SMALL  LOOP  ANTENNAS

An Overview of the Underestimated Magnetic Loop HF Antenna

Leigh Turner VK5KLT 

It seems one of the best kept secrets in the amateur radio community is how well a small diminutive magnetic loop antenna can really perform in practice compared with large traditional HF antennas. The objective of this short paper is to disseminate some practical information about successful loop construction and to enumerate and summarise the loop’s key distinguishing characteristics and unique features. A magnetic loop antenna can very conveniently fit on a table top, in an attic / roof loft, an outdoor porch, patio balcony of a high-rise apartment, or any other space constrained site or antenna restricted community. 

A few facts:
A properly designed and constructed small 1m diameter loop will outperform any antenna type except a tri-bander beam on the 10m/15m/20m bands, and will be within an S-point (6dB) of an optimised mono-band beam mounted at an appropriate height above ground. 

So where’s the catch; if the small loop is such a good antenna why doesn’t everyone have one and dispense with their tall towers?  The laws of nature and electromagnetics cannot be violated and the only price one pays for operating with an electrically-small antenna is narrow bandwidth. Narrow instantaneous bandwidth rather than poor efficiency is the fundamental limiting factor trade-off with small loops. Any small antenna will be narrow band and require tuning to the chosen operating frequency within a given band. Users of magnetic loops must be content with bandwidths of say 10 or 20 kHz at 7 MHz or a little more than 0.2%. They are content as long as the antenna can be easily tuned to cover the frequencies that they wish to use.  For a remotely sited or rooftop mounted antenna this requires just a modicum of that ingenuity and improvisation radio hams are renowned for.   

A small transmitting loop antenna is defined as having a circumference of more than one-eighth wavelength but somewhat less than one-third wavelength which results in a uniform current distribution throughout the loop and the structure behaves as an inductance.  The doughnut shaped radiation pattern is in the plane of the loop with nulls at right angles to the plane of the loop. The loop self-inductance can be resonated with a capacitance to form a high-Q parallel tuned circuit.  The attainment of a high-Q tells us that the loop antenna is not lossy. When power is applied to the loop at its resonant frequency all of that power will be radiated except that portion absorbed in the lumped I²R conductor and capacitor losses manifesting as wasteful heat. With proper design these series equivalent circuit losses can be made negligible or at least sufficiently small compared to the loop’s radiation resistance that resultantly high intrinsic radiation efficiency and good performance can be achieved.

 Current through the loop’s radiation resistance results in RF power being converted to electromagnetic radiation.  However, since the small loop’s radiation resistance is very small compared to that of a full sized resonant ½ λ dipole, getting this favourable ratio of loss to radiation resistance is the only “tricky” part of practical loop design and homebrew construction. Through utilizing a split-stator or a butterfly style air variable capacitor construction or a vacuum variable capacitor, low loss can be achieved in the tuning capacitor. Conductor loss can be controlled by optimal choice of the diameter of copper tubing used to form the loop element and paying very careful attention to low ohmic interconnections to the capacitor such as welded or silver soldered joints, etc.  Even with 100 Watts of Tx drive power there are many tens of Amperes of RF circulating current and Volt-Amps-Reactive (VAR) energy flowing in the loop conductor and tuning capacitor. 

Capacitor losses are further minimised by welding the rotor and stator plates to the stacked spacers to eliminate any residual cumulative contact resistance. When connected across the loop terminals the butterfly construction technique inherently eliminates any lossy rotating contacts in the RF current path. The configuration permits one to use the rotor to perform the variable coupling between the two split stator sections and thus circumvent the need for any lossy wiper contacts.  Since the fixed stator plate sections are effectively in series, one also doubles the RF breakdown voltage rating of the composite capacitor. In view of the fact the loop antenna is a high-Q resonant circuit, many kilovolts of RF voltage can be present across the tuning capacitor and appropriate safety precautions must be taken.

 Although loop antennas have deceptively simple appearance, they are complex structures with radiation patterns and polarisation characteristics dependent on whether they’re fed in a balanced or unbalanced fashion.  The method of feeding and matching the loop resonator, ground plane configuration, as well as the form factor and proportions of the loop element itself are all fertile ground for experimentation.  Various matching methods include series capacitor, transformer coupled subsidiary loop, and gamma-match; each with their merits.

 Small loop antennas have at least two simultaneously excited radiation modes; magnetic and electric folded dipole modes.  When the ratio proportions of loop mode and dipole mode radiation are juggled to achieve equal strengths some radiation pattern asymmetry results and a useful degree of uni-directionality can be achieved with a typical front to back ratio of about 6dB or so.

