GW4ALG's 136 kHz Pages

[ GW4ALG went QRT in February 2007 ]

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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


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:


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!

Anchoring the first turn                                    Use of a cable tie                                      The completed coil (I2 in the Mark II loading coil)


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)

Mk 1 Loading Coil    Mk 1 Coil Information











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.


Base of the loading coil (note the charred wood!)

The burnt end!

Loading coil (covered with waterproof jacket)

Loading Coil in Jacket

Problems with the Mark I version
This tall loading coil, with its high centre of gravity meant that it tended to be blown-over on windy days!

In fact, to work the GU expedition station on 25th November 2000 (operated by G0MRF; G3XTZ; and G3YXM), I needed to use guy ropes to prevent the loading coil from being blown over in the high winds of that day

.The guyed Mk 1 loading coil

But the main reason for retiring this coil was due to the collapse of the coil former.   The tension of the winding eventually caused the 22 mm sticks to distort, and allow some of the turns to work their way down the former, and overlap other turns.   If this had continued, the slippage would have eventually resulted in arcing between turns. 

It had seen better days . . .


This problem would not have occurred if I had made a more stable structure at the outset.   Nevertheless, the coil did provide good service for over 2 years, and enabled me to work 16 countries, and 100 different stations - all on two-way 136 kHz CW. 



Mark II Loading Coil

I built this loading coil to replace my original loading coil described above.  In use, this new monster loading coil is positioned on top of my remote-controlled variometer to provide fine tuning of the antenna from the operating position in the shack.  It has a mass of 18 kg, and a DC resistance of 2.6 ohms, as measured from the start of  winding 'LA' to the junction of 'UD' and 'UE'  (the tap used for the 12 m vertical).

The lower part of the assembly features a built-in variometer and two 'nested' coils made of coaxial cable (with the inner and outer conductors of the coaxial cable connected together).  The nested coils increase the total inductance, and also maintain close coupling into the variometer coil, even when the taps on LA-LC and/or UA-UG are such that little mutual inductance is being contributed by these outer coils.

General arrangement of the Mark II loading coil

Mark II loading coil

The lower outer coils (LA-LC) and upper outer coils (UA-UG) were wound on plastic 'muck buckets' (Type 2, HD-PE) purchased in the UK from one of the branches of 'Countrywide' stores (at a cost of 8 pounds each).  These have a stepped and tapered profile and are very robust.  The bucket is 400 mm tall x 500 mm diameter.  Four holes were drilled in the lip of the bucket so that the two halves could be held together with 6 mm cable ties.

Coil details

Outer Coil Details
Mean diameter = 470 mm

Reference Turns Multi-strand
PVC covered
LA & LB 33 50/0.25 mm 128 mm
LC 14 50/0.25 mm 54 mm
UA 17 50/0.25 mm 65 mm
UB 12 32/0.2 mm 40 mm
UC 15 32/0.2 mm 44 mm
UD 7 32/0.2 mm 23 mm
UE 15 19/0.2 mm 30 mm
UF 21 19/0.2 mm 40 mm
UG 18 19/0.2 mm 34 mm

Inner Coil Details
: 32 turns wound with thick coaxial cable, 290 mm diameter, winding length of 260 mm.

I2: 29 turns wound with URM70 coaxial cable, 250 mm diameter, winding length of 175 mm.

V1 & V2: 25 turns each, wound with multi-strand 32/0.2 mm, 180 mm diameter, winding length of 80 mm with 44 mm spacing (for the axle).

The picture below shows the variometer coil during construction.  Here the balance of the coil is being checked so that the coil will remain in a horizontal position.

Balancing of the variometer


To adjust the balance, some lead was put into a plastic tube and held in position at the open end of the plastic paint kettle using hot melt glue.

Location of balance weight



In the picture to the right, notice how connection to the variometer coils is made via a hole adjacent to the axle.


Location of wire entry hole

Retaining the wires out of harm's way

The picture at left shows how the connections to the variometer are wound loosely around the plastic-covered axle.   In fact one wire is taken clockwise around the axle, and one anticlockwise.   This helps to retain the wire at extreme ends of the rotation, while preventing work-hardening of the conductors.

The picture to the right shows the connections to the extreme ends of V1 and V2.  Because the two conections come together to pass through the hole in the side of the coil, black sleeving has been used to provide additional insulation.  A 'jubilee' clip has been fixed to the axle and hot melt glue liberally applied around the clip to fix the coil to the axle.

Close-up of the variometer and axle

The picture below shows the inside of the upper assembly.  Notice how synthetic 'Polyflex' pipe lagging (9 mm thick) has been used as a spacer to prevent the connections to the taps from getting too close.

The upper bucket


The picture below shows all the lower coils and the variometer mounted in position.  The 22 mm plastic pipe covering the axle between the outer bucket and the variometer coil can also be seen.

The lower bucket

The inside of the lower assembly can be seen in the picture at right.  The control wheel and protractor can be seem.

The lower bucket and control wheel




Portable Loading Coil

Alternative set-up for /P operation (for kite/balloon-supported/fixed antenna)


In Preparation


Portable Loading Coil Coarse Tuning Variometer