Topband: how high are very high voltages on antennas?

[Grant Saviers]

It is often mentioned that "very high RF voltages" are present at ends 
of dipoles, voltage nodes on loops, etc. but I haven't been able to find 
any guidance in the form of numbers.  How high is "very high"?  Assuming 
QRO power into a full size apex up delta loop, corner fed, vertically 
polarized, what will be the voltage at the apex?  I haven't found a way 
as yet to have EZNEC give me a value.

If I wanted to put a relay at a voltage point to switch a stub, how 
would I float the 12v DC coil from the RF present?

Grant KZ1W