by Graham Maynard.
Radio waves radiate from an electrically excited transmitting mast, and ripple outwards at almost the speed of light. It is displacement of the dual electrical/magnetic equilibrium that transfers energy, not a tangible flow, and this is why radio signals are almost all pervasive.
Salt water causes little daytime path loss, inland water and wet ground provide the best overland conditions, while dry, rocky or sandy soils absorb medium frequency signals at a much greater rate. Darkness brings strong ionospherically refracted long distance signals, with dusk and dawn, both far away and over our own location, producing constantly changing focussing effects. Hills and large buildings affect reception by disturbing wave patterns over and around them. Upright structures absorb wave nenergy by transducing the field and dissipating it as heat and/or a parasitic re-radiation.
Now only rarely can we choose our listening site, so such major factors must be accepted and efforts turned to optimise the situation. When erecting medium-frequency wire antennas try to visualise;-
* (a) your fields of interference, i.e. from power cables,
garages, dwellings, boiler rooms, sheds, outdoor lighting, telephone
cables etc., and in the case of loop antennas the direction of your
local transmitters or most wanted signals.
* (b) the predominantly overhead radio field, it's nearby roof and tree shadow horizon, and ground capacity losses. Then place as little of your antenna as possible in theinterference field, and as much as possible in the radio field, by aiming to;-
1. Construct antennas as high and clear as possible from
buildings and cables; as little as 30ft (10m) free space around them
quietens much neighbourhood interference; also aim to take as much wire
as possible above your radio field horizon, by at least 10ft (3m).
2. Complete the antenna circuit with a clean radio frequency
earth; a separate outdoor metal earth stake, well away from the noisy
ground currents that surround mains electricity and water pipe
3. Keep noise out of the antenna circuit by using radio frequency isolating transformers between it and any cable that enters noise fields, especially feeders running into the listening room.
The most simple way of fixing antenna wires is with nylon string and p.v.c. tape; three tight binds will last a long time, though don't knot the cable or introduce sharp bends as tension will snap the conductors. Take a look at telephone poles to see how cables are woven with tensioning supports - these chaps have wire hanging down to a fine art, so copy them. When using insulated 16/0.2 wire, nylon string, p.v.c. tape and treated timber posts for long to tropical frequency reception there is no need to install further insulation.
Our primary concern is the wanted signal-to-noise ratio, so (1) above deserves more consideration. A random wire antenna produces random performance - might be OK; "inverted-L" or "T" installations do much better if two high supports can be provided away from interference; a vertical, with ground stake at it's base, requires only one high insulated support and performs well because it can more easily be sited away from electrical interference; a counter-inductive, helical top winding can almost double the usefulness of a vertical antenna by increasing the length of wire raised into the radio field and away from interference and ground losses. For me, 20 metres of wire on a 10 metre support, with 15 metres wound on the upper half, has outperformed all other back garden designs, including active. Vertical supports may be treated timber, or a timber base with p.v.c. rainwater downspout or glassfibre pole upper; screw the base to a shed or good fence. A counter-inductive helical top is easily made by laying the vertical down and taping wire at the mid-point and top. Pull the 15 metres of loose wire away from the support to find it's mid point, and tape this to the support three-quarter point. Repeat division until say eight equal wire triangles are formed, then roll the support ~ver them to wind the wire up. Tighten and secure each centre point with tape. If tip corona discharge causes "engine ignition like" interference when black clouds are overhead, then mount a wire ball on top and connect to the antenna wire; three reshaped and equispaced wire coathangers are ideal.
The earth should be directly beneath the antenna, as both its ground reference and lightning guide. This is not a power circuit so a length of pipe or old digging-fork head hammered into clay through regularly watered soil will suffice. Antenna impedance rises as frequency falls, and taking as example a figure of 2000 ohms at IMHz, this would be grossly shunted and noise modulated by direct connection to any low impedance cable feeding into a domestic environment. For optimum low noise signal transfer and antenna circuit isolation a matching transformer must be used, with core and windings carefully chosen to match impedances and frequency range. The transformer should be sheltered from sunshine and rain; sunlight degrades plastic and frost crumbles ferritel Feeders for low to tropical frequency reception need not be expensive, indeed, some figure-eight twin cables are quite suitable if used in balanced mode with a receiver transformer.
Interference is induced equally on both conductors and cancels on a balanced receiver transformer primary; only difference signals induced by the antenna transformer are coupled to the receiver input. Feeders also should be shaded from sunlight and routed wellaway from all other cables to prevent electrical noise induction. Those running close to soil will break down more readily to earth in the event of a lightning strike, though they ought not lie wet and should have ends sealed against moisture.
While careful antenna siting and construction are always beneficial, clean AM reception cannot be guaranteed. People today are generally unaware of and uninformed about the interference that their mains powered equipment generates, and this is our problem. Interference can be carrier or commutating, e.g. televisions or dawn-to-dusk dimmers, and when radiating their noise is unavoidable.
