[Fwd: LF: Properties of PVC.]
Wed, 28 Jan 1998 09:25:18 -0500
Please note the remark regarding "at least one" instance of sky-wave reception
of 137KHz transmission.
There is hope yet
> Re comments on PVC Wire -
> The dielectric properties won't matter too much at 73 / 137kHz as far as
> absorbtion and losses is concerned. What does affect its usefulness for
> loading coils is the increase in capacitance due to the permitivity of the
> insulation, and the fact that the turns are necessarily spaced apart by the
> thickness of the covering.
> Any additional distributed capacitance is very undesirable in a loading coil
> as it leads to circulating currents which add extra losses. The turns
> spacing apart is also serious since inductance is (very roughly !)
> proportional to the inverse of the coil length for a given number of turns
> leading to increased size and more losses.
> If PVC wire is as available and cheap as you say (it certainly isn't in the
> UK) and you have the space available, you could try a large loop. Make it
> of multiple parallel strands - a sort of homemade heavy duty Litz wire, say
> 10 - 20 strands - with a loop area of a few 10s of square metres this will
> approach or even exceed the efficiency possible with the same total amount
> of copper used in a Tee antenna plus loading coil. There is not a great
> deal to be gained in using wire of a diameter greater than 0.5mm as the skin
> depth at 73kHz is 0.2mm and at 137kHz reduces to just over 0.1mm. More
> turns of thinner wire is a better route to choose than thicker wire.
> However, I still maintain that if antenna size is restricted by a small
> house / garden and massive high masts are not possible, wire antennas have
> the edge over loops. There must be a break even point somewhere and my gut
> feeling is that this is somewhere in the region of a garden 30 - 40m long
> and a maximum height of 10 - 15m. I certainly couldn't back these figures
> by any more than a few back of envelope calculations. Furthermore, this
> breakpoint will be different as the frequency band is changed.
> HOWEVER - Since we have already demonstrated some skywave reception on
> 137kHz and a possible report of it on 73kHz, there may be other advantages
> to loops as these have a significant mid to high angle component to the
> radiation pattern that vertical wire antennas are less likely to generate.
> G2AJV uses a very large loop of mains cable - something like 100m^2 I
> believe - and puts out a signal comparable to the medium sized 'electric'
> Andy G4JNT