>> The first step in winding a coil is to select a coil form.    
>> The coil form should be a low loss material (we are talking   
>> RF losses) like polyethylene, polystyrene, or polypropylene,
>> Lexan, or Plexiglas (acrylic): but a common material is PVC,
>> which is high loss. Thin wall tubing is best regardless of 
>> material.

 CA> If one is using a "good" material such as HDPE, is the wall  
 CA> thickness as critical?  

The thinnest wall possible is best, regardless of material.
Obviously the losses are going to be higher with thin wall 
PVC pipe than HDPE pipe that is slightly thicker. 

 CA> Am I better off just using very thin wall PVC and sealing?
 CA> PVC is considerably cheaper.

Yes but prep time may be a factor to some. No exceptional prep
is required if HDPE is used as a coil form. PVC must be sanded
smooth; and must be very, very, dry (even gently baked to remove
residual water vapor); before applying several coats of a high Q
sealer to the bare form.

By keeping a PVC coil form thin, and throughly drying and sealing
the bare plastic before winding, we can approach the Q factor of
the sealant. This method prevents the windings from coming in
contact with the PVC at any point. The moisture content of the
plastic beneath under the sealant is very low if properly dried,
and will not reabsorb moisture if properly sealed. Using this
preparation technique followed up by sealing the completed coil
(top and bottom capped hermetically), and not allowing the wire
to enter the coil at any point (no holes drilled); you will end
up with a very high Q secondary construction of exceptional
electrical strength. It will produce sparks four or more times
the actual winding length without breaking down if the proper
discharger is used. This is pretty good considering PVC is so
lossy and electrically weak.

The reasoning behind keeping ANY coil form as thin as possible to
to keep the distributed capacity as low as possible.

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12