However. if we receive the interference twice; on our main antenna with the wanted signal, and on a second antenna by itself; and we subsequently subtract the interference from the signal plus interference; then the wanted signal can be resolved alone. The interference from a single radiating noise souce can be fully nulled using this method, but where time delayed re-radiation components exist, or other noise sources are also present then received noise can only be minimised. To combat distant interference directional nulling is necessary, this topic is covered later.
A convenient interference antenna, that picks up nearby noise with respect to the receiving system and is well screened from broadcast radio fields, is the feeder itself. If we deliberately collect feeder interference, and manually adjust its phase and amplitude to null or counter interference transferred in the balanced or screened mode, then reception will be improved.
This noise cancellation technique can sometimes fully clean genuinely weak signals during periods of strong nearby interference, on other occasions only a best comprmise is possible. Each location has its own noise signature so results will differ, though manually adjustable noise nulling should always be beneficial. Obviously the feeder should not be too short or coiled at the receiving end, this impairs its noise collecting ability. The feeder/receiver transformer case can be used to house the phase and amplitude controls to complete a neat, desktop, noise nulling control box.
Without doubt the best form of indoor medium wave antenna is an Active Tuned Loop: They have long been appreciated for their advantages:-
* may be used at the listening desk with any receiver, (i)
* for their size have a high natural gain, (ii)
* provide an easily rotatable figure of 8 receiving pattern to favour wanted signals, or minimize (null) unwanted ones, (iii)
* when balanced are much less affected by nearby electrical in terferenc.e, (iv)
* their large tuned circuit improves receiver selectivity, (v)
1. We must however remain aware of the limitations imposed by
equipment interference, building wiring and structural surroundings.
Induced noise can be a problem, as can the skewed null plane caused by
structural wave distortion and re-radiation. So, for reception in
different directions be prepared to try a loop in several places, just
as you would with a good portable.
2. Active connection is essential for trouble free listening.
Loops with a coupling turn have much reduced "0" and subject receivers
to secondary short-wave breakthrough. True transduction occurs only
across the tuning capacitor, and signals should be amplifier buffered
at this point to produce low impedance output for receiver connection.
High gain results from their multiple turn, wound area and natural
resonant "Q", 7t x 1m x 100.
3. A loop antenna has maximum sensitivity to stations in line
with the plane of the loop. Minimum sensitivity, the so called null
position, occurs on the line of winding axis. All loops are broadly
sensitive either side of the wire line, hence accurate alignment is
unnecessary for wanted stations; only better loop antennas are capable
of producing deep and sharply defined nulls, here accurate axial
alignment is essential to avoid unwanted signal reception.
4. Loop windings act as an antenna wire with respect to the
tuner and its connections; tuners are normally grounded to avoid hand
capacitance effects; if one end of the loop winding is grounded with
the tuner then the other end picks up interference. Balanced tuning
with respect to ground allows both ends to pick up interference so that
there is no difference between ends, and therefore no Jifferential loop
output to be amplified.
5. Most of today's consumer quality radios, especially digital, suffer from poor front end performance; local stations spread across the dial and mask weaker signals. A sharply tuning loop is of assistance here, it can peak the weaker signal and hopefully null a stronger, overpowering one.
Better for DXing purposes are outdoor loop antennas; these can be tuned, but stable remote control is very difficult to achieve, and besides, active aperiodic (wideband) designs are more simple. A large untuned outdoor loop of convenient size, say twenty metres circumference - 12ft high by 20ft long - is over 40dB quieter than a normal active tuned loop, but requires a low noise amplifier to recover its 25dB gain deficit. A single loop of 300 ohm balanced feeder provides two accurately spaced turns, good for MW reception but poor above. Additional turns cause medium frequency resonance, limited bandwidth and amplifier linearity problems. Larger turns are inconvenient to erect and only of advantage for low frequency reception at remote locations; it is better to use additional spaced aperiodic antennas in a phased array than just one very large loop.
Thus, a broadband, active, 20 metre circumference outdoor loop with carefully designed amplifier could cover Long to Tropical frequencies with mid-band resolution to sub uV/m field strengths. Ideal for transatlantic MW DXing if mounted on a GB NW-SE bearing, or, erect broadside-on to a troublesome local or regional transmitter.
Comparative measurements of signal output from an active tuned loop and an active aperiodic loop, the former 11 turns of 30 ins square, the latter I loop / 2 series connected turns of 300 ohm cable 5 metres square, show that, while both produce the same output the latter clearly resolves to field strength levels that are weaker by a factor of four. From 14.00 to 24.00hrs background noise and increased ionospheric propagation generally mask this difference, but from 04.00 to 10.00hrs. the larger loop has distinct advantage; especially during the winter months.
Being as sensitive as a 10 metre Counter Inductive Helical top wound vertical, a 20 metre circumference active outdoor loop will similarly benefit from feeder driven noise nulling. The loop, a balanced closed circuit without external (ground) connection, can then perform well even if inconspicuously clipped to house or garage walls. For optimum results site away from noise with a circular or 16ft square shape; impedance is lower so the bottom wire need not necessarily be raised above ground level and terminations can be at any low point. Support the top as if a wire between poles, clip to a fence, trees or walls, or hang from rainwaten gutter supports or upstairs windows.
