The antenna forum

This article was first published in the mid 1970s. It is reproduced here in its original format, further notes appear at the end of the article, Webmaster.

THE ANTENNA FORUM

This publication has been edited and published by Ken Brownless, at YORK, working in association with Charles Molloy and Pat Dobbs. Our aim is to try to present accurate and authentic information on the subject of Antennas.
The Loop Notes and Plans which follow are the result of several months of thought, construction and testing. As we approached the anticipated Sun-spot Minimum, it has to be remembered, that at the last one in 1974, the European QRM was not (quite) as BAD as it is now. So in order to help DXers get the beat results we have this extended feature, with details of the M.W.C. LOOP MARK 2. It incorporates what is known technically as the ALTAZIMUTH system - tilting for superior nulls. We have two variations on an original theme (to paraphrase things in musical language).

* (1) Loop for use with a Differential Matching Amplifier (D.M.A.)
* (2) Loop for use without a D.M.A.

 


DIAGRAMS are available for those who wish to build the M.W.C. Loop Mark 2 and may be obtained on receipt of return postage or an I.R.C. from K.B. in York. The diagrams are Loop Outline drawing and D.M.A. Construction details.


LOOPS - HISTORICAL NOTE. A Loop may to defined as a length of wire wound a frame in the form of a Loop, i.e. with a feed from each end to the receiver. The two Italian engineers Bellini and Tossi did the first experiments in Direction Finding work using a Loop system. Marconi was also involved in SOME D.F. experiments. The first application of the Loop system, on which today’s models have their original inspiration, dates back to the late twenties from "Frame Aerials" as they were then called were used on top of superhet receivers. They were actually large aerial coils connected direct to the receiver circuit and no Induction Link was used. Mostly these were small in comparison with today’s models. One of the largest then was 30 x 12 inches and the shapes varied - box, diagonal, etc - some having a M.W. winding and 90° away a L.W. winding. The earliest portable radios came out around 1930 and their aerials wound round the inside of the case were of the Loop variety. In those days portable radios were VERY MUCH less portable than they are today.

The practicability of the use of Loops for M.W. DX purposes became apparent as the result of experiments carried out on the West Coast of the U.S.A. in the late fifties. The details which were published in a magazine called "DXing Horizons" (now no longer published) were of a 40 inch 7 Turn plus Link Turn Box Loop tuned by a 350pF capacitor with a 500pF fixed capacitor to cover the L.F. range 540 to 620 kHz, and it was this publication that lead to the use of Loops in this country with the first two being built by Al Slater and Ken Brownless.
The value of a Loop is well known from our DX experience of the Directional Properties, proved Signal to Noise Ratio values - to the value of 35% over a 30ft wire at a height of 20ft - and the ability to separate DX stations. The trouble with the early "Frames" and also with the re-development of the Loop has been the "Vertical Effect" which is referred to in the later text, and in recent times much development has occurred in this kind of Antenna, especially by Gordon Nelson, and today we have Loops which reflect the research done over many years.

 

FACTS AND FIGURES. The AREA of a Loop is very important a factor if you are concerned with DX RECEPTION, and it must be remembered that the LARGER the area of the Loop the STRONGER the SIGNAL INPUT to the receiver will be and also it must be remembered that many DX SIGNALS are WEAK. If you take a Loop Antenna with the ordinary Inductive Link Turn of 40 x 40 inches you have what we call a STRENGTH FACTOR of 80.Then a Loop of 34 x 34 inches will have a S.F. of 68, one of 31 x 12 will have a S.F. of 43, and one of 21 x 9 will have a S.F. of 30. So the comparative relationship of a 40 x 40 and 21 x 9 Loop means that the latter will give you LESS THAN HALF THE SIGNAL INPUT of the former. You can increase the A.F. Gain in many cases, but it stands to reason that if you are using a small Loop the amount of A.F. Gain available for a DX SIGNAL might not be sufficient for PROPER audibility. If you are only interested in European reception, you can stick to a small Loop.

Loop Tuning is normally done with a variable tuning capacitor. A commercially built A.T.U. has been tested in the past but when compared with results using the V.T.C., it was found that the latter gave 6 to 12 decibels more strength than the A.T.U. Using an A.T.U. the number of turns in the Loop is not too relevant as the tuning of the Loop turns wound round the frame is done by the A.T.U., while in the case of the use of the V.T.C. the governing factor is the number of turns on the Loop.

Directional Effect on Loop Antennas extends from the Long Wave Band to the Medium Wave Band and up to about 2000 kHz with effective Maximum to Minimum strength facilities over 90°. There is a degree of signal strength reduction on Loop rotation up to 2500 kHz, but above that frequency there is no directional effect. So, in this respect, a Loop designed for Short Wave use is of little value.

Short Wave LOOPS are normally tuned by an A.T.U. and the number of turns needed is not relevant. You could have six or twenty six turns wound on a frame to make a S.W. Loop and tune it with an A.T.U. As a general rule the strength of DX signals on Short Wave is such better than the equivalent on Medium Wave. You can attach a very short length of wire to an A.T.U. and get some DX signals - more signals will be obtained if you have some wire wound round a frame. The results obtained will not compare with those obtained with a proper outdoor antenna.

