Topband: Shunt Feed Questions

[kb9cry@attbi.com (Phil - KB9CRY)]

I'm planning on using the design shown in K1ZM's book to shunt feed my 
tower for 160M.
Two questions:
1.    He specifies using vacuum variable capacitors for the matching.  
Why those types?  Can I use air variables?
2.    He shows weatherproofing the matching stuff in a rubber (non 
metallic) enclosure.  What if I use a metal box instead?  Will the 
capacitors be affected by the metal enclosure?

73  Phil  KB9CRY