Given adequate receiver performance, especially with regard to front end dynamic range and selectivity, this type of antenna is the most convenient way of achieving really good medium wave reception.
A summary of three antenna types will focus thoughts;-
1. The outdoor passive wire antenna;- sensitive, quieter if it
is a counter-inductive helical-wound vertical with noise nulling;
requires some effort in construction but no running costs; trouble free
and no generated products; omni-directional so co-channel signals are
2. The indoor active tuned loop;- high gain; selective;
portable; negligible running cost; not the quietest for early morning
reception but entirely adequate at night; rotatable nulls resolve major
co-channel stations, also weak signals adjacent to powerful locals;
easy to use though can be cumbersome; performance limited by supply
cable interference so try it in different locations.
3. The outdoor active broadband loop;- sensitive; very quiet, more so with noise nulling; most convenient; requires about 500mw of power; a figure-of-eight sensitivity pattern so can counter powerful locals by erecting broadside-on, and/or favour TA by lining NW/SB; normally fixed so cannot alone be used to separate co-channel signals.
Of normal sized antennas only the loops can favour a wanted transmission whilst simultaneously nulling an unwanted one, though of course the stations must not be roughly in line. When transmitters are in line though in opposi te directions, and we wish to null one without affecting the other, then signal mixing techniques must be used. Here indoor controls to vary the phase and amplitude of feeder signals from two or more fixed external antennas can produce (a) alternative sensitivity patterns that are quite unlike those produced by each antenna alone, or (b) a single deep and accurately directable null. These new responses can only be generated when the feeding antennas produce signal voltages that are similar in amplitude but different in phase, with the phase difference relating to transmitter direction.
Normally, broadband outdoor antennas are used; a tuned antenna has sharply changing signal phase and amplitude components about resonance, therefore any generated pattern also changes with frequency and has narrow working bandwidth; an indoor design is likely to be less quiet and will degrade overall performance to its own level. There is little point in nulling an unwanted carrier if non-coherent sideband sibilance remains or the resultant has poorer signal-to-noise ratio; also, a transatlantic cardioid pattern is more useful when adjacent European channels are reduced with the centre frequency.
There are two ways of obtaining phased antenna signals;-
1. by spacing antennas so that the distance between them is a
reasonable fraction of a signal wavelength, e.g. quarter of wavelength
spacing in line with a transmitter, circa 75m., introduces a ninety
degree received signal differential;
2. by using antennas that have different directional sensitivity patterns; directional antennas already have phase responses that are related to incoming signal bearing, and dissimilar patterns produce a relative differential that changes with incident direction.
When a transmission is separately received by more than one antenna, we can manually adjust the phase and amplitude of each incoming feeder signal, to either add or cancel prior to receiver detection. When the same signal is added in phase its resultant has greater amplitude and an improved signal-to-noise ratio, though other transmissions are not necessarily reduced or nulled. When added out of phase and with equal amplitude the signal is very deeply nulled, though other transmissions are not necessarily boosted. It is only when the cardioid ( heart shaped) or unidirectional ( balloon shaped ) reception patterns are generated that simultaneous deep nulling in one direction is oppositely accompanied by doubled sensitivity and signal-to-noise ratio.
The most simple of possible mixed antenna arrangements is a co-sited loop-pIus-wire combination. The loop is closed, has a figure-of-eight sensitivity pattern, and transduces with output phase and polarity related to the angle between transmitter direction and the loop winding plane; conversely, a wire is open, omnidirectional and uni-phase.
Both are electrically small with respect to signal wavelength, i.e. the wire does not resonate, and they transduce electro-magnetic radiation differently, such that their antenna charge redistributions are 90 degrees out of phase, and said to be in quadrature. This 90 degree difference reverses when signals arrive from the opposite direction with respect to the loop, so that switched reversal of loop output can easily provide a bi-directab1e, mixergenerated, cardioid response. Note, however, that antenna and feeder reactances modify the original quadrature, so a good range of mixer phase adjustment should be available to compensate, as well as to generate, the desired responses.
This received quadrature is important, not just for directional control, but for the reduction of unwanted nearby noise fields, so here a little fundamental side-tracking should be worthwhile.
Transmitting antennas and their drive circuitry are reactive, as also is building wiring and the electrical equipment connected to it. Both types of system are capable of producing electric and magnetic fields, plus electro-magnetic radiation that is out of phase with the original excitation.
The altenating electric and magnetic fields follow normal laws and are basically unwanted; often, when one field is at maximum the other is zero, and vice-versa, with a cyclical exchange of energy between electrical and magnetic. Close to an unbalanced antenna or conductor these static fields are intense; they follow excitation, may be capacitively or inductively coupled, do not radiate electro-magnetically and decay inverse squarely with distance. Induced energy thus falls inversely to the power of four with distance, and this is why an outdoor receiving antenna is so quiet, especially with regard to domestic interference.