Connecting Loop to Receiver. The connections from the Loop to the Receiver should go to the DI-POLE TERMINALS. This gives a PROPERLY BALANCED INPUT with equal nulls and peaks 90° from NULL/PEAK. If you connect the Loop to ANTENNA and EARTH Connections, the result is that you get an UNBALANCED effect with 180° from Null to Peak in some cases, and in others - usually with strong stations - the effect is of two unequal Peaks and two unequal lulls.

The Shape of a Loop in some ways does not matter i.e. it can be a Box type or a Spiral type. It does, however, pay to have equal lengths all round with, turns - your outer area of 49 square inches means that the turns are 40" x 4. If you have a Loop of say two vertical sides of 20 inches and two horizontal sides of 10 inches, then your vertical pick up will be more than the horizontal pick up. In such a case, it would be better for the measurement to be 15" x 4.

 


LOOPS – SIGNAL/NOISE RATIO AND A.T.U. The Loop which is designed for DX WORK has an effective Signal to Noise ratio which has a beneficial effect on DX RECEPTION. Note that such an Antenna is designed to be tuned from its MAIN Turns by a Variable Tuning Capacitor. This maintains the S/N ratio which brings us to the point of the Antenna Tuning Unit which is now being incorporated in some similar Antenna systems. When you use an A.T.U. in place of a V.T.C., you are ADDING some extra strength to the input from the Loop to the Receiver which is caused by the signal pick up in the A.T.U. itself and can counteract the effect of the Signal to Noise ratio and INCREASE the Noise to Signal level factor. The use of the Feeder Cable to carry the Loop pick up to the receiver is also designed to minimise signal pick up between the two.

Antenna Tuning Units. For the outdoor antenna, the A.T.U. definitely represents an EXCELLENT strength booster. Highly recommened is the data on this subject in "How to Listen to the World" 7th Edition. The commercially made Joy-Match for use with the Joystick Antenna or other antennae is an excellent A.T.U. which gives considerable gain. These designs are for S.W. and M.W.

The Loop can also be used as an A.T.U. although it is not really designed for that purpose. We experimented with a 34 inch 11 turn Spiral Loop - Loop range was up to approximately 1830 kHz but linking an outdoor antenna to the Tuning Capacitor for use as AN A.T.U reduced the range to approximately 1440 kHz. Increasing the tuning range in such circumstances could be done by tapping back one or two or more turns - trial and error method. It does give a moderate gain on strength when used with an outdoor antenna.

 


LOOPS – WITH AC/DC RECEIVERS. There are some receivers available for use on AC/DC mains and these include the Eddystone 840 series. No earthing facility is provided because if you try to connect one to such a receiver, you can get a SEVERE electric shock. The Loop with the D.M.A. can be used quite safely in such cases as no DIRECT earth connection is involved.

In order to use a Loop without a D.M.A. on an AC/DC receiver, the following procedure can be used. Do not tap the centre of the Link Turn but only take a lead from the outer conductor on the two lengths of coaxial cable and take this to GROUND outside. TAKE GREAT CARE that NONE of the outer conductor (braid) comes into contact with the inner conductors which go to the receiver DI-POLE connection, otherwise you will be in trouble. We now look at the reasoning behind the use of either and/or the Coaxial Cable lead to the receiver and the Link Turn Centre Tap. It is to cut down on the Vertical Effect about which some notes follow.

Vertical Effect. A Loop picks up both a Horizontal (directional) and Vertical (omni-directional) signal component. The problem is that of the "Vertical Effect". In order to minimise this effect there are two recognised methods which are as follows.

* (1) A centre tap from the Link Turn to Earth.
* (2) The use of twin coaxial lead.

Both have the effect of cutting down on the Vertical Effect and in the new Loop, when used without a D.M.A we hove employed both these methods.

 


LOOPS - PEAK AND NULL FACTORS. The Peak - the best reception can be affected by other Antenna leads which may be near to the Loop and the solution in this case is to earth these antennas, Another point to remember is that, although you can use a Transistor Radio with a Loop, using it with another receiver when the Transistor Radio is nearby can affect -the performance, so the Transistor Radio should be at least ten foot away from the Loop, It is the Ferrite Rod that causes the problem. Another factor that can affect performance is that of metal objects such as window frame - one could try to earth these.

The Null — the minimum reception point can also pose a problem. In general, the Null is better on a larger Loop, There is the Telephone and Powerline bugbears. As you will know, interference (man made) is re-radiated over these lines and a similar re-radiation of signals can affect the Null values that are obtained. This is best descried as an “uncontrollable” factor.

 

OTHER TYPES OF ANTENNA FOR MW. Invert “L”. These were very much the fashion in the early days up to or over 150ft in length. Key factor with this antenna is that it does have a measure of directivity. The higher such an antenna is, then the better the results and also the use of insulators is advisable, as is a continuous length of wire. Sometimes the downlead is in the form of Coaxial Cable - this is usually the "Anti-Interference" Antenna system with a Screened Step Down Transformer and a similar transformer at the receiver end to "step up" the signal. Mainly, this type of antenna has its application Short Waves to help solve the car ignition problem. This kind of antenna - with or without the coaxial download - may be the second choice after the Loop. The signal strength will be greater but so will the NOISE LEVEL, especially on DX.

Vertical Antenna. One great value of this type of Antenna is its use in areas where local M.W. reception is tricky. The antenna may take the form of a length of vertical wire from the house guttering to the receiver. There is also the commercially made Vertical Rod Antenna consisting of a rod with a coil incorporated, and an A.T.U. is necessary when using this system. Good results can be obtained, but the main disadvantages is that they are omni-directional. The coaxial cable from a FM/TV antenna can be used as an antenna also.

