[HBR] PVC coil forms for the W6TC HBR

[Tim Shoppa]

W6HHT writes:
> Does anyone on the reflector have first-hand experience with, or direct 
> knowledge of, the performance of PVC tubing with a typical 0.066-in wall 
> thickness in an HF receiver front end application as typified by the 
> Crosby HBR?  Floating around in various places on the net there is some 
> vague commentary about general unsuitability of PVC for coil forms due 
> to "excessive losses".  No specifics are cited, nor linkage to sources, 
> and as a result it is then impossible for me to decide if the commentary 
> is rumor, speculation, or has some measure of substance.

PVC does have comparatively high losses... as compared to say air or vacuum.

This matters a lot if you have a very high-Q tank with multiple kilowatts
flowing around, or a trap in an antenna that you hope will take a killowatt
continuous.

Doesn't matter much in a receiver.

I suspect any slight decrease in Q that comes from using a PVC form as
opposed to the styrene forms is because the PVC tends to be thicker.
I despise styrene forms largely from my frustration when I was a kid
trying to machine and solder to pins stuck in it.

> The reason for the query is my intent to move forward (or perhaps not) 
> with some experimental work on HBR coil forms using PVC NMT (non 
> metallic electrical tubing) and PVC Schedule 40 pipe parts.  I have a 
> preliminary physical design using a standard PVC pipe connector, plus a 
> 5-pin connector customized from a PVC pipe cap, and a coil winding 
> surface using the NMT.  Stability of the physical design looks 
> promising, but I would prefer not to waste time on coil experiments if 
> there is significant truth to the "PVC excessive loss" story.

I have been having good luck with phenolic and glass-epoxy tube in
making custom HBR coils. I used to have a local plastic supply house
that had a retail section with remainders and cutoffs, but they're
not nearly as good as they used to be. Most of my stuff comes from
McMaster-Carr ( http://www.mcmaster.com/ )  recently. But the reason
I use those instead of PVC is the superior machinability of phenolic/
glass-epoxy, the fact that it doesn't melt (and many grades have
flame-suppressing properties) and availability of a wide range of
sizes, no electrical reason.

> My in-house references, texts and ARRL handbooks do not shed useful 
> light on the subject.

You might look at books and articles about antenna traps if you want
to know what happens to lossy coil forms at high power. A couple
have very good pictures of the spectacular results - I remember a QST
or QEX article in the past year or two that actually shows the trap on fire!

Tim.