Electro-magnetic radiation is an alternating disturbance of the ambient electric/magnetic equilibrium that transfers energy when acting upon fundamental particles. Transduction is related to frequency, intensity and p article excitation levels. For example, by energising the free charge carriers within a conductor, radio frequency electro-magnetic radiation generates a replica electromotive force.
Electro-magnetic radiation has individually observable electric and magnetic identities which arise from excitation phase shifts about a transmitting antenna system or radiating conductor, but here the generated fields are'in phase. Unlike individual electric and magnetic fields, these components are unable to pivot in energy exchange about conductor current, because the alternating excitation, being forcedly faster than is natural, reverses and repels them as they establish.
The fields are cast off as a dual electric/magnetic alternating disturbance, and, as with water ripples, it is their wave nature that transfers enrgy. Like light, they radiate freely through space with a field strength that is inversely proportional t6 distance; received energy falls inverse squarely with distance. This is why night-time medium frequency sky-waves can be so strong beyond their intended service area. Layers of energised particles above the atmosphere can either smoothly refract signals to produce good DX, or, resist wave motion, dissipate signal energy and add their own noise to give us a bad night.
While observable electro-magnetic wave components are in phase, the radio frequency field component~ generated about a loop or open wire alternate out of phase - unless the conductor is resonant. We may transduce either wave component using an antenna that is insensitive to the other, but cannot detect one component without the other being present. If one component is dissipated, transduced or reflected, the a local shadow or interference pattern is formed which gradually fades wi th distance by diffraction, . diffusion and surface wave recombination effects.
A wire loop responds to change in magnetic flux; hence magnetic fields and the magnetic field component of electromagnetic radiation are capable of producing radio frequency e.m.f. at the loop terminals. An open wire responds to electric field changes; hence voltage fields and the electric field component of electro-magnetic radiation are capable of producing r.f. e.m.f. at the wire terminal with respect to another separately-referenced terminal. Any electrically small loop that is balanced, or an isolated system, is little affected by electric fields; similarly, any electrically short wire ( ?/8) responds little to magnetic fields. However, just as a small loop can null magnetic noise fields, a small balanced dipole, of say two metres length, may be used to nul! unwanted electric noise fields, because it also has a figure-of -eight sensitivity pattern and no separately referenced input. Remember, at 1MHz? is roughly 300 metres and ?/8 at 1600kHz only 23 metres.
A receiving antenna close to a transmitter, or to building wiring, cannot avoid receiving both induced and electro-magnetically radiated fields; the resulting phase mix of differently related components (directly induced plus radiated/transduced) shifts the perceived field polarisation axis. Thus, to achieve nulling near radiating or re-radiating conductors, an altazimuth (tilting) loop is necessary, or a loop-pIus-wire mixing system.
A. loop produces maximum e.m.f. when the rate of change of magnetic flux is maximum; that is, when the magnetic component passes through zero value. A wire produces maximum e.m.f. when the voltage component is at maximum. Now, since the magnetic and voltage components of electro-magnetic radiation are in phase, loop and wire e.m.f.s are out of phase, and 90 degrees apart. So, if we receive an electro-magnetic signal on equally sensitive individual loop and wire antennas, phase shift the feeder signal of one by 90 degrees and then add, we double the sensitivity in line with the loop in one direction, and cancel it in the other to produce the cardioid reception pattern.
Cardioid sensitivity patterns are generated by mixing equal amounts of phase matched wire and loop signals. Forward sensitivity is virtually doubled, and nulls shift from the loop axis to align with the winding wire. For a given reduction in signal strengtp the cardioid null is wider than the sum of bi-directional null angles and therefore adjustment is much less critical.
|Approximate depth of null dB||-||6||20||40||60||80||100|
|Necessary alignment accuracy||Loop Degrees||30||6||0.6||difficult||difficult||difficult|
|Necessary alignment accuracy||Cardioid Degrees||90||36||11||3.6||1.1||difficult|
Suppose now that we are receiving an individual nearby electric/magnetic noise field, with, as outlined its components in quadrature. Our loop-wire combination produces from it two noise outputs that are either wholly in or out of phase, which is ideal if wanted reception is acceptable when phased addition causes total cancellation. If, as a by-product of cardioid generation, one feeder signal is shifted by ninety degrees prior to mixing, the action of each antenna circuit loading the shifted output of the other often reduces the noise resultant to a level below that with either antenna alone, even when this nearby interference is in the same direction as a wanted signal. Depending on conditions, feeder-signal phase shifting thus provides an opportunity to reduce either nearby or distant noise, and, with additional feeder noisenulling a chance of eliminating nearby interference whilst retaining a useful reception pattern.
The mixed and co-sited, loop-pIus-wire system has another advantage that complements its directional response; at night, and with all weak signals, it sounds better than with either antenna alone, because both of the received electro-magnetic field components are less often simultaneously distorted by ionospheric polarisation changes and path disturbances - rather like spaced antenna diversity reception. Co-sited antenna interaction is not a problem; if both are aperiodic at wanted frequencies then mixer adjustmentstill provides stable wanted reception.
