[Fwd: LF: Re: [Lowfer] Question about coil wire]
Wed, 28 Jan 1998 12:34:58 -0500
FYI, interesting stuff on 180 KHz antenna coils
Lyle Koehler wrote:
> Johan Bodin wrote:
> > Hello LFers
> > PVC has quite bad dielectric properties at higher frequencies,
> > but does it matter in a coil for 73 or 136 kHz?
> Actually the dissipation factor may be higher at 100 kHz than it is at
> 10 MHz. From an old copy of Reference Data for Radio Engineers, the
> dissipation factor of PVC is given as 0.093 at 1 kHz and 0.055 at 1 MHz.
> The dielectric constant is shown as 4.8 at 1 kHz and 3.5 at 1 MHz. There
> can be quite a difference in loss and dielectric constant for various
> compositions of PVC, but none of the numbers are very encouraging.
> > Has anyone investigated the loss of a loading coil made of
> > PVC insulated wire? Is there a considerable difference in
> > loss compared to a similar coil made with the same wire
> > area but without the PVC?
> Brian Beezley, K6STI has a program called COIL.EXE that lets you specify
> the coil form material. This isn't quite the same as having the wires
> surrounded by the dielectric material, but it gives an idea of what
> happens with a lossy dielectric. The loading coil for my 186.75 kHz
> LowFER beacon is made from #14 (1.63 mm) insulated house wire in a
> basket-wound, air-supported configuration. The coil has about 75 turns
> and is 18 inches diameter by 9 inches long. For a straight air-supported
> solenoid coil with these dimensions, COIL.EXE predicts an effective
> inductance of 2.93 mH and a Q = 748 at 190 kHz, with a distributed
> capacitance of 23 pF. The same coil on a PVC form would have L = 3.22
> mH, Q = 322 and distributed C = 44.6 pF. I measured a Q of over 400 on
> my basket-wound coil. The basket-wound configuration is probably worth
> the extra effort when using wire with lossy insulation, because I would
> expect a coil wound with PVC-insulated wire to have a lower Q than a
> coil on a PVC form.
> > PVC insulated wire for mains wiring in houses is very cheap
> > in Sweden. The most common variety has 1.5 mm2 cupper
> > area. Outside diameter is about 3 mm while the cupper dia.
> > is about 1.5 mm.
> The same thing is true in the US. 500 feet of #14 bare copper wire costs
> $35 (plus shipping), but I can get insulated #14 at the discount
> building-supply store for about $15. That's why I use it, even though it
> isn't ideal for low-frequency loading coils. Besides, if I used bare (or
> enamelled) wire, I would have to find some kind of large, low-loss form,
> and wound have to figure out how to wind it with the proper spacing
> between turns. (Best Q for a given wire size is usually obtained with a
> spacing between turns of twice the wire diameter.) A basket-wound coil
> of any size can be constructed by putting an odd number of dowels into
> holes into a plywood board. The PVC insulation provides the right wire
> spacing so it isn't too difficult to wind. After "weaving" the coil
> lacing the crossovers with something like Dacron kite string, and doping
> the lacings with silicone bathtub sealer, you can pull the whole thing
> off the dowels and have a reasonably efficient loading coil at low cost.
> One more note: Farm supply stores often carry 3/8 inch diameter, 4 foot
> long fiberglass electric fence posts that make nice slippery dowels for
> the coil-winding operation. Probably they could be left in place to help
> keep the coil rigid, without too much degradation in Q.
> Lyle, K0LR