1) “MAGNETIC LOOP OR SMALL FOLDED DIPOLE?” by M J Underhill, M J Blewett University of Surrey, U K, – G3LHZ, 7-10 Jul 1997
INTRODUCTION – The single turn tuned loop antenna as shown in Fig. 1 is a remarkably effective ‘small’ transmitting antenna.
Small is defined as having a maximum physical dimension which is considerably less than a wavelength of the frequency of operation. Effective performance can be obtained for loops with diameters down to as little as two percent of the wavelength. For the HF band of 1.5 to 30MHz two percent corresponds to loop diameters of 4 metres down to 20 centimetres. For deployment in the confined space allowed to many if not most of the users of the HF band such a small size is of great importance.
For reception the tuned loop antenna also performs effectively. In this case there is little difference in performance between a small and a full size antenna, where full size means having dimensions around half a wavelength or more. This is because the most important criterion for effective reception is signal to noise ratio and not antenna efficiency. In the HF band, particularly at the low frequency end, external manmade and galactic noise is dominant, and large antenna losses can be tolerated with little or no loss in received signal to noise ratio.
Most of the evidence for the ‘effective’ performance of the tuned loop is anecdotal and qualitative, not quantitative. Some of the reasons for this are explained in the following sections. Theoretical predictions remain pessimistic with respect to the performance actually being achieved by users of the antenna, but it is not easy to find evidence adequate to be able to prove whether there is a real discrepancy between theory and practice.
2) “Small Loop Antenna Efficiency” by Mike Underhill – G3LHZ, May 2006
A good, practical work focusing on the physics of small transmitting loops before leaping into modelling and getting wound up in the ‘old’ theory that isn’t holding a lot of water these days.
3) “All sorts of small antennas – they are better than you think – heuristics shows why!” By Professor Mike Underhill – G3LHZ, 4 Feb 2008
4) “The Midnight Loop, An Experimental Small Transmitting Loop ~ Theory & Practice ~” by G. Heron, N2APB & J. Everhart, N2CX,
Massachusetts QRP Convention 2010
Excerpt from one of the slides:
The Efficiency Controversy
Prof Mike Underhill G3LHZ has challenged established loop design theory claiming very small loop efficiency is actually many dB greater than claimed (Ref 8).
Noted antenna expert John Belrose VE2CV has countered with NEC antenna modeling and real-world measurements upholding the established means of deriving small loop efficiency. He concludes that G3LHZ used flawed experimental data and reasoning to uphold his unorthodox views.
Note: Authors above fail to address the practical ‘heating’ tests Underhill uses to demonstrate the temperature increase a small loop would see were it to actually dissipate a large percentage of the RF energy (power) supplied to it.
5) “Tuneable Coupled (Multi-) Mode Small Antennas—CFA, CFL, EH etc?” By Professor Mike Underhill, G3LHZ
6) “Radio Wave Properties”
A well-done demo showing how current and voltage varies along a dipole and a loop antenna. This is a practical demo using a series of still pictures showing current intensity using pilot lamp bulbs mounted in various positions along a half-wave dipole antenna.
Note well also this observation of the authors of the above webpage/demo:
With the impedance properly matched, the bulb brilliance for this “magnetic antenna” is equal to that of the λ/2 dipole antenna.
Peter Heller’s thoughts on this last statement: That this is true, despite the fact that the magnetic “loop” is only a tiny fraction of a wavelength in linear extent, beautifully demonstrates the truth of the “antenna theorem”: the absorption cross section of a resonant loop depends on its directivity pattern, and is of the order of the square of the wavelength, rather than the square of the linear dimensions, as one might have thought.
At the level of an intermediate course in electromagnetism, the fact that the resonant loop has an effective cross section many times as great (e.g. a hundred) as the square of its size, can be discussed by showing how the energy (Poynting) flux is “funneled” into the loop. This is due to the way the incident field combines with that of the loop itself; the point here is that the loop, although it is functioning as a “receiving antenna,” is also producing its own radiation field. This field is superimposed on the original incident (plane-wave) field. The Poynting vector field corresponding to the total field has the property that its “field lines” in a region of area of the order of the squared wavelength ultimately terminate (i.e. “flow to”) the receiving loop, even though the latter is physically very much smaller than the wavelength. He published this in the Am. J. Phys. 65(1), pp 22-25, 1997.
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Dubious ‘old school’ sources:
1) “The Truth and Untruth about Electrically Small Antennas” by John S.(Jack) Belrose, VE2CV, VY9CRC
Comment: The above work by Belrose represents a viewpoint voiced by many but remains basically an un-studied piece exc in a narrow specialized area; Belrose does not in any way address the issues raised by Underhill such as the absence of heating in a small loop with the application of large amounts of RF energy (high power); three-digit power levels over 10s of minutes should literally ‘burn up’ a small loop yet does not and the Belrose’s piece does not address this key issue raised by Underhill.
2) Anything by Mr. JI on loops. This source is usually correct on most subjects as when seen strictly from a ‘ham’ (as in ‘ham and egg operator’) point of view, but ultimately falls short in understanding (or explaining) the underlying physics. When solving a problem, he considers only a limited number of solutions to a problem, usually finding the ‘inside the box’ answer as opposed to anything outside the box (i.e. outside his realm of experience). Woe be unto him who relies *only* on this source as a basis for understanding RF radiation from an antenna.