Deliberately imbalanced feeder-signal mixing with fixed antennas causes one of the generated figure-of-eight lobes to be larger than the other, thus creating intermediate sensitivity patterns with new nulls at angles between those normally on the loop axis or the single cardioid one in line with the winding wire. Sufficient asymmetry to indicate a directional sense gave rise to the term unilateral reception, and its new nulls may be mixersteered in any direction with no more than fingertip effort on the knobs of a listening desk mixing box.
The new response might have a rather indistinct minimum (a), or two minima less than 180 degrees apart (b), nicknamed - cottage loaf. Navigators avoided using unilateral responses because accurate determination of transmitter bearing was not possible. Sensitivity pattern (b) is generated by mixing loop antenna signal with less wire antenna signal than is required for the cardioid response. The relationship between the nulling angle with respect to the loop winding wire, and the level of wire signal as a percentage of loop output is tabulated below:
|Wire signal, percent||0||26||50||71||87||97||100|
|Null angle, degrees||90||75||60||45||30||15||0|
Nulls at ninety degrees is the unmixed figure-of-eight loop response; the single, zero degree null is cardioid with equalised mixing.
As with normal axial loop minima, mixer generated nulls may simultaneously be aimed in two different directions, again with symmetry about loop plane, however, if one null is optimally sharpened then the other automatically becomes shallow and poorly defined.
Summarising;- an ability to switch, reverse and match the phase and amplitude of fixed loop and wire antenna feedersignals is the basis for generating five choices ofreception; (i) omnidirectional; (ii) figure-ofeight; (iii) deep nulling in any direction; (iv) cardioid in line with the loop wire; and (v) shallow nulling in two directions symmetrical about loop plane.
For serious listening (iii) and (iv) are' the most useful.
With loops that can rotate about a vertical axis the full cardioid pattern is similarly rotatable, also, shallow minima may be empirically generated in any two directions, though with no certainty of improved reception.
Passive and active indoor tuned loop nulls can be greatly improved by using a second,movable, "zero cleaning" winding that is connected to another antenna system. Every situation is different, so try ten turns of any wire on a 1ft (30cms) square frame; connect the winding ends to the output of a second loop or an outdoor antenna and earth. Tune and adjust the receiving loop for best reception, then move the zero cleaning winding around it until improvement is noted; the field combines before it is transduced and therefore full-channel steady-signal reductions of 60 to 80dB are easily achieved without touching or tilting the main tuned loop. Sadly the same method is not recommended for countering mains wiring interference; some reduction in noise is possible by producing a cancellation field from the interference difference between mains earth and an outdoor ground stake, but direct interconnection is hazardous.
If you have a long outdoor wire antenna, say 100ft or more, then additional induction with the ten turn winding might be capable of directly generating a fully tuned cardioid response; here though, the induced field must be very strong, and necessary close mutual coupling often causes ruinous de-tuning.
Cardioid can also be generated by phasing and mixing the signals from two vertical wire antennas, spaced at a quarter of a wavelength, in line with wanted and unwanted transmitters. If your garden runs to 50 metre antenna spacing then this is a fair medium-wave compromise.
With similar in-line spacing and their winding planes in the direction of wanted/unwanted transmitters aperiodic outdoor loops are even more useful. When signals are mixed the system retains axial insensitivity to produce a balloon-like pattern with two small rearward side-lobes. This is more directional than cardioid and similar to a terminated one-wavelength Beverage with its much reduced side and rear noise plus good forward sensitivity, though of course without lengthy wire supporting problems. Limited mixer controlled null steering is also possible with spaced antennas, but strange intermediate sensitivity patterns develop. ?? and two, spaced, triple mixed co-sited loop-wire systems?? - even better with virtually four times forward sensitivity and negligible to the side and rear.
A fully rotatable figure-of-eight sensitivity pattern is easily produced by mixing the signals from two co-sited aperiodic loops erected at right angles, e.g. on corner boundary fences. Mixer controls rotate the resultant pattern, and can produce very deep nulls in any direction; a useful alternative when the vertical antenna cannot be erected to construct a loop-wire system, though not as sensitive as this and other combinations.
And what about two, independently mixed, co-sited loop-wire systems, again ?/4 spaced in line with wanted reception. Here there are two interesting possibilities; first, by phase mixing both system resultants we can achieve virtually four times forward sensitivity, with little side and negligible rear pick-up; and second, using diversity reception, or pre-receiver ultra-sonic feeder sampling techniques to average individually fading antenna system resultants, it is possible to provide directional, fade reduced reception with ordinary receivers.
True long distance reception rarely suffers from multi-path carrier notching, so a synchronous receiver used with the triplemixed, twin loop-wire option, will make the best use of a plot that is 50 to 75 metres long in the great circle direction of main interest.
