FAQ on MW Loops

TIPS ON BUILDING A MW LOOP ANTENNA by Steve Whitt

Although numerous designs for MW loops have been published there are some general questions that aIways seem to crop up. It is not my intention in this short feature to give step by step instructions on how to build a loop. Rather I will try to clarify some some common problems.

 

Can I use a metal frame for my loop?

No! This will most certainly disrupt the aerial performance. Ideally the. frame on which the wire is wound should be made of a rigid insulating material like wood or plastic. Try to avoid metal although a few metal bolts or screws are acceptable as fixings.

 

Is it better to build my amplifier into a metal box?

If you are using a differential matching amplifier (DMA) (why aren't you??) then it needs to be physically located as close to the end of the loop Winding, usually very close to the tuning capacitor. When I say close I mean ideally less than 20cm. This is to prevent stray signal pickup that would disrupt the intended figure-of-eight directional pattern of the antenna. Any direct signal pickup by the amplifier would also have the same effect on the directional pattern. Therefore, yes, it is a good idea to screen the amplifier in a metal box. To be effective the box must be connected electrically to your receiver earth via the outer screen of the coaxial cable used to connect the output of the amplifier to the aerial input socket of your receiver . This should be the only electrical connection made to the box and the connection between the loop winding and amplifier input must go via insulated feedthrough connections (do not accidentally use feedthrough capacitors) fitted in the side wall of the box.

 

How long should the lead to my radio be?

As short as physically possible. If you are using a DMA then theoretically you can run a very long length of cable back to your radio. Unfortunately this connecting cable tends to act like an aerial unless great care is taken in its design. This unwanted pickup on the cable interacts with the desired pickup from the loop and can compromise the depth and direction of loop nulls. Try to keep the lead to no more than 3 metres.

 

How many turns should I wind on my loop'?

It depends on the size of the loop and the spacing between turns and it's best to follow the guidance of a published design. If in doubt, initially wind too many turns since it is easier to remove wire than add it later. A simple test to check if you have too many turns is to see if the finished loop will peak a received signal at the top end of the band

 

Can I build my loop fixed directly on a wall?

Yes, but make sure it points the way you want it to since walls do not rotate! A wall is good since it is rigid and if you are building a spiral loop rather than a box loop it is a simple matter of pressing pins into the wall around which the loop is wound. However a wall can contain hidden problems. Firstly pushing pins into a wall can be a safety hazard if you hit mains wiring. And any mains cabling in the wall is likely to be be a serious source of interference and noise. If the wall contains a significant amount of metal (either reinforcements or expanded grills used for plastering) then this could completely disturb the performance of the loop. The best walls are outdoor garden or garage walls but if you want to try indoors then the outside walls of the house are best.

 

What sort of wire should I use for my loop?

The very best wire is known as Litz wire which will ensure the lowest losses in the winding and will result in the sharpest tuning. Litz wire is unfortunately very difficult to obtain these days and I know some people who have made their own! Commercially available Litz is made of a bunch of many (up to 250!) strands of very thin (typically 48swg or less) wire each individually insulated with an enamel coating from every other. The whole bundle is wrapped in cotton or silk . It is only at each end of the Winding that all the strands come into electrical contact with each other when the bundle is soldered and the enamel is burnt off. Commercially made Litz looks like an ordinary multi-strand wire but a pocket ohm-meter will quickly show that all the strands are in are insulated from each other. If Litz wire is not available then almost any other convenient flexible wire is suitable, as second best, for loop winding. The wire does not need to have an outer insulation since, if the loop is correctly wound, the tension in the Windings will stop wire sagging and touching adjacent turns. The latter situation is most undesirable.

 

Every time I touch my tuning knob it affects the loop tuning!

This is known as hand capacity effect and is inevitable if you are using a tuning capacitor with metal spindle. Its effect is most severe on a better loop since it will disturb the very sharp tuning. If you can touch the metal of your tuning capacitor and it has no effect on tuning or received strength either you are made of plastic or something is seriously wrong with your loop! The simple answer is to place a large plastic knob on the tuning spindle of the variable capacitor. With the best loops this may still be insufficient and a plastic spindle exlension (say 5-10cm long> will be beneficial.

 

Can I use an outdoor loop?

Yes, but you will probably need a different design to the commonly published loops that employ a simple hand-turned tuning capacitor. The prime advantage of on outdoor loop is that it avoids the intense interference generated in most domestic households and carried around by the mains electricity wiring. Unless you want to run to the bottom of the garden to retune the loop every time, you wIll either need a remote control system of some sort or an untuned loop, both of which are beyond the scope of this article.

There are many more articles like this one on the Medium Wave Circle Re-print CD, which is available from the Medium Wave Store.