Barrett wrote:
I have seen some silver mica capacitors for sale @ 56pF they are, Capacitor
rad silver mica 56pf 400v 1%10mm 273 itt.
I was told that the voltage had to be higher than 6KV and those are only
400V. What is the Voltage that I can use and why?
Thanks
"Cecil Moore" wrote in message
...
Barrett wrote:
Some one sent me an email and said these will do the job. Ceramic
capacitor disk 10KV/1000pF. Do you think these would work?
I don't know about those particular ones. Some ceramic
capacitors are meant to be DC biased for bypass service.
I once tried to use disc ceramic caps across my ladder-
line. They caught on fire because they were not rated for
AC service and lit up the night sky.
Someone told me the ceramic insulator was actually
trying to physically vibrate at the RF frequency and
friction heated it up. I don't know if your capacitors
are rated for AC operation.
A friend of mine used silver mica capacitors for his
traps and they worked fine.
This is my first attempt at making traps and I do not fully understand
how they work. I am willing to learn as this is what makes the hobby
great but I cannot find enough step by step info to lead me in the right
direction.
-------trap----------FP----------trap-------
An ideal trap is parallel resonant with a high Q. That
gives it a very high impedance at the resonant frequency
and