By mixing the feeder signals from two co-sited aperiodic loops mounted at right-angles, for example on corner boundary fences, a fully rotatable figure-of-eight sensitivity pattern may be generated that is capable of deep signal nulling in any direction; this is a useful alternative when the vertical antenna cannot be erected to construct the loop-wire system, though is not quite as sensitive.
When a vertical aperiodic wire antenna and second mixer are added to this combination then cardioid generation becomes possible, only now it has twice sensitivity and can be aimed in any direction; more complicated but very versatile. But don't be thinking that this is something new - long before thermionic valves made any impact, Marconi, Bellini, Tosi and others researched many aperiodic loop-wire antenna systems for navigational and telegraphic purposes, they used powerful spark and alternator transmitters, goniometers and magnetic detectors.
An active indoor loop, or a passive loop with differential matching amplifier can also be used to produce rotatable cardioid, the pattern is generated in line with the loop wire. Signal is phase mixed with that from an outdoor wire or vertical antenna to produce levels twice those of the weaker source - usually the loop because it is in the vicinity of noisy equipment and domestic wiring. Often the loop Q is too high and needs to be spoiled with a potentiometer to widen null bandwidth and facilitate set-up, sensitivity is impaired though.
Let us now look at the mixer. This is a radio frequency phase shifting and amplitude adjusting arrangement that controls one feeder signal with respect to the other by plus to minus ninety degrees, and each individually from maximum level to zero. Plus to minus 45-degrees shifting on both inputs is more efficient than 90-degrees on one alone, and provides mixing for directable single signal nulling, or forward lobe generation in either direction along the loop wire line.
The phase shifts may be introduced using variable RC, RL and LC circuits, or by switch selecting cut lengths of feeder to delay the signal. The latter is most efficient, though inconveniently complex and bulky, especially if balanced. Also useful are switches to compare individual antenna inputs with the newly generated response, and for reverse phasing of inputs to quickly direct the response in an opposite direction.
To optimise weak signal reception in all situations, and to quieten nearby electric or magnetic noise fields, each signal input should have independent level and phase adjusting controls; also, the mixer could provide valuable pre-receiver tuning and notching facilities.
A radio frequency mixer literally adds new horizons to mediumwave DXing, and, while basically simple to operate, some 'hands-on' experience is necessary to achieve good results. For example, we soon learn not to adjust controls for maximum sensitivity, or the highest carrier-meter readings; our ears are the best devices for discerning weak signal to noise ratios, and so, with fixed and possibly reduced receiver gain, we should mix aurally to detect the slight changes that can then be trimmed to optimum.
To use a mixer each antenna feed must first be individually selected and peaked or tuned, with any noise-nulling controls set to mid position; make a mental note of which feed is the stronger, as this is the one that can better afford to lose amplitude when initial phase adjustments cause slight level reduction. Turn both inputs on together, then switch one between its positive and negative phase options; normally the S-meter indicates signal addition and subtraction, though sometimes the difference can be hard to detect.
For signal boosting simply leave the switch in the position that shows addition, then re-adjust both input phase or tune controls with the levels set to maximum; with clear signals watch the S-meter; with weak ones, or those suffering from interference, use your ears - sometimes slightly reducing the level of one signal can improve overall reception.
For nulling, leave the phase switch in the position that shows subtraction, then (1) adjust the phase or tune control of the stronger input to further reduce your S-meter reading, (2) improve by adjusting the weaker input phase, (3) adjust the amplitude of the stronger signal to deepen the minimum, and (4) do the same with the weaker input. Repeat 1-4 until sharp nulling is achieved. Cardioid is where the developed null is in line with the loop wire. When you have nulled down to the level of nearby man-made noise sequentially adjust any noise nUlling controls; then repeat everything. Yes! There are many knobs to turn, but it's cheaper than moving house, and more convenient than countryside Beveraging.
Where antennas have individual noise-nulling controls, then mixer adjustments disturb the noise nulls, and nulling controls disturb the mixer response. This interaction is not deleterious because the nulling adjustments become less critical, and received noise residuals can better be balanced out. Just as cardioid generation renders a signal intelligible when it is not so with either antenna alone, noise-nulling plus mixing can perform similarly compared to mixing alone when nearby noise is a problem.
During the day, mixer generated sensitivity patterns are virtually as they appear on a flat drawing; signals are mostly groundwave and nulls sharp in the horizontal plane. At night however, the nulls effectively become distorted conei-of attenuation aimed at incoming ionospherically returned signals; they are then much less effective against all other transmissions. Indeed, constant two handed mixer adjustment is often necessary to counter the cyclically shifting, multi-path resultant of midevening, high power Europeans. This is where the Beverage has, and, to a lesser extent, spaced loops and spaced loop-wire systems can have, advantage.
THE WAVE ANTENNA
Beverage antennas are so named after their main exponent, Mr.Beverage. They are long enough to transduce static electric andmagnetic fields, but the lack of independent reference, ground proximity, line impedance and longitudinal conduction limit transduction to electromagnetic radiation propagating in line with the wire; we can imagine that charge carriers within the conductor are mobilised as waves progress along the wire line, and that this produces an alternating e.m.f. at wire ends. Such action is quite unlike the individual electric or magnetic field transduction of wire or loop antennas. Stations in line with the wire are automatically favoured against those on other bearings and static interference fields in any direction.
