Included is my experience-driven rationale for what is in this kit and what is not, and why.
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 Antenna wave propagation 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 ft 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. 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.
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.
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? 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.
ROTATABLE CARDIOID 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.
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 degrees shifting on both inputs is more efficient than degrees on one alone, and provides mixing for directable single signal nulling, or forward lobe generation in either direction along the loop wire line.
srmvision.comrthulu Lecture Notes Antenna & Wave Propagation CREC Dept. of ECE P a g e | 5 1. Fundamental Concept Introduction: An antenna (or aerial) is an electrical device which converts electric power into radio waves, and vice versa. Antenna and Wave Propagation [Kamal Kishore] on srmvision.com *FREE* shipping on qualifying offers. The aim of this book is to give an introduction to the fundamental principles of antennas and wave propagation. Unlike other books available. Such a travel of the wave from transmitter to the ionosphere and from there to the receiver on Earth is known as Sky Wave Propagation. Ionosphere is the ionized layer around the Earth’s atmosphere, which is suitable for sky wave propagation.
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.
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 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.
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