Indoor Antennas. With European signals at S9, plus, plus, PLUS, these can be useful and one idea is to fix one round the picture rail in the room - if you have a picture rail. You can hide a length of wire behind the curtain sides and run it along the curtain top length behind the curtain rail, and a short length of wire stuck into the A-terminal will bring you SOME kind of result, The most practical of the "Indoor Types" is the wire in the Loft, however this has a similar defect to the indoor wires in that they are "screened" by the house. The Loft gives you a measure of height and greater lengths of wire can be fixed to the rafters than is normally possible to erect outside.

The "V" Antenna. This Can provide a solution where you have limited erection space outside. Two 25ft lengths in a “V” shape will give you the equivalent of a 50ft wire with a feed from one end. There is a slight directional effect with the "V" Antenna, i.e. one facing with the apex to the North will have a slight gain on signals from the South.

Long Wire/Beverage Types. The Long wire Antenna may be up to or over - sometimes well over - 1000ft erected at a height of up to 10ft or so. The longer the wire the sharper the directional effects are. When a Long Wire is unterminated it is bi-directional. Another Long Wire type antenna is the Beverage which is named after its New Zealand inventor. This antenna is unidirectional and is terminated at the far end with a resistor which matches the Input Impedance of the antenna stage in the receiver. Directivity is along the path away from the end of the wire. Gordon Nelson has a Long Wire 6600ft long, 30 miles North of Boston, Mass.

 


ANTENNA ERECTION. In early January 1974 the importance of proper erection of antennas was brought home to some people when we had gale force winds and storms. An outdoor wire should be erected in as high a position as is possible, and the anchor points should be set off by Insulators and supporting wire or nylon rope apart from the main antenna. These should be at the far end and the lead in end. The antenna wire should not be taut but a small degree of slackness should be used. If it is too taut it could easily break in a storm. Use insulated wire as if this is not used, bare wire is affected by atmospheric pollution which can gradually lead to the wire being weakened to breaking point. The point where the wire enters through the window frame should be protected to prevent grazing and possible breaking of the wire. A continuous length of wire should be used wherever it is possible, but should two lengths be used make sure the joint is soldered and protected by a weatherproofing solution.

 


EARTHS. Where there are earthing facilities on your receiver - this does not apply to AC/DC types - it is a great help to use them. A damp place is the best to use for an Earth connection and if you can get down to the clay level with your rod/s this is a good idea. An efficient earthing system consists of three copper earthing rods placed in the form of a triangle in the ground with a soldered connection from your receiver to the first rod which is duly soldered to the other two rods and then back to the first rod. Once again, the soldered connections should be covered with a waterproofing solution to protect the soldered connections. It is a very good idea to make periodic inspections of your antenna and earth systems to ensure that you are getting their full capabilities and that there are no potential sources of bad connections which can cause noise. The leads from ground to the receiver should be kept as short as possible.

There is another potential earthing device which is a COLD water tap, and the telephone system is sometimes linked to this. The hot water tap should HOT be used.

 

 


WIRE. For a Loop this needs to be of a moderately pliable nature with not too thick a covering of insulation - winding could be more difficult that way. Wire is obtainable in 7 or more strand types and the number is unimportant. Loop wire does not need to insulated.

In some shops one may see wire that is uninsulated and this should be avoided for you have to remember that if this kind is used outside, there is the fact that atmospheric pollution will corrode the wire and weaken the strength of it, so if there is a storm or high wind the wire could break after some time or erection. To see the effect of corrosion, you can take a piece of wire, bare one end, then leave out in the open for a few months and see the effects of pollution, etc.

There are various types of Insulation on the wire that we use. There are assorted colours of plastic covering which will be familiar to many of you. We have seen also a wire covered with what seems to be a synthetic rubber compound which is too pliable to be of much use with Loops. The advantage of both those types is that they are CLEAN to handle. There is also a wire covered with strands of criss-crossed fibre thread which appears to be coated with some thick "preservative” solution, but this is very dirty to handle. Again, wind, weather, and pollution can damage the insulation and lead to breaking of the insulation and possible damage to breaking point of the wire.

When doing any work on antennas it la highly advisable to use ONE length of wire for the job - if not you may encounter cross modulation. If you MUST use two lengths, make certain that the joints are properly soldered and covered with a waterproofing solution. If experiments involve adding or subtracting turns, when the experiment is complete RE-WIND with a continuous length of wire.

 

 


DIRECTION FINDING. On a Loop which is properly constructed, properly balanced and properly connected to the receiver, there are TWO null (minimum signal strength) points each 180° away from the other, and TWO peak (maximum signal strength) points also 180° apart. The correct link to the receiver is via the Di-pole Terminals. A form of D.F. can be managed if you link the leads to the Antenna and Earth Terminals on the receiver, and on weak stations you will obtain a relationship of 180° from Null to Peak, but on strong stations it was found that you get one peak with greater strength and another at less strength, and also two unequal nulls. This form of D.P. is only possible in daylight hours when the Ground wave dominates.

D.F. work at night poses problems, for we have both the ground and Sky Waves to take into account. This produces what is called in simple terms "The Night Effect". It is difficult, even in daylight, to get a true bearing on B.B.C. Radio 4 stations with the sharing of frequencies, and at night there is a tendency for stations on the same channel to "pull” the bearing on the Loop away from its real position. The Night Effect causes nulls to tend to vary and in order to obtain a fairly accurate bearing on a given signal, the best plan is to take a series of bearings in degrees and average out the total number of bearings taken.

