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relays and capacitors Re: [TowerTalk] relays

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Subject: relays and capacitors Re: [TowerTalk] relays
From: (Jim Lux)
Date: Wed Aug 13 12:21:59 2003
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:
>1361 pF
>1640 pF
>...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 ( ?) 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 
are corrugated)

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.

Jim, W6RMK 

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