 The small loop with its doughnut shaped pattern exhibits a typical gain of 1.5 dBi over average ground and a gain of 5 dBi when deployed with either short radials (the length of each radial need only be twice the loop diameter) or mounted over a conductive ground plane surface. By comparison a large ½ λ horizontal dipole mounted ¼ λ above average ground has a gain of 5.12 dBi and a ¼ λ Vertical with 120 radials each ¼ λ long has a gain of 2 dBi over average ground.  The front to side ratio of a loop is typically 20 to 25 dB.

However the small loop has one very significant advantage over any other antenna due to its unique radiation pattern.  If the vertically oriented loop’s figure-8 doughnut pattern radiation lobe is visualised standing on the ground the maximum gain occurs at both low and high angles, radiating equally well at all elevation angles in the plane of the loop, i.e. radiation occurs at all vertical angles from the horizon to the zenith. Because the loop radiates at both low and high angles, a single loop can replace both a horizontal dipole and a Vertical.  This is particularly beneficial on 160, 80 and 40m where the loop will provide outstanding local / regional coverage and easily match and often outperform a tall ¼ λ Vertical for long haul DX contacts, i.e. an exceptionally good general purpose antenna.

 Energy radiated by the small loop is vertically polarised on the horizon and horizontally polarised overhead at the zenith. It will be quickly realised that a loop has the distinctive property of providing radiation for transmission and response for reception over both long distances and over short to medium distances.  This is achieved by virtue of low angle vertically polarised propagation in the former case and by means of horizontally polarised oblique incidence propagation in the latter case.  In contrast, a Vertical monopole is useful only for low angle vertically polarised propagation since it exhibits a null overhead and poor response and radiation at angles in excess of about 45 degrees. Such antennas are of course very useful for long distance communication by means of low angle sky wave skip propagation, or for short range communication via the ground wave propagation mode.

In further contrast, a horizontal ½ λ dipole (or beam arrays comprising dipole elements) at a height above ground of a just a fraction of a wavelength (as opposed to idealised free space or mounted very high) exhibits maximum polar response directly overhead (good for NVIS) with almost zero radiation down near the horizon.  Such popular “cloud warmer” antennas in residential situations as the surreptitiously hung ubiquitous G5RV, End-feds, dipoles, inverted-V, etc. are thus most useful for short to medium range communication in that portion of the HF radio spectrum where oblique incidence propagation is possible.   

Importantly it should be noted when comparing small loops with conventional antennas that a 20m Yagi beam for example must ideally be deployed at a height above ground of at least one wavelength (20m) in order to work well and achieve a low take-off angle tending towards the horizon for realising optimal no compromise long-haul DX operation.

 Unfortunately such a tower height is impractical in most residential zoning rule situations imposed by municipal councils and town planners.  If the Yagi beam is deployed at a lower 10m height then a diminutive loop will nearly always outperform the beam antenna.  This writer never fails to be amused by folks who acquire a potentially high performance Yagi HF beam and sacrilegiously deploy it in suboptimal installations in respect of height above ground or a metal roof.  The problem worsens on the lower bands below 20m where the resultant lobe pattern direction is not so conducive to facilitating good DX communication.

 In comparison to a vertically mounted / oriented loop, the bottom of the loop does not need to more than a loop diameter above ground making it very easy to site in a restricted space location.  There is no significant improvement in performance when a small loop is raised to great heights; all that matters is the loop is substantially clear of objects in the desired direction of radiation!  Mounting on an elevated roof ground-plane yields excellent results.

 A good HF antenna for long haul DX requires launching the majority of the Tx power at a low angle of radiation; things a good, efficient and properly installed vertical, a properly sited small magnetic loop, and big multi-element beam atop a very tall tower do very well.

 Receiving properties:
In a typical high noise urban environment a loop will nearly always hear more than a big beam on the HF bands.  The small magnetic loop antenna (a balanced one) responds predominately to the magnetic component of the incident EM wave, while being nearly insensitive to the electric field component; which is the basic reason why loops are so impressively quiet on receive; often times dramatically so. They will pull in the weak signals out of the ambient noise and you will very likely receive stations that you’d never hear when switching across to a vertical, dipole or beam antenna.

 In a propagating radio wave the magnitude of the electric vector is 120π or 26 dB greater than the magnitude of the magnetic vector, the difference being due to the intrinsic impedance of free space (377 Ohms).  On the other hand the induction fields associated with man-made noise have electric E-field components many times greater than a normal radiation field (radio wave). While a dipole or vertical antenna is sensitive to both the electric and magnetic components of a wave, the small loop is responsive only to the magnetic H-field component and it will be substantially “blind” and offer a high degree of rejection to pickup of undesired man made noise and atmospheric disturbances.   