On some European stations it is possible to get a Null on one end and a Peak on another and vice versa. The ease of doing this will depend on where you are located - you need to have about a 90° bearing between the two stations. In one town you may be 90 between Station A and Station B, but somebody else may be only 40° between the two stations and no matter what you do you will be unable to separate the two stations.

It should be noted that when you try D.F. in daylight with the Loop connected to Antenna/Earth you reduce the H.F. range and increase the L.F.

The difficulty of D.F. work during the hours of daylight was highlighted at the time that the I.B.A. took over the 1151 kHz frequency - which had previously boon used for Radio 4 at Stagshaw and Scarborough - and went onto 908 kHz. Until then the latter frequency was in use at London and Clevedon and Nulls and Peaks were obtainable. The position changed when Stagshaw and Scarborough came on 908, and we now get London and Stagshaw off the two ends of the Loop and Scarborough at about 90° away from these two, so nulling the signals out is very difficult. Once again, the location from which you ore listening is important. In some places you can separate two stations on the same frequency, but try doing that in SOME other places. It just cannot be done.

BASIC DIFFERENCES THE LARGE LOOPS as they are normally called originated in U.S. A. in the sixties and were designed specifically with DX in mind, hence the size. They have been tested, experimented with, and found to be ideal for the purpose for which they were designed. The "Mini" Loop is simply a revised version using an inductive link with a variable tuning capacitor. The original version of this model dates back to the thirties in the days of the first superhet receivers, when the "Frame Aerial” as it was then called stood on top of the "Wireless Set". The connections from the Frame went direct to the inside of the set and, in fact, was the actual Aerial Coil in those far off days. We had some experience of using one of these "Frames” in the thirties in Hull. Those antennas were on the Loop idea but with an area too small for DX as we know it today. The inspiration for the current "Mini" is to be seen in the Newnes "Wireless Constructors Encyclopaedia" of around 1930.

 

 


LOOPS - SPACING BEWEEN TURNS. This must be taken into account in Loop construction. With the larger Loops the spacing is usually half an inch between turns. If you have a half inch between each turn and then reduce it to a quarter inch. the tuning range of the Loop can be affected. This is because of the self-capacitance of the actual wire in the turns - 7 main turns for a 40" Loop, 14 for a 20" Loop, and 28 for a 10" model. In some models the spacing Is close for reasons of neatness and the turns needed are best found by empiric methods. For full efficiency it is essential that you should be able to PEAK reception on the Loop Tuning Capacitor over the full frequency range of 519 to 1602 kHz. Once more on the theme of the "Mini" Loop – let’s say a 10" model - with Europeans one can make use of such a Loop but you must differentiate between DX and EUROPEAN reception. On a larger Loop for DX, the LARGE models are in general use for obvious reasons. With European stations on such a Loop, one finds that on the S-meter the pin stops the pointer with the S9+ signals, so it is difficult to obtain an accurate strength reading in such cases. "Mini" Loops REDUCE the available signal strength and at the same time reduce, the Vertical Effect thus making a sharper cut in signal strength as you rotate the Loop through the 90 from Maximum (PEAK) to Minimum (NULL) Signal Strength.

The following questions should be asked BEFORE contemplating the construction of a Loop Antenna.
(1) The Area of the Loop, i.e. the length of the four Motions of a BOX Loop or the length of the first outer turn on a SPIRAL Loop, for a 50 inch Loop the area will be 250 square inches. (2) The number of turns on the Loop. (3) The capacity of the Variable Tuning Capacitor, (4) The spacing between each turn. (5) The frequency range coverage.
If you are quoted details on a Loop with UNEQUAL side lengths, i.e. 20 x 10 inches Area Factor the total of the TWO sides will be 30, then modify it to 15 x 15 inches.

 

 


LOOP EXPERIMENTS. These can be done with several sizes of Loop. A 40" Box Loop normally has 7, main turns, a 20" one 14, a 10 inch one 28 (too small, except for Europeans) , or a 15" one approximately 21 turns - again rather too small. A 48" model will have about 6 turns when tuning is by a 500pF capacitor. For a 40" SPIRAL Loop 9 turns are needed. In order to calculate the SIZE of the cross-pieces for the basic frame of the Loop, the following procedure is suggested.

Let us suppose that you wish to make a 20" Loop, then you draw a SQUARE to scale - say 1 inch to 10 inches. This gives you the plan for the windings. Next you draw two DIAGONALS across the square that you have. Diagonals will be 29 inches long, however note that if you are making a BOX LOOP, the actual length of your cross-pieces should be 28 inches in order to allow for the spacer bars which are placed on the end of each cross-piece - with notches in them to hold the turns in position. A pair of centre supports will be needed for the supporting of the frame. If ONE INCH dowel is used one of the cross-pieces needs to be shortened by one inch and cut in half to fit properly. The holding dowel is then mounted to support the entire frame.

A similar process can be followed if you are making a SPIRAL LOOP, but an extra three inches on each end of the two cross-pieces is necessary because one of the cross-pieces is mounted Into the rotating stand and the other can hold the tuning capacitor.

