The Underestimated Magnetic Loop HF Antenna articl

1 1An Overview of the Underestimated Magnetic Loop HF AntennaIt seems one of the best kept secretsin the amateur radio communityis how well a smalldiminutivemagnetic loop Antenna canreallyperformin practicecompared with objective of this articleis to disseminate some practicalinformation about successfulhomebrewloop construction and toenumeratethe loop skeydistinguishing characteristics and unique loop antennacanveryconvenientlybe accommodatedon a tabletop,hiddenin an attic/ roof loft,an outdoorporch, patiobalconyofa high-rise apartment,rooftop,or any otherspace small but efficacious HF antennafor restricted space sites is the highly sort after HolyGrail of manyan amateur radio andinterest is particularly strongfrom amateurs having to facethe prospect ofgiving up theirmuch lovedhobby as theymove fromsuburban residential lots into smallerrestricted space retirement villages andother communities that have strict rules against erecting elevated Antenna structures.

2 Inspite of these imposed restrictionsamateurs do have a practical andviablealternativemeans to actively continue the hobby using a covertin-door or portable outdoor andsympathetically placed small Magnetic paper discusses howsuch diminutiveantennascanprovide an entirely workable compromise that enable keen amateursto keepoperating their HF station without anyneed for their previoustalltowers andfavouritebeam antennas or unwieldy G5 RVorlong practical difference in station signalstrength at worst will be only an S-point orsoif good design and construction is making acursory investigation into the subject of Magnetic loop antennas usingthe Google internet search engine willreadilyfindanoverwhelmingand perplexingabundanceof article will assist readers in making sense of the wide diversityofoftentimesconflictinginformat ionwith aview tofacilitate the assimilation of theimportantessence ofpractical knowledgerequired to make an electrically-smallloop workto its

3 Full potentialand yieldverygoodon-airperformancewithacapab le account of few(sobering)facts:Aproperly designed,constructed, and sitedsmallloopof nominal 1m diameterwillequaland oftentimesoutperformanyantennatypeexcept a tri-band beam on the 10m/15m/20mbands, and willat worstbe within an S-point (6dB)or soof an optimised mono-band 3elementbeamthat smounted atanappropriateheightin wavelengthsabove loops really come into their own on the higher HF bands from say 40m throughto 10m; frequentlywithabsolutely stunning performancerivalling the best field deployable and fixed sitetunedloops havebeenthe routine antennaof choice formany years in professional defence, military,diplomatic,andshipboardHFcommun icationlinks where robust and reliable general coverage radiocommunication 80m and 160m top-band theperformance of asmall loopantennagenerallyexceeds thatachievable from a horizontal dipole.

4 Particularly one deployed atsub-optimal height above is a common site limitation realpracticaladvantage of the small loop compared tosaya short vertical whiptunedagainst earthor a full sized vertical antennais the loop sfreedom from dependence on aground planeand earthforachievingefficient operation;thisuniquecharacteristic hasparticularprofoundsignificance for smallrestricted spaceantennas operatingon the80mand 160mHF where s the catch; if the small loop is such a good Antenna why doesn t everyone haveone and dispense with their tall towersand traditional antennas?

5 The laws of nature andelectromagnetics cannot beviolated and the only unavoidable price one pays for operatingwith an electrically-smallantenna is narrow bandwidth. Narrow instantaneous bandwidthrather than poorefficiencyis the fundamental limiting factor trade-off with small smalland compact(in terms of a wavelength) Antenna willinherentlybe narrow bandand require tuning to the chosen operating frequency within ofmagnetic loops must be content with bandwidthsof say 10 or 20 kHz at 7 MHz or a littlemore than They are content as longas the Antenna can be easilytuned to cover therange of spotfrequencies that they wish to a remotely sited or rooftop mountedantennaimplementingthistuningagil ityto QSY across the bandrequires just a modicumof that ingenuityand improvisationradio hams are renowned for.

6 Figures 15 and the efficiency of a small vertical with a small loop Antenna one tradesgrounddependency andearth losses formucheasier controllableconductor losses in both the loopradiating element and ohmic loss in the associated tuning capacitance and smalltransmittingloop(STL)antennais defined as having a circumferenceof more thanone-eighthwavelength butsomewhatlessthan one-thirdwavelength which results in anapproximatelyuniform current distributionthroughout the loopand the structurebehavesas a a short vertical or dipole Antenna .

7 The looppresentsaninductive reactanceat itsterminalsso tuningand matchingis convenientlyaccomplishedwith a single can be resonated withcapacitance to formahigh-Q parallel Antenna Q is very high because the radiation resistanceis small compared to the reactance of the loop and theVSWR bandwidth is very attainment of ahigh-Q tells us that theloopantenna is not lossyand is appliedto the loopat its resonant frequencyallofthatpower will be radiatedexcept that portion absorbed in thelumpedI2 Rconductor and capacitor lossesmanifestingaswastefulheat.

8 With proper designandcarefulconstructiontheseseries equivalent circuitlosses can be made negligibleorat leastsufficiently smallcompared to the loop sradiationresistancesuchthataresultantly highintrinsicradiation efficiencyand goodantennaperformancecan be achievedfrom a relatively small HF Antenna !Thevertically orientedSTL Antenna sfigure-of-8 doughnut shaped radiation patternmaximumis in the plane of the loop with nulls at right angles to the plane of the oriented loops function perfectly well close to ground horizontallymounted, the antennapatternis omnidirectional with nulls straight up and straight oriented loops should be elevateda significant fraction of a wavelengthabovegroundto prevent significant ground losses.

9 Consequentiallythis orientation is the loop s radiation resistance results in RF power being converted toelectromagnetic wave must comprise both Magnetic andelectric field componentsin orderfor itto exist. In the case of an STL, a strong magneticfield is generated by passing a substantial RF current through the loop conductor and thismagnetic field in turn generates a corresponding electric field in spacethus providing thetwo essentialE and is where the term Magnetic loop Antenna , since thesmallloop sradiation resistance is very small compared tothat ofafullsizedresonant dipole.

10 Getting afavourableratio of loss to radiation resistance is theonly tricky and challengingpart ofpractical loop design current flowing through an Antenna s radiation resistance when squared and multipliedbythat resistancedetermines the amount of power that s actually to therelativelylow value(few hundred milli-Ohms)of a small loop s radiation resistance, highcurrents(tens of Amperes)are necessary to enable even moderate powers to be radiated;soit is essential that allcontributingsources of loss must be kept low in comparison with theradiation resistance in order to achieve high efficiencywith the majority of the antennainput powertransmutedinto the radiation field,rather than being dissipated as heat energyinall of the contributing sources ofdeleteriousloss utilizing alarge sizedsplit-stator(see Figure 1)orasimilarbutterfly styleairvariable capacitor construc