frequen­cies and so is actually quite efficient in spiteof its small size. Its efficiency ranges fromabout 70% on 20m to as high as about 9S%on 10m.

The second principle difference is the ca­pacitor in the lsoLoop. Designed for transmit­ting, it is capable of about 150W. Its spl it sta­tor design avoids the moving contacts re­quired by conventional designs. The IsoLoopachieves the ideal of placing the tuner at theantenna. This antenna tuner does what itsname says: tunes the antenna! Because theIsoLoop is actually resonant, it easily outper­forms practical dipoles mounted at the sameheight. There is some misunderstand ing con­cerning the ability of a small antenna to per­form well in the HF bands. The fact is, what isimportant is resonance- and this antennaresonates.

Insta ll ing the IsoLoop

Unpack ing the IsoLoop is easy: it ispacked in a box slightly smaller than theIsoLoop's diamete r. Two small ca rdboardboxes contain the lC-2 controller, its powersupply, and male-to-male 1f8-inch phonepatch cord . The antenna slides from the boxwith nme effort, and its 18·pound weight is nottoo difficult for one person to handle. Out ofthe box the antenna is set up for axial mount­ing, parallel to the earth. In this configuration

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Photo B.The fsoLoop 's default mounting posi­tion is parallel to the earth and provides anomnidirectional pattern.

the antenna's radiation pattern is omnidirec­tional. A bidirectional pattern is also possibleusing the alternate mounting position. whichplaces the antenna pe rpendicular to theearth. Since the antenna is especially goodlor restricted space installations-like apart­ment buildings-using the al ternate (radial)mounling positio n may prove useful for in­stalling the antenna on high-rise balconiesand cut of windows.

When choosing a mounting location, keepin mind that the Isoloop will on ly performproperty when mounted at least lour feet fromlarge-e-especially metall ic-objects. Th is in­cludes four feet from the ground, which, whileit sounds like a relatively poor location, is notnecessari ly that bad. The lsoloop Is a loopantenna and not a dipole. It does not sufferfrom the problems of a dipole located closerthan a half wavelength to the earth. Whilefour feet off the ground is clearly not ideal.the IsoLoop's radiation angle is about 37 de­grees. while only a quarter wave from the

earth. Th is low radiation angle insures betterOX performance by delivering most of thetransmitter's power at an angle that will takeadvan tage of ionospheric propagalion. Re ­member. ThQ angle of incidence equals theangle of reflection.

While the tsoLoop was be ing tested here,it spent most o f its time o n a fo u r- footaluminum stepladd er in the midd le of thesecond -floo r ham shack . Even in th ismakeshift instal lation the antenna performsexceptional ly. In any case, while the lsoloopis more forgiving than other antenna designs,it still woos better mounted higher in the air.Its relative ly small size allows for mountingwith standard TV mast and hardware. andits low profile is unl ikely to cause too muchconstemation among the neighbors.

orce the mounting location is chosen, andthe antenna phySically mounted. the feedlineand control cable must be routed back to thetransceiver. Supplied with the antenna is aright-angle adapter for the SO-239 input tothe antenna. This allows the coax to be rout­ed at 90 degrees to the antenna which mini­mizes induced currents in the teedune . Asmall piece of Coax Sear" is incluc:led to pro­tect the antenna connection. Fifty feet of coo­trol cable comes install ed on the antenna. Ifthis is not enough. AEA can supply SO-footextension cables. The S-pin DIN connectorused on the control cable is a common type•and the cable itself ls a shielded S-conductorcable , so build ing o ne yourself of arbitrarylength should be no problem.

Once the cables are routed back to th eshack. the coax is connected to the transce iv­er and the control cab le is connected to theS-pin DIN connector on the back of the LC-2control box. The LC-2 will also need its powersupply connection. The supplied patch cord isused to connect the ng's speaker output tothe input on the back of the unit. and an ex­ternal spea ker is plugged into the adjacentoutput. These connections are only neces-

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18 73 Amateur Radio Today · October. 1992

Photo C. The special split-stator tuning capacitor built into the IsoLoop is capable 01 handlingup to 150 watts. The capacitor is remotely tuned via a motorized gear-driven reduction unit.

Diameter$I1ipping weight

sary if you intend to use the LC-2's b.Jning in­dicator-which is especiaJty useful for olderradios (see the description below). Once alllhese connections are made, the antenna isready for use.

Using the IsoLoop

Without so me understanding 01 how touse the IsoLoop, you could spend seve ra lfrustrating hours getting the antenna to wc:>rk.Because of tile extremely narrow passbandof the lsoLoop, it is important to understandhow to tune the lsoLoop ancUor to develop atechnique of you r own.