 

 


LOOP REMOTE CONTROL. Remote Control of a Loop Antenna with the Tuning Capacitor mounted alongside the receiver is possible but only advisable when a Loop WITHOUT a pre-amp or D.M.A. is used, as in this case ALL leads need to be kept as short as possible. Sometimes it is more convenient to tune the Loop from beside the receiver and the system used is as follows. Approximately a 5ft length of LOW LOSS twin-feeder cable is necessary (300 ohm twin-feeder is best, having only a capacity of 4pF per foot), with connections from the ends of the main tuning turns via the feeder to the Tuning Capacitor. If a 500pF T.C. is used, the use of the feeder will increase overall capacity to 520pF and may result in a cut in the tuning range on the H.F. end of the band, and if this happens, the removal of one turn should restore the lost part of the range.

LOOP TESTS were made with the S680X to find out RELATIVE PEAK SIGNAL STRENGTHS on the 34" Spiral Loop and a "Mini" Loop 26 x 10 inches (Box type). Area Factors involved are 34 plus 34 equalling 68, and 26 plus 10 - 36. Frequencies tested with relative strengths in decibels being:- 647 - 66/48, 719 - 54/30, 746 - 42/26, 791 - 48/24, 1007 - 42/18, 1160 - 54/30. TOTALS of the dB readings are 306 and 176 and the average of the six readings is 51 and 29. Ratio of 51:29 Is similar to 17:9 (if you divide 51 and 29 by 3). Multiply 17 and 9 x 4 and you get 68 and 36.

With reference to the “Mini”Loop, some people would have us believe -that this particular size of frame is suitable for all usages. The data shows that a "Mini" Loop gives LESS signal input to the receiver than a larger Loop does. We dispute the claim of the use of a small frame for what is described as "ALL usages". The standard Loop was designed for DX, that is the reason for its size. It must be remembered that on a normal large Loop a DX signal can be VERY WEAK, so what would it be on a "Mini" Loop?

Nulls. We next turned our attention to the DIFFERENCES between Peak and Null reception points on the two Loops as measured in decibels. We checked the following frequencies during daylight - 692, 746, 908, 1187, and 1214. We now list the decibel readings on the 34" Spiral Loop followed by those for the 26 x 10 inch Loop, Following those figures, we have the comparative reduction in decibels between the two Loop nulls.

692 kHz 72/42 48/36 30 12
746 kHz 36/0 30/12 36 18
908 kHz 42/0 39/18 42 21
1187 kHz 42/6 24/0 36 24
1214 kHz 54/0 42/6 54 36

TOTAL of dB reductions for the 34” Loop is 198 - average 39, and for the 26 x 10 inch Loop 111 - average 22. The percentage relationship between 22 and 39 is about 56% and that of 29 to 51 (Peak Signal Average) is about 57% this indicating that there is a close relationship between PEAK and NULL values and NULL REDUCTION values based on the Area Factors of the two Loops.

Following the Strength/Null tests we discovered that the "Mini” Loop had ANOTHER disadvantage. One of the features of the normal Loop is its excellent Signal to Noise Ratio but in the case of the "Mini" Loop with respect to noise of the "man made" variety (Electric motors or vacuum cleaners and TVI for example), it was found that the "Mini" Loop degree of rejection was definitely POOR in comparison with that of the normal Loop.

 


“MINI" LOOP NOTES. This temporary Loop was round around two fibreboard cartons joined together which gave sides of 26 and 10 inches. 15 Turns (no spacing) were used plus one Link Turn which was connected - without any feeder cable -to the Di-pole terminals on the receiver and the exact centre of the Link Turn was tapped and a lead taken to Earth on the receiver and then to Earth (Ground outside). Tuning the Main Turns was by 500pF Variable Tuning Capacitor. Reason for Link Turn tapping is to improve the Electrostatic Balance which was bad on the original version. Diagonal measurement for a 26 x 10 inch Loop is 28 inches.

The simplest method of construction is to form a frame for the Loop with two pieces of wood 26 x 2 x ½ inch and then glue them together in a rectangle. A 3" square piece of three—ply can be used to hold the tuning capacitor, with this being fixed on an 8 inch length of half inch dowel which is fixed to a centrally placed piece of wood (on bottom section of frame when wound) of 2 x 2 x ½ inch size. A stand is needed of wood built up to 6 inches square -four inches deep with a half inch hole, 2 inches deep to hold the rotating dowel. If your main interest is the powerful Europeans this Loop is O.K. but dubious for the weaker stations such as some of the Spanish ones.

 


FERRITE RODS. These are now standard in domestic receivers - both Transistor and Valve types - for the Antenna. A Loop can easily be used with a receiver incorporating a F.R. Antenna. It can supplement the results. Placing a Loop in close proximity to such a receiver, you will find that you can peak the signal strength further on the Loop. If you have a Transistor portable with facilities for external Antenna connections, you can use these for connecting the Loop DIRECT. This must be done correctly for proper results - via the Di-pole terminals in this case (if you have V.H.F. facilities), the Antenna lead to the M.W. Antenna input and the other lead to CHASSIS via the VHF input on the opposite side to the Antenna input. If you wish the Loop to be fully effective, it is advisable for the Loop frame to be kept five foot away or so from the Transistor outside the field range effectivity of the Ferrite Rod. WARNING - if you connect a Loop to the Antenna/Earth connections you get an unbalanced input.