The wave antenna is thus broad-band with medium source impedance, and has a sensitivity pattern that changes with end termination impedance and frequency. They can be made unidirectional with suitable end termination impedance. Working in reverse, one that is excited by radio frequency energy generates full waves of electromagnetic radiation and can be a useful directional transmitting antenna. By comparison, driven ferrite, loop and whip antennas are mostly reactive, they tend to produce only one or other static field, and unless sizeable with tunable reactive drive make poor medium frequency electromagnetic radiators. We could all benefit from Beverage reception, but alas, large tracts of open land are necessary to properly erect them.
This text is approaching completion and I've not yet covered any antenna related technical developments. For example, what about active whip antennas? Yes, these are effective, easy to install, can be made to cover a wide frequency range, and.reduce susceptibility to feeder noise ingress - ideal for every-day listening. However they are not quieter than the counter-inductivehelical top-wound-vertical with feeder noise-nulling as described earlier, and are less trustworthy with regard to pre-receiver distortion products. When used alone with any receiver these factors might not be considered a problem, but for directional reception, where one feeder-signal is used to cancel another feedersignal, any accompanying noise or non-linearity products cannot cancel and this degrades the generated nulls. High performance radio frequency amplifiers producing as little as 0.005% noise plus distortion from local signals or, night time Europeans, that is 86dB down, still generate products that are greater in strength than real DX.
First class active whips are generally remotely tuned and cost dearly, but still don't out-perform a decently erected and impedance matched length of wire. Remote tuning can also introduce long term phase stability problems.
Nor have I mentioned ferrite cored antennas where the rod is used to concentrate radio frequency magnetic flux through a high-Q antenna coil. These transduce more effectively than small equivalently sided air-core loops, and may easily be wound in a manner that counerts static electric noise fields - interference. However at medium frequencies diminishing returns occur beyond rod sizes of approximately 12 x lin. dia. (30 x 1.2cm), and here a traditional 12in. square loop is both better and cheaper.
Ferrite bar antenna sensitivity is limited by tuned-Q-multiplication of coil winding, thermal agitation noise; the higher the values of inductance and Q - the greater the noise; for example, in normal temperatures at IMHz, a perfect 220UH resonant winding with circuit Q 120, generates 4uV noise. Wound antennas, with their additional shape, form, conductor, core and nearby material .losses, are worse, so extra amplification or increased receiver sensitivity are not the answer. For better reception an antenna should have lower inductance - fewer but larger turns, and, lower Q - be of aperiodic or optimally impedance matched design.
There are regenerative designs that use carefully controlled fractions of antenna amplifier radio frequency output as positive feedback to increase tuned Q. Regeneration simultaneously increases output and sharpens the tuning selectivity characteristic of small antennas; especially useful with receivers that cannot hear loop winding noise, or have inadequate front end performance. However if you have a good receiver, one sensitive enough to detect tuned winding noise, and the antenna is regeneratively peaked, the receiver hears a noise "carrier" with quiet sidebands and automatically reduces gain without any signal being present. Under these conditions the S-meter might even produce a reading as the loop is tuned to receiver frequency; sounds like a quiet background but actually the system is deaf.
Regeneration can also re-constitute Q that is lost when loop antenna turns suffer dielectric losses by being wound directly against a support, or where the wire is fine and has less than optimum winding form; it does not however reduce the equivalent resistive noise.
Under weak signal conditions regeneration always degrades the received signal-to-noise ratio, while with stronger signals we note sideband narrowing. Another unavoidable disadvantage is that feedback gain re-adjustments alter the reactive balance, this shifts the resonant frequency and makes fine re-tuning necessary for each new feedback setting. When weak signals adjacent to powerful ones are regeneratively peake~cross modulation is often observed as amplitudes increase beyond stage linearity. Small regenerative antennas produce surprising output for their size, but equivalent effort and money applied towards a better antenna and receiver is generally the more positive way forwards.
Also bear in mind that this type of antenna is not the best for mixing applications; there is an even greater change of phase and amplitude between a carrier and its sidebands, and often the loop needs to be tuned 50 to 100kHz away from a wanted signal to generate a decent null.
We might also consider the possibilities of varicap tuning. with indoor designs the varicap is more convenient and reduces hand capacity problems, however, it is a semiconductor device with less Q than its original mechanical equivalent and voltage tuned values can drift annoyingly with temperature; this point is especially important when unattended time-switched cassette-recorder monitoring of weak channels is required at night when the heating has been turned down.
Greater ambient temperature variations occur with outdoor designs, virtually precluding varicap use with any form of phase stable pre-set system. So, unless precisely ovened, tuning diodes should be avoided as part of a sensitive, directional nulling system, and, if ovened components are used, don't overlook the possibility of intermodulation effects from other strong signals.