Tuning the loop is accomplished with theLC-2 controlle r. The capacitor is driven by thestepper motor through a 30:1 gear reductiondrive. This allows very fine adjustments of thecapacitor, which are necessary because ofthe very tight resonance of the loop. Thespeed contro l adjusts the pulse rate of the...ignal to the stepper motor with the slowestsetting providing me tiny adjustments neces­sary to fine -tune the SWR, while the highestspeed witt move the capacitor'S rotor throughits entire rotation in about 15 seconds. Tuningworks like this:

1. Remove any antenna tuners from tilefeedline, and jf the rig has a built-in tuner­tum it off.

2. With the speed control in the fastest po­sition, press either direction button until thenoise in the receiver peaks. This will happenquite suddenly, and the peak is very small.You will probably pass through the peak, butlet go of the button as soon as you notice it. Itwi ll sound like a burst of noise. (Note: If youhave an older rig with a mechanical a-meter,and you ha ve connected-what AEA calls­the audio-visual LE Os, you can use these toobserve the peak. Adjust the sensitivity con­trol until just the left-most LED gloWS, and thefirst of the two center LEOs flicker. As you ro­late the capac itor, you can watch l or thepeak-on the LEDs-as well as listen for it Ifyou have a modem rig with an electronic VUmete r-one that supplements the a -meter.showing the audio level-you can use this in­stead of the LC-2's LEOs.)

3. Adjust the speed control to about middlespeed . Pre ss the oth er direction b ulto n,wh ich will bring the capacitor back the otherway, toward the peak you passed. You willnot have to wait OOg. You will probably passthrough the peak in the other direction. Alter­nate the directions--adjusting the speed ceo­trot downward if necessary-until you feelyou have peaked the noise (or signal) as bestyou can.

4, Adjust the speed control to its slowestposition and, using an SW R meter, repeat theprocedure above fo r the lowest reading . Acorrectly installed IsoLoop ShOuld tune down10 about 1.5:1 or less from 1().3O MHz. Re­tuning will be necessary every 1().100 kHz ,the bandwidth increasing w ith frequency.Keep in mind :

-The capacitor has no stop, it rotates freelyand there is no absolute up and down relatedto the directional controls.

-The peak is very small ; you will have to

practice to make the antenna work.-rumoff your antenna tuner! You will try

forever to get the Isoloop tuned with no suc­cess if it is on.

-ae sure 10 mount the antenna at least fou rfeet from large objects if at all possible.

As you can see, the tuning proced ure­while not necessari ly complex--is specific.Once you get the hang of the procedure youwi ll probably find yourself using faster andfaster speeds fOf" all but the final tOlJCh.up forSWR. You will also become belter at hearingthe peak. This antenna becomes better asyou do. Some of you may remember a similarprocedure-at least in leel-from the daysbefore au tomatic antenna toners.

Performance

I was interested in the lsoLoop because ofmy limited space and restrictions against oct­side antennas. Connected to a Kenwood T8­45OS, the antenna performed brilliantly. I hadthe opportunity to work some band openingson 10m--and got universall y excellent signalreports. Running about 25W, I was able towork the East Coast from my Indiana OTH. Ihad a hard time convincing some of the sta­tions I contacted that I was us ing the IsoLoopand 25W--but I was. Keep in mind, too , thatthe antenna was indoors on an aluminumstepladder. The lsoloop consistently outper­formed a 50-foot longwire using the automat­ic antenna tuner in the Kenwood. [ was ableto monitor packet OSO on 30m, and CW andsse QSO on 20, that were not even audibleon the Iongwire.

Who Should Use an IsoLoop?

The IsoLoop is extremely nexible. It is theperfect limited space HF antenna, usefu l forapart ment dwellers, those w ith restrictivecovenants, and those with aesthetically sen­sitive neighbors . It is also useful for mobileapplications, such as mobile homes, emer­gency command vehicles, and boats-butwith its 15 pound weight. I would be hesitantto put it o n a car (though I have heard it'sbeen done). Even if you don't have space re­strictions, the lsoloop works better than wireantennas , is easy to install and use , a ndmight just be the ideal an tenna to supplementyour tribander.

Conclusion

The IsoLoop is one of those products thaiis a pleasure to use. It is an elegant applica­tion of a traditional design with modem engi­neering. Its performance is exemplary; it willnot disappoint you. iii

IsoLoop 10-30 Specifications

Frequency coverage 10 to 30 MHz (c:ontroous)

Nominal inpedance 50 etmsCol. llldoI S().239

Power hatding 15(1ON

VWSR 1.5:1 or less acrossoperaling range4J'

25 pounds. may be shipped UPS

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73 Amateur Radio Today- OCtober, 1992 19