In , Lancer
wrote:

On Mon, 14 Jun 2004 05:20:37 -0700, Frank Gilliland
wrote:

In p7jpc05ugaf983cn5th32r7rd99cgvbh71@2355323778, Lancer
wrote:

snip
Wrong, the coil has very little radiation. The open air coil has less
loss because the losses are less in an open air coil.
(resistive losses, capacitive coupling losses, form loss)
Do a search on coil Q

Care to argue that point?


I would. What happens when you bring a solid sheet of conductive
material close to the end of a coil? Eddy currents -- it has
tremendous losses, the Q drops like a rock, and it's inductance is
unpredictable. It's a royal bitch to design a shielded IF/RF coil or
transformer to be used for high frequencies, and shielding is almost
-never- used for power RF coils and transformers unless there is some
serious space between the inductor and shield, hence the popularity of
toroid cores for those applications. And since a bigger coil makes a
bigger field, you need much more 'free space' to maintain a high-Q.
Vertically mounting a big coil above the sheet metal of a vehicle
results in a very lossy coil.


I would think that 4 feet would be more than enough distance.


It would depend on the diameter of the coil. I remember there is an
equation somewhere to determine the loss, but it involves some heavy
calculus and it's too early in the morning for integrals.


So you would say that a large air wound coil has more loss than a
small diameter coild wound on a solid form?


No, not necessarily. If the wire diameter is the same, the smaller
coil might easily be more efficient due to lower DC resistance.
Distributed capacitance isn't much of an issue at 27 MHz unless you
have lots of close-wound turns, and even then it's not necessarily a
loss but a factor that affects the coil's reactance. Regardless, there
are very effective winding techniques to reduce that problem. There
are many low-loss coil form materials available, such as Teflon, some
of the better quality ceramics, and even grooved hardwood makes a
decent core at RF frequencies (however, PVC sucks above a couple
hundred kHz). There is also the issue of weather conditions that can
significantly affect an open coil regardless of whether or not the
wire is insulated -- rain/humidity, snow, air pollutants (ozone can
make a -=BIG=- difference!), etc. Even the speed of the vehicle can
have an effect on the properties of an open coil. For the sake of
consistency and longevity, a smaller coil that is sealed from the
elements (as a whole) is a far better choice for anyone serious about
the issue.






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