Loading Coils for 136 kHz

This page starts by showing how to make loading coils for LF, and goes on to describe two of the loading coil assemblies used to resonate the experimental vertical antenna used at GW4ALG.  Please don't think that the loading coil has to be as big as these! 

The page concludes with a suggested loading coil arrangement for portable work, especially kites and balloons.  The coils used in this latter design are more typical of the size of loading coil generally used by experimenters.

This page comprises quite a few pictures.  To reduce the download time to your PC, they have been included as 'thumbnails'.  To enlarge any picture of interest, simply click on the thumbnail.

How to make LF loading coils
Mark I Loading Coil
Mark II Loading Coil

How to make LF loading coils

Dimensions

For most of us, the dimensions of our loading coil will be determined by the actual materials to hand.  A useful equation for determining the inductance of a proposed coil is as follows:

Materials
Plastic buckets; plastic paint kettles; and PVC pipe make excellent formers for winding big LF coils.  Most plastics have excellent RF properties and keep their shape - even under the significant tension imposed by several turns of tightly-wound copper wire.

For really large coils, I find it much easier to use plastic-covered multi-strand wire than solid conductor wire.  I have also joined together the inner and outer conductors of surplus 'end-of-reel' lengths of coaxial cable to wind high Q inductors.  But, you don't get many turns per centimetre with coax, so you need very long lengths of coax to make a coil of appreciable inductance!

All the coil details provided below are included to provide food for thought.  But you don't really need to make such big coils.  In most circumstances, smaller coils (with thinner wire) will result in just as many QSOs!

Coil losses
At the end of the day, the losses in your coil are likely to a lot less than the losses in other parts of your antenna system.  Aim to get the DC resistance of your loading coil below 15 ohms: if you get below 5 ohms, then you're doing really well, but you're unlikely to notice any improvement in overall performance.

Although the Q of the coil is related (among other things) to the ratio of the coil length to the coil diameter, the stability of the coil construction is far more important.  In practice, you will have to use those materials that are convenient to use, and readily available - at the right price.  So don't worry unduly about the overall coil dimensions, but do wind single-layer turns - the beginner should not attempt any form of 'pile winding' construction due to the possibility of voltage breakdown between turns.

This is not to say that you cannot try a 'Russian doll' approach, with three or four nested formers, as a way of conserving space.  This will work fine if you maintain an adequate spacing between each former.  Indeed, this is exactly what I did in my Mark II loading coil (see below).

In use, assuming a vertically-orientated coil, aim to elevate the cold end of the loading coil about 300 mm above the ground.

Tapping points
The tension in the completed winding means that providing taps after the coil has been contructed is not easy.  It is far better to consider how and where the taps will be provided at the outset.  If you plan to try lots of different antenna configurations then, ideally, your variometer will have an inductance swing that, together with the tapping points, will allow any given inductance to be selected within a range of, say, 3 to 8 mH.  The exact amount of inductance required to series-tune the antenna will depend upon the antenna's capacitance.   It is indeed a fortunate experimenter who has an antenna big enough that it needs less than 3 mH inductance to achieve resonance!

Construction tips

Before winding your first LF coil, you must determine your preferred direction for winding the coil - and stick to it!  There's no point in winding some series-connected coils where one of them is in anti-phase to the rest.

When using a former that does not have parallel sides (such as most buckets), start the winding from the narrow end.  Be sure to anchor the first turn very well - it is well worth the effort.  The picture in the centre shows how cable ties can be used to hold the first turn in position.  For a wire of 2 mm diameter, a spacing between cable ties of 80 mm would be appropriate.  For thick wire or coax (as in the case shown below), a spacing of 150 mm should be adequate.

To help anchor the wire, I usually pass the start and end of the coil (plus any tapping points) through a small hole in the coil former.  I will then drill another hole alongside the first if I need to have the wire exit the coil former at that point.

For wire under 2 mm diameter, another good method of fixing the first turn is to drill pairs of holes around the circumference so that, you can weave the wire into the former and out again every so often.  A hole spacing of 10 mm every 70 mm works well.   Again, it is especially important to maintain tension in the wire for the first turn.  The last thing you want to happen is have the turns to start spilling off the end of the former like a child's 'slinky' toy!

The completed coil is shown on the right (it was eventually used as I2 in my Mark II loading coil described below).  Note how all the turns are side-by-side (or, 'contiguous', as we used to say in the days of wire-wrapping technology) to maintain stability.  Note how the ends of the coil are passed into the coil former to help secure the first and last turns. 

Mark I Loading Coil
This loading coil was dismantled in April 2001 after providing over two years service. 

The former for this 6 mH loading coil was made using two plywood end-pieces from a cable drum, joined together using seven lengths of  1 m  x  22 mm broomstick handle.   This resulted in a (mainly) air-cored coil former, having a diameter of 400 mm.

The picture below (left) shows the general arrangement.  Details of the coil information is shown to the right (number of turns/winding length in mm).  The turns were held in place using a combination of cable ties and hot melt glue.  The DC resistance of the loading coil was 3.4 ohms.  (The resistance of the 1.6 - 2.4 mH variometer was 1.8 ohms)

Loading coil (shown with the main variometer on top)

Prior to the legs being mounted underneath, this loading coil was originally mounted directly on the patio slabs, with the variometer connected at the earthy side of the loading coil.  This worked fine until it started raining one day (while working EI0CF).  The voltage developed across the 2 mH variometer coil was enough to cause the damp wood to ignite when the insulation on the lower turns of thick coax broke down.  Fortunately, the SWR meter at the shack end of the feeder alerted me to the problem, and no serious damage resulted.

The picture below (at left) shows the resulting combustion products.  To prevent this occuring again, a number of steps were taken, including the addition of three legs at the base of the coil; re-location of the main variometer (to the hot end of the loading coil); and the fabrication of a waterproof cover.