At the link below is an example of the shortcomings of this source (the infamous ‘loading coil current distribution’ debate):
Remember: Experimentation (measured, quantifiable experimental results) triumph theory (the abstract ‘thinking’ on a subject) *or* modeling.
3) “Small Antennas for High Frequencies” by Iulian Rosu, YO3DAC / VA3IUL
I must take issue with a couple of early points made in this paper, in particular, these two items in light blue below.
Forming a radio wave
· When an alternating electric current flows through a conductor (wire), Electric-E and Magnetic-H fields are created around the conductor.
· If the length of the conductor is very short compared to a wavelength (<< λ/4), the electric and magnetic fields will decrease dramatically within a distance of one or two wavelengths.
· However, as the conductor is lengthened, the intensity of the fields enlarges and part of the energy escapes into the space. When the length of the conductor approaches quarter of a wavelength (λ/4) at the frequency of the applied alternating current, most of the energy will escape in the form of electromagnetic radiation.
There is impossible to make a small antenna to radiate like a big antenna.
Comment: Apparently the author of this paper has never worked with anything but linear, standing-wave wire-type antennas composed of the ‘usual’ multiple quarter wavelengths, and certainly not loops.
After spending some time in this field (researching antennas, and testing antenna them, and realizing how efficient antenna radiation is achieved) one comes to the conclusion that antenna ‘radiation’ is a function of the ‘stored’ energy field present at distance from the antenna such that when field reversal occurs (remember, we are exciting this antenna with an AC signal source!) the ‘fields’ (the energy stored and present in the fields) are far enough away that they cannot ‘return’ to the antenna; larger structures (larger antennas) enjoy an advantage in this ‘functional’ area, but, are by no means the only way to accomplish this ‘radiating’ phenomenon.
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1) “HF Copper Loop for 40, 80 and 160m” by VK4AMZ
This is a very entertaining and instructional read, and any amateur seriously looking to build a loop for transmitting on top band should read this article and learn from Mike what pitfalls to avoid, such as all capacitors (including “doorknob” style capacitors) are not created equal.
I think Mike’s testimony in his article above really goes to show what these loops can do when properly built and sited.
2) “KK5JY Small Transmitting Loop Project” by Matt Roberts, 2012-12-04
Comment: Practical construction of small transmitting loops including a 5 ft diameter loop (almost a QW) for 20m (and thereby putting putting this loop into the Edginton Loop category).
The use of 15 ft of 5/8″ copper for a 20 m tuned loop represents significant overkill for a tuned Quarter Wave loop (i.e., this size was completely unnecessary. 10 gauge copper wire would have more than sufficient.)
3) “Magnetic Loop for 80-20 mtrs“, by Frank Dorenberg, N4SPP
Frank has a nice website that details his experience with loops, and also shows a few pitfalls to avoid as well.
4) The G0CWT Edginton Loop, A/K/A the Edginton QW Loop
Theory and practice on implementing the Edginton Quarter-Wavelength (1/4 wave) Loop, including the winding of the ferrite transformer and the impedances (Z) seen at various points around the loop by the inventor himself.
I (the author of this website) have built multiple QW Edginton Loops for 160 meters through 20 meters with good results on each. My largest QW Edginton loop is a vertically-oriented loop 16 ft (vertical height) by 40 ft (horizontal run) hung (or supported) between two stacks of (quantity) 6 ea. of those 4 ft mil tent poles (as found on ebay) which yielded a height on each end of 22 feet.
My first Edginton loop was a 10 ft x 22 ft loop hung between two 16 ft tall mil tent pole stacks (only quantity 4 ea. of those 4 ft mil tent poles on each end and no guying required – for support a couple fence “T” posts were used, driven with a sledge hammer into the ground). 10′ x 16′ resulted in a QW on 80 meters and 1/8 wave on 160 meters … it worked well for local groundwave comms on 160 with the ferrite transformer 9T:2T ratio and the proper resonating capacitor. Initially, to get started, I used air variable caps, then a vacuum variable and finally short lengths 1 7/8″ Heliax in parallel with ‘gimmick’ style trimmer caps.
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The ‘usual’ (linear vertical structures requiring ground radials et al as part of the resonant or resonating structure):
1) “Antennas By N6LF – 630 meter antennas” by N6LF
Of note at present two of his papers:
Fair use excerpt from page 1:
Most amateurs do not live on extensive acreage and/or have unlimited funds and in many cases their antennas will have to fit in urban backyards.This multi-part series of articles on 630m antennas discusses limitations inherent in small antennas, makes suggestions for optimizing short antennas and gives practical examples which have been field tested.
The free space wavelength (λo) at 475 kHz is ~2,071 ft so a λo/4 vertical would be ~500 ft high! It’s unlikely many amateurs will put up anything near that height. This series looks at verticals with physical heights (H) between 20′ (~0.01λo) and 100 ft (~0.05λo), with emphasis on the shorter end of the scale (H=40′-60′). By any definition these are “small” or “short” antennas, which will have very low radiation resistance (Rr), narrow bandwidth and high losses even with the best design and construction.