Yes; excellent practical suggestion Peter...this will work well and the
Duroid substrate is easy to guillotine to precise dimensions to get the
readily calculable target C values.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of peter chadwick
Sent: Tuesday, 29 April 2014 5:45 PM
To: amps@contesting.com
Subject: [Amps] Low Pass Filter capacitor failures
Rogers 'Duroid' Teflon PCB material is very good for making suitable
capacitors, and you can actually calculate quite easily the size and loss.
It has an advantage over sheet Teflon and aluminium that there aren't any
airgaps and it doesn't need clamping together, which can lead to 'creep' of
the Teflon. At 2 metres, the skin depth is 0.00022 inch and so 1 ounce
copper will be about 6.5 skin depths, which should be adequate: 2 ounce
copper might offer a measurable improvement, but I doubt it. The only point
to watch then is the expected current density in the metal and the loss in
the dielectric, which you can calculate from the loss tangent.
Spice will give you answers on voltages and currents, but if you don't have
Spice, it's not that hard to calculate, especially with a programmable
calculator, let alone a computer.
Jim said the band is 144 to 148 MHz: not in ITU Region 1. Where the OP
lives, it is 144 to 146MHz. One needs to define what is meant by cut off
frequency, too.
73
Peter G3RZP
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