Hence the widely used term “magnetic loop” antenna to signify this field discrimination to the components of the incoming incident EM wave. Antenna theory treats the loop as the electrical conjugate of the dipole, i.e. the loop is a “magnetic dipole” while an ordinary dipole is an “electric dipole”. 

 Significantly, a small loop antenna will typically produce a signal-to-noise ratio / SNR that is some 10 to 20 dB greater than a horizontal dipole in a noisy urban environment and an even greater improvement in SNR when compared to a vertical antenna as a result of the man-made noise comprising a strong electric field component and being largely vertically polarised.  The most important criterion for reception is the signal to noise ratio and not antenna gain or efficiency.  In the HF band, particularly at the low-mid frequency end, external man-made and galactic / atmospheric noise is dominant.

 The magnetic loop antenna has one other important practical advantage in receive mode. The aforementioned high-Q resonator imparts a very narrow band frequency selective bandpass filter ahead of the Rx front-end stages.  Such an incidental preselector comprising the antenna itself imparts greatly improved receiver performance on the congested lower HF bands with high power broadcast stations and particularly when lightning strikes and atmospheric electrical discharges are present in the regional area. Unwanted overload causing and adjacent-channel QRM interference signals are rejected or heavily attenuated.

 As well as eliminating strong-signal overload and intermodulation effects, the filtering dramatically reduces the amount of lightning induced broadband impulse energy fed to the Rx front-end and weak signals can still be heard when reception under such adverse conditions was previously impossible.

 It is these collective characteristics of small loop antennas that enable them to often very significantly outperform their large dipole, Yagi or Quad beam counterparts during direct A/B comparative testing. Conversely in Tx mode the antenna’s filter action selectivity causes any transmitter harmonics to be greatly attenuated and not radiated.

 Construction and siting issues:
Without a good quality low-loss split stator or butterfly or vacuum variable capacitor of adequate RF voltage and current rating, it is quite futile building a magnetic loop antenna and expecting it to yield the impressive results it’s potentially capable of.  Minimising all sources of loss is particularly important in Tx mode.  The butterfly style has slightly lower rotor loss than the split-stator construction style.  The tuning capacitor is undoubtedly the single most critical component in a successful homebrew loop project.  Although more expensive and harder to find, vacuum variable capacitors have a large capacitance range in respect of their min/max ratio and allow a loop to be tuned over a considerably wider frequency range than that achievable with an air variable capacitor. Vacuum capacitors also have lower intrinsic losses than most air variables. Good quality Jennings vacuum variable capacitors and a multitude of Russian made equivalents can be readily found on the surplus radio parts markets and eBay, as can their silver-plated mounting and clamp hardware to ensure a low contact resistance connection to the loop antenna conductor. 

Other creative means can also be used to fashion high VAR rated low-loss capacitor such as trombone, piston, or interdigitated meshing plate configurations. Air is always the preferred dielectric as most other materials have high loss tangents and dissipation factors.Whether a vacuum or air variable or homebrew capacitor is chosen, their mechanical shafts are readily interfacable to a reduction gearbox and motor drive to facilitate easy remote tuning of a roof top or covert loft mounted loop.  The tuning can be manual or automatic based on VSWR sensing and a self-tuning servo system to control the drive motor.   

Failure to pay very careful attention to construction details in relation to eliminating all sources of losses and making bad siting choices such as close proximity to ferrous materials are the two main reasons why small magnetic loop antennas sometimes fail to live up to their performance potential and instead behave as a proverbial “wet noodle” with associated poor signal reports.  Conversely a well built / sited loop is an absolute delight. 

Transmitting loop antennas intended for optimal coverage of the HF spectrum from 3.5 MHz to 30 MHz are best segregated into at least 2 distinct loop sizes. A nominal 0.9m diameter loop for covering all the upper HF bands from 20m through to 10m (and perhaps also tunable down to 30m depending on capacitor min/max ratio), and a 2m diameter loop for covering the lower bands 80m through to 30m. For best operation down at 160m and improved 80m performance an increased loop diameter of 3.4m should be considered.   

The performance on the 160/80m bands will be highly dependent on what antenna you use as a reference comparison, e.g. a centre-loaded mobile whip or full size dipole/monopole, etc. and what path is used, NVIS, ground wave, sky wave, etc.  The conductor diameter is determined by the desired loss resistance due to skin-effect, and choices can range from 6mm copper tubing to large bore 100mm copper or aluminium tube.  Commonly used conductor diameters used to fashion the loop are 20mm and 32mm soft copper tube.