The use of a Ferrite Rod Antenna with a Communications Receiver is also worthy of consideration if you are interested in European reception and some years ago Charles Molloy did some experiments along these lines. There is a field for regular experiment here as, in particular, the sharpness of separation on such an antenna is superior to that of a "Mini” Loop. Details are:- some 60 to 80 turns close wound on an eight inch F.R. 3/8 inch diameter with a coupling link of 5 or 6 turns - using 25 SWG wire - with a 300pF variable tuning capacitor. This will cover the M.W. range. It is for low impedance output and co-axial cable is used to connect to the receiver Di-pole terminals. The long Ferrite Rods give better results.

 


M.W.C. LOOP MARK 2. This new model Loop is of lightweight design and is SPIRALLY wound, This new Loop is a 34 INCH SPIRAL which with or without a pre-amplifier gives good results. The basic pattern for a Loop antenna is the "cosine pattern” in which the NULLS are equally deep and 180° apart. The NULL" is the minimum strength position and the PEAK is the maximum strength position, so the NULL and PEAK are 90° apart. Me have already remarked that the signal pickup and the NULL value on a larger Loop is better than on that of a smaller Loop. The ALTAZIMUTH Tilt Principle has been incorporated. One major Loop problem is that of the "Vertical Effect" - Electrostatic Imbalance - part of the received signal is from the magnetic pickup of the Loop and another part is from the behaviour of the entire Loop structure as a short vertical antenna. How this Loop is presented as an ordinary model and also as a model with a Differential Matching Amplifier incorporated. The model that has been used is that designed by Steve Money and described in PRACTICAL WIRELESS for April, 1973. A Link Turn is used with the ordinary model and this has been designed to get better results than with the older Loops. No Link Turn is used with the D.M.A. model, for the D.M.A. servos as the equivalent of the Link turn.

A Note on the Differential Matching Amplifier. The D.M.A. is a balanced input amplifier using transistors, and in order to maintain a high Q value in the aerial circuit a pair of FET's are used in the Input Stage. A matched pair of 2N3823 transistors are used with a 2N706 being employed to provide a constant current feed. Another 2N3823 is used as a source follower to match the low impedance of the coaxial cable into the receiver input circuit. FET's have a much higher Input impedance than the normal bi-polar transistor.

Constructional Data of Frame and Stand. The Frame Support consists of a length of broom handle or one inch dowel to a length of up to 5 foot. Broom-handle or Dowel plus the Support must fit plumb into an adequate support stand. This could be a block of wood 12in x 12in x 6in with a one inch hole drilled in it to a depth of four inches. THREE holes are drilled in the dowel one inch from the TOP, I7½ inches from the TOP, and 22 inches from the TOP of the dowel. These are to act as support for the top of the Tilting Bracket (smaller one) and the Locking Plate (larger one). The dowel which supports the frame of the Loop and fixes into the stand should NOT be glued into position.

 

Frame Construction.

* (1) HORIZONTAL CROSSPIECE is one length of wood 56in x l in x ½ in.
* (2) VERTICAL CROSSPIECE is one length of wood 56in x 2in x ½ in.

Join these together in the shape of a CROSS by cutting out from the VERTICAL crosspiece a piece of wood a HALF INCH long and ONE INCH deep. Mark the wood at a quarter inch - to left and right of the exact centre - that is 28 inches, so the cutting marks should be at 27 three quarter Inches from each end of the VERTICAL crosspiece. Make two cuts to 1 inch depth and with a chisel cut out the one inch depth of wood. This will enable you to fix the two crosspieces plumb together. However, do not join them together yet. Note that the windings should be done on the side where the joint is made. On the other side at one end of the HORIZONTAL Crosspiece attach a piece of three ply wood 9in x 4in to hold the D.M.A. and the tuning capacitor - one on top and one below with the vertical length of wood being the longer one. JOIN the two crosspieces and the piece of 3-ply wood together with Araldite (an epoxy resin). Note that the supporting bracket for the D.M.A. capacitor, etc, should be mounted on the reverse side to the windings section.

Prior to Winding. We have allowed four inches of free space on each end of the Crosspieces. First of all, take a measurement over the sections of the crosspieces to cheek starting points for the outer 34in winding. Then attach a ruler to the points you will have marked, tape it in position, and mark off the required number of half inch spaces. Then put in 1 inch tacks and put a twist of plastic adhesive tape on each. You are then ready for the WINDING STAGE.

Turn Winding. When the D.M.A. is not used the main turns are wound first and the link turn added afterwards. The D.M.A. and tuning capacitor should be mounted on a small board at the end of the Horizontal crosspiece which will be in front of you if you tilt the Loop to the left. So the first turn will begin near to the capacitor with HALF INCH spacing between turns with the winding being supported by 1 inch tacks covered with plastic insulating tape. Winding is done like this. The outer turn is 34 inches per section -34 inches up to the top of the Vertical Crosspiece, another 34 inches to the opposite, end of the Horizontal Crosspiece, and then to the bottom of the Vertical apex another 34 inches. You now have THREE 34in sections and the next winding section goes to the point where you begin - but to the second tack inward on the Horizontal Crosspiece where you began. Proceed along those lines with the remaining turns until the windings are complete. The end leads go to the Tuning Capacitor. When the D.M.A. is NOT used, they terminate there. When the D.M.A. is used they go to the Input and from the Output to the Receiver. Winding Hint. At each tack on the frame make the wire taut by pulling it and then twist it around the tack, and then continue winding.

