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
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!
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
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.
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!
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!