 Note that the radiation efficiency is not related to the loop size.  Loop efficiency is determined by the conductor tube diameter and its conductivity.  This conceptual notion is counterintuitive for many folks.  A small loop will also be efficient and radiate power very effectively on 80m and 160m but the resultant L/C ratio and stored energy will often be such that the loop’s Q factor will be so high as to yield an impractically small instantaneous bandwidth that’s not useful for SSB communication purposes.  Achievable bandwidth is roughly proportional to loop size / diameter and Q is inversely proportional to the loop diameter. Depending on construction a small loop of nominal 1m diameter can exhibit an intrinsic radiation efficiency of 90% over the 1.8 to 30 MHz frequency range.

 Copper tubing is the preferred material to fabricate the loop as it has a higher conductivity than aluminium. Large bore rigid Heliax coax such as LDF550 will conveniently make excellent loop construction material for the smaller diameter 20m to 10m HF band loops when run at the 100 Watt power level.  In relation to resistance and conductivity, small loop antennas inherently exhibit very low radiation resistances, which compete with the ohmic resistances of the loop conductor and the resistances from connections and welds, including the tuning capacitor connection.  Magnetic loop antennas will typically have a radiation resistance in the order of 100 to 200 milliohms.  This means that every additional milliohm caused by a poor contact will cost you one percent efficiency. That is why professional magnetic loop antennas for transmitting purposes will never have mechanical contacts and everything including the capacitor plates should be welded or soldered.  It is not uncommon to experience 60 Amperes or more of RF circulating current in the loop and capacitor when fed with several hundred Watts of power.

 There are extrinsic factors of both a beneficial and deleterious kind affecting the radiation and loss resistances when the loop is not strictly deployed in a free space scenario.  When the loop is mounted over a perfectly conducting ground plane reflector or copper radial wire mat an electrical image is created that effectively doubles the loop area.  This in turn beneficially increases the loop’s radiation resistance by a substantial factor of 4 times.

Conversely if the loop is placed over average ground (a reasonable reflector) the radiation resistance increases but a reflected loss resistance is also introduced due to transformer effect coupling near-field energy into the lossy ground. Similarly when ferrous / iron material is too close, the magnetic near-field of the loop will induce by transformer action a voltage across the RF resistance of the material causing a current flow and associated I²R power loss. This situation might for example arise when the loop is mounted on an apartment balcony with nearby iron railing or concrete rebar etc; the deleterious influence can be minimised by simply orienting the loop to sit at right angles to the offending iron or steel material.  Another loss contributing component is due to current flowing in the soil via capacitance between the loop and the soil surface.  This capacitive coupling effect is again minimised by keeping the loop at least half a loop diameter above the ground.

 The transformer analogy for the loop antenna is a good one.  The HF communication link may be visualised as a reciprocal “space transformer” with the loop acting as a secondary “winding” loosely coupled to the distant transmitting antenna. The magnetic field component of the incident electromagnetic wave induces a small RF current to flow in the loop conductor by means of induction that in turn gets magnified by the loop resonator’s high Q that’s appropriately impedance matched to the coax transmission line.

 A freestanding loop is best supported a metre or two in height on a short non-metallic mast section of 100mm diameter PVC drainpipe and pedestal foot fashioned from plastic plumbing fittings.  The loop can also be placed on a rotator drive and turned for best signal strength or it can be oriented in angle to null-out particularly bad QRM.

 Care must be taken not to touch the loop when transmitting and to keep a safe distance away from the loop’s magnetic near-field to ensure conservative compliance with EMR standards for human exposure to EM fields.  A distance equal to or greater than one or two loop diameters away is generally a safe field strength region.  RF burns to the skin from touching the loop while transmitting are very unpleasant and take a long time to heal.

 Concluding remarks:
The proof of the pudding is always in the eating so experimentally inclined amateurs are encouraged to gain some first hand experience by getting into the shack and constructing some homebrew loops.  Such empirical validation is always very gratifying, particularly when a VK station can have a solid 5 and 9+ QSO on 20m with a Californian or Canadian station from an elegant looking Lilliputian indoor loop sitting on a table fed with a modest 50 Watts! What we ultimately seek from any antenna is reliable HF communication at all times when a band is open and, simply put, that means radiating most of the RF that’s applied to the antenna in a useable direction and take-off angle.  The underestimated magnetic loop antenna satisfies that basic criteria very well.

 © Leigh Turner VK5KLT

7-June-2008

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