I think Tom means one wire diameter between turns.
73 Al W6DZ
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <w7ti@ispwest.com>; <topband@contesting.com>
Sent: Thursday, May 08, 2003 10:39 AM
Subject: Re: Topband: 160 Meters Mobile Antenna
| > >PVC and PBT and other such plumbing pipe plastics are very
| > >lossy for RF.
| >
| > The traditional test for lossy plastic is to put a section of about
| > six inches in the microwave for 20 seconds or so and see if it gets
| > hot under full power.
|
| The largest effect of dielectrics (insulation) on 160-meter mobile loading
| inductors is the insulation noticeably increases unwanted stray
capacitance
| between turns and across the coil. Any increase in capacitance shunting
the
| coil decreases Q of the system, and can even decrease bandwidth at the
same
| time efficiency and Q is reduced! That's why high inductance loading coils
| work better with turns spaced one turn diameter with only air between the
| turns, and are optimized when somewhat longer with a smaller diameter. All
| of this minimizes capacitance across the coil.
|
| The dielectric constant, and this is something that does not show as heat
in
| a microwave test, is generally more important than dissipation factor. The
| higher the dielectric constant, the more capacitance is added.
|
| For mechanical support nearly any material (even materials unacceptable in
| capacitors, like fiberglass, Delrin, and PVC) can be used that is
| mechanically strong enough, as long as air is in the area immediately up
| against the coil turns.
|
| Capacitors and transmission lines are another matter, of course.
Dissipation
| factor is critical in them. Fiberglass or PVC would be very lossy in an HF
| capacitor, even if a microwave showed little or no heat.
|
| 73 Tom
|
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