The need for a good antenna is quite understandable when noteworthy medium wave reception is required, however, it is .not,at first sight, obvious that a good receiver must also be chosen: Connect an effective antenna to the average radio and you've got problems; the dynamic range of listenable signals far exceeds the linearity of unbalanced front ends and single-superhet image rejection and selectivity.
Look out for a second-hand professional receiver or good, recent, double conversion type with roofing filter and choice of IF passbands. A. few 'World Band' portables are satisfactory, but many others generate their own noises, whistles that change in frequency far more quickly than the tuning rate, and stations on the dial where they should not be.
All receivers benefit from tuned loop input or an efficient and passive, in-line tuned LC stage, much like the track tuned front ends that served old valve radios so well. So do aim to include at least one stage of tuned radio frequency pre-selection between antenna and receiver, and for comfortable resolution of weaker signals when receiver noise exceeds antenna system noise, consider a regenerative pre-selector at the receiver input.
Some thought is also necessary with regard to antenna-receiver coupling. Portables may be inductively coupled to passive tuned indoor loops by positioning the receiver beside or within the loop winding ; the degree of coupling is easily adjusted by rotating the receiver with respect to the loop. Close coupling is achieved by mounting the portable within the loop with its ferrite rod against the winding and parallel to the loop axis, preferably in a corner; here loop tuning swamps the receiver's own tuned circuit and is good for weak signals that are several channels away from stronger locals.
Where weak signals adjacent to stronger ones are to be resolved critical coupling can be acheived by setting the radio beside the winding though 3 to 6 inches from it, again with the ferrite rod parallel to the winding axis. This set-up allows the loop and the receiver to tune radio frequencies independently and without interaction, better selectivity is acheived but sensitivity is lower than possible. Regular listeners soon learn to adjust a suitable compromise.
Portables can be coupled to active loop and outdoor antenna feeders, either by using the set's own external antenna socket or via some form of radio frequency induction to the internal ferrite or frame antenna. The feeder may be connected to turns of wire wrapped around a ferrite rod placed on top of the portable, or to turns of wire that are wrapped around the entire radio in the same sense as internal antenna turns. The ferrite coupler may be in the form of an old MW rod with coil - easy to move about and adjust for best results; the external winding say 2 to 10 turns of any wire not neat or as flexible, and often induces shortwave breakthrough.
Mains powered receivers are'less critical; some offer low impedance screened input sockets and separate antenna + earth connectors, so try all options and combinations. Night-time' signals are strong so be prepared to attenuate as necessary to preserve working linearity; easily checked by tuning to two or three times the frequencies of strong LW + MW locals, and if you hear something odd with a passive antenna then try a known good receiver before blaming any transmitter.
GO FOR IT
The rest is up to you; real DX rarely comes freely, so learn to organise a planned listening strategy. Medium wave reception changes hourly, daily, monthly, annually and every eleven years, so arm yourself with a log book, UTC clock, calendar, copy of- the World Radio and TV Handbook, monthly sunrise/sunset charts., plus a permanently connected cassette recorder with electro-mechanical time switch. Don't buy an electronic timer, these might be quieter mechanically, but cause radio frequency interference and have fewer programmable on-off periods.
Also join a major DX club to see what is possible, different locations - different catches. Chart dawn/dusk grey line progress around the globe, it enhances propagation; list frequencies in a region of interest and if one is observed look for others. In full darkness listen for steady distant signals or heterodynes - the foreign beacon and transmitter beats that produce a note which does not change as it is tuned; this indicates a relatively smooth ionosphere, so make extra effort and try to resolve signals with directional antennas and narrow receiver passbands.
The dawn period that follows a good night is normally very rewarding during winter, this is something that you can plan for by checking reception before you go to bed. Learn to recognise noisy nights where signals sound choppy with waves of noise, these are unlikely to reward the dozy plodder; if it is very noisy and cloud free you might observe an Aurora about 0l.00hrs. - these conditions sometimes boost trans-equatorial reception - something to look out for if you have a directable antenna system.
So we do hear the European, Asian, American and African continents, but who will be the first GBer with a non-Beverage antenna to record Australian MW 7 They've heard us on indoor loops! But all is not equal.
Well, once again I say "That's it". This time my mind churning effort has been to understand and develop the results possible with simple, small back-garden, mixed loop-vertical antenna systems. It really is not as complicated as might first appear, and those willing to try could enjoy as I do, listening to other people's locals ··· Ontario ·· New York ···
These pages have been long in writing - they summarise many years of enquiring study and thoughtful co-ordination with determined and diligent empiric effort; I'm grateful for the support of my wife Andrene, son Simon, encouraging friends, and those Medium Wave Circle members who have freely influenced my thoughts and writing. Many Thanks.
G S Maynard, Newtownabbey, N.Ireland.
How to become an Antenna Master was first published in Medium Wave News 1993. It is reproduced here in it's original form although the drawings have been re-drawn for clarity. Circle webmaster December 2006.
There are many more articles like this one on the Medium Wave
Circle Re-print CD, which is available from the Medium