Loop/D.M.A. Receiver Connections. Loop Turn Ends, one to first connector on capacitor and from there to INPUT on D.M.A., the other one to second connector on capacitor and from there to other INPUT on D.M.A. The Link from the D.M.A. is by a single length of coaxial cable. Inner conductor to A terminal and screening to AE (Chassis) Di-pole terminal.

How for a look at the board on which the capacitor, etc, is mounted. If you are using the Loop model WITHOUT the D.M.A., then a board half the size (see notes on materials needed) can be used. If you are using a D.M.A., then the full size board is advisable. On the board, which is mounted on the end of the Horizontal Crosspiece, you need an equal space above and below. Above the crosspiece mount the D.M.A. on the front, and on the reverse side mount a PP9 Battery with a snap-clip connection from the Battery load from the D.M.A. - a low voltage switch can be incorporated if you wish. This can go above the D.M.A. and will save disconnecting the Battery when not in use. Mount the Tuning Capacitor on the lower half of the board. Mount the D.M.A. so that you have space on the board to drill holes for the leads from the D.M.A. One hole about one quarter of an inch diameter for the leads to the Battery and another-hole the same diameter to take the Co-axial Cable Output from the D.M.A. These two should be side by side. Then at the other end of the D.M.A. drill two one eighth of an inch diameter holes to take the Loop Input to the D.M.A. In order to prevent "give" or "pull" on the connections which could result in the breaking of the soldered connections, put some glue in and on top and bottom of the holes when the connections have been soldered. When turning the Loop around it is likely that wear and tear" in the form of "pull” which could eventually lead to a broken connection from the D.M.A. Therefore, at the point where the co-axial leaves the horizontal section of the crosspiece, USE a good length of plastic insulating tape to hold it FIRMLY in position.

Now to the Leads. When the Inductive Link is the feeder to the Receiver the length of co-axial cable is not too important and we have quoted 5 foot as a length, but if you can use less you can do so. When a D.M.A. is used, it is necessary to keep the loads - ALL of them - as SHORT as possible. The longest lead is the D.M.A. output to the Receiver - co-axial cable. You can calculate the approximate length needed. Though the Loop swings through 360°, in effect, it is only necessary to be able to swing it through 180°. The required length can easily be calculated by (l) measuring the shortest distance from Loop/ D.M.A. output, and (2) measuring the distance 90 away -from this position and the total of the two lengths will be the amount of cable required. Allow a few inches on either end.

The Link Turn on a Spiral Loop is wound OVER the centre turn. If there is an ODD number of turns and if there is an EVEN number of turns then it goes over the half way turn - say No.6 out of 11 turns - the sixth turn from the outer winding. The Link Turn is tapped at the exact centre and earthed, so first of all fix the Link Turn to the frame on a temporary basis and measure the length round the frame and allow six inches of Link Turn extra on each end for attaching to the co-axial cable link to the Receiver. Remove from frame and tap the EXACT centre of the Link Turn. CAREFULLY remove a half inch of the outer covering on one piece of the co-axial cable and take the lead from the tapping to the Screening Braid, having previously replaced the Link Turn. The braiding outer cover should be removed about 6 inches along.

Linking the Loop to Receiver. This is done with TWO 5ft overall lengths of Co-axial Cable. From each end of the two pieces of cable, remove 2 inches of outer covering and screening braid and then bare one inch at the ends of the two lengths of cable making sure that the screening braid and inner conductor do not touch. Join the two lengths of cable together with tape at six inch intervals. Connect the two ends of the cable to the ends of the Link Torn on the Loop frame. Six inches from the other ends of the dual cable, bare half an inch of the braiding OB each cable taking CARE NOT TO CUT the screening braid when cutting the outer cover of the co-axial cables. Connect a length of about eight inches of antenna wire (plastic covered) around both pieces of exposed braiding. Connect this to EARTH on Receiver. Then connect the two inner conductors of the Co-axial Cable to the DI-POLE terminals on the Receiver. NOTE that the Link Turn system only applies when the D.M.A. is NOT Used. When using the D.M.A. system, a single length of co-axial cable is used as a feeder with one conductor going to the A terminal and the other to the AE terminal (i.e. DI-POLE connections). No earth connection is needed. No Link Turn is used.

Tuning the Loop. Tune to a station on your Receiver with your Loop pointing in approximate line with the station and then adjust the tuning capacitor for Maximum resonance (PEAK) on the Loop. Your minimum signal strength point (NULL) will be 90° away from the peak. Your Null provides a good guide to an accurate bearing on the direction of the signal being received and if you have a direction plotter on the base of your Loop, you note Null position and to get D.F. bearing swing Loop through 90° .

Tilting. Some comments from Gordon Nelson. Starting with the Loop Frame vertical, rotate the Loop until the station pickup is minimised. Now unlock the Tilt Arm and tilt the Loop about 20 . Rotation will now give a deeper Null on one side than on the other. Continue to increase the tilt angle while rotating back and forth through the Null position. At one particular combination of tilt and rotation (and one position only) the local signal will suddenly drop to a very low level or completely disappear into the background noise. This setting is EXTREMELY CRITICAL. Movement of' the Loop frame by only a fraction of an inch from this setting often changes the pickup by 20 to 40dB. This deals with a local station elimination, now onto a distant station elimination. Adjust both rotation and tilt to minimise the undesired station signal, and because the polarisation figure for a skywave changes with time, frequent adjustments will be necessary. This is also known as the "Night Effect".

Transistor Receiver Note. To follow the notes on tho Altazimuth/Tilt system, the Ferriite Rod as is well known has directional properties which are similar to the Loop's directional properties, i.e. NULL to PEAK is a matter of 90° . We did some tests with European stations in the Null (minimum strength) position and then tilted the Yacht Boy Receiver to an angle of about 15° and with it in that position of tilt, we turned it slightly to the right and then to the left and with it facing & little more rightwards the signal disappeared.

Modifying Existing Loops. To add tilting facilities to your present Loop we suggest the following procedure. You will need the two Tilting Brackets and the Dowel Stand as for the M.W.C. Loop Mark 2. The existing dowel on your Loop should be removed - this will also reduce the weight. The TOP BRACKET will be attached to the centre-piece support piece on your Loop. If the area involved is 6 square inches, then we suggest the following setup be used. The dowel position will be at the centre of the Loop and by the side of the dowel will be the Upper Tilting Bracket, which, in this case, we suggest should be 13½ long with the hole drilled centrally 1½ from the top. For conversion only use those above figures. If your centre-piece support is 6 square inches, to give extra support to the Upper Tilting Bracket, glue (with Araldite) 2 pieces of wood - one 2in x 2in x ½in and one 3 ½in x 2in x ½ in to the Frame Centre Support. These measurements will allow for the Upper Tilting Bracket to fit, between the two pieces of wood as described above, and be glued in position. Note - 11 Turns are used if the Link Turn is tapped to Earth, in addition to use of the dual Co-axial Cable. If the Earth tap from the Link Turn is NOT used but ONLY the dual co-axial lead to the Receiver then 13 turns are needed as with the D.M.A.

Performance. This is satisfactory with useful decibel gain up to 36 decibels and the overall merit is excellent.

Frequency Ranges. The original 34 inch Spiral Loop used in York some years ago was very similar to the new Loop that we have been working on. It had 13 turns at half inch spacing with 500pF tuning. This previous model had an ORDINARY link turn. When you earth the Inductive Link Turn, it has the effect of increasing the LF range and decreasing the HP range and to counteract this the number of turns was reduced from 13 to 11 which covers from just below 480 to about 1800 kHz. LOOP WITHOUT D.M.A. – 11 Turns with Link Turn over centre turn; Range 480/1800 kHz. LOOP WITH D.M.A. - NO Link Turn is used; Range 480 to about 1800 kHz. The ranges that we have quoted are those on the Eddystone S680X and they may VARY with other receivers. If the tuning on the High Frequency range does not cover up to 1600 kHz, then you REMOVE one turn (or two) and if the Low frequency range-does not cover 500 kHz, then you ADD one turn (or two). Empiric methods can be used to determine the needed number of turns. Temporary connections are advisable when testing for range coverage.

 

APPENDIX

Wood 1 Vertical Crosspiece 56 x 2 x ½ inch
" 1 Horizontal Crosspiece 56 x 1 x ½ inch
" 1 Dowel Shaft 1 x 3ft length (minimum) of 1 inch Dowel.
" 1 Tilt Bracket (Upper) 12 x 2 x ½ inch
" 1 Tilt Bracket (Lower) 23½ x 2 x ½ inch
Wood - 3 Ply 1 Mounting Plate (DMA, etc) 9x4 inch.
" 1 Mounting Plate (without DMA) 4x4 inch.
" 1 Locking Plate (for Lower Tilt Bracket) 5½ x 2½
Bolts* 3 x ¼ inch Diameter - Length 3 inches For use with Tilt Brackets.
" 3 x ¼ inch Wingnuts For use with Tilt Brackets.
" 6 x ¼ inch Washers For use with Tilt Brackets.
Wire Lengths For 11 Turns 115ft 2in - ALLOW 125ft Without D.M.A.
" For 13 Turns 134ft - ALLOW 144ft With D.M.A.
" For Link Turn 13ft Without D.M.A.
" For Link Turn Tapping to Co-axial Cable - 5ft Without D.M.A.
" For Co-axial Lead - 2 lengths of 5ft Without D.M.A.
Araldite 1 tube This is a very strong Epoxy Resin

Note Allowance has been made above for extra length of wire in each case so as to allow lee-way. Seven or more strand single plastic covered antenna wire can be obtained through large dealers in lengths of 100 Yards or Metres.

*Also needed if converting Loop in Current use, a quantity of Screws.

 

Webmasters Comment

This article was originally distributed with Medium Wave News. The original paper is undated but, from the comments about storm damage, was probably written in 1975. In publishing this I have tried to keep to Ken's original layout as much as possible. All the underlining's are just as he typed them on to the stencil, 11 pages in all. The only concession to modern layout is the appendix table at the end. In the original this is a somewhat confusing list.

Although written 30 years ago much of what Ken has to say is as relevant today as it was then. There is no reason why the M.W.C LOOP MK 2 could not be built today. No original plans exist, as far as we are aware, but there is enough information in the text to allow reconstruction of the Loop. The origional D.M.A. that Ken used could also be constructed although there are much better designs available. The circuit diagram (schematic) is reproduced below. There is also an audio recording of Ken discussing the design of this Loop.

Further information on Loop Antennas and D.M.A's can be found in our Reprint CD which is available from the Circle Store, priced £9.00 including postage and packing.

If anyone does rebuild this loop design, the Circle would be interested in hearing from you.