At 11:49 AM 8/13/2003 -0400, Jim White, K4OJ wrote:
>OOoooh OOoooh - me too me too
>I also am working on a project... I am going to be needing some big
>caps... please also include in your reply to the group a good source for
>the following capacitors:
>...sort of dove tails in with N2EA's needs.
>Sorry to break your pileup, Jim
At what frequency? What sort of RF current capability and voltage rating do
you need? How much loss are you willing to tolerate? How stable does the
capacitance have to be?
For instance, will standard transmitting doorknob type work? Granted, they
only come in standard values (usual 100, 220, 470... kind of sequence). RF
Parts in San Marcos, CA carries this sort of thing (and there are copious
Ebay sources of widely varying reliability and quality)
Relay wise, the owner of MaxGain Systems (www.mgs4u.com ?) has vacuum
relays at a reasonable price, but I don't recall the voltage ratings.
To a first order, the voltage rating is going to be determined by the
spacing of the contacts when open. Figure something around 15 kV/cm (35
kV/inch) as a starting point, subject to test. Most relays are kind of
ugly for HV use... wide thin straps as blades are great from an inductance
and structural standpoint, but horrible from a corona off the edges
Watch your creepage distances, too.. Usual rule of thumb is to assume you
need 3x the free sparkover distance along any surface (why HV insulators
You can get remarkably high voltages with fairly low powers in a High Q
kind of system. Any physically small antenna (relative to wavelength) is
probably going to be in this category. Say you're putting 100 Watts into
your 1300 pF cap at 3.5 MHz.. that's around 35 ohms impedance, so the *RMS*
voltage is about 60V, with no resonant rise effects. However, if the Q of
your system is 100 (bandwidth of 35 kHz), then that voltage could get up to
6kV... 8.5kV peak!
Start running 1.5 kW, and you're looking at voltages on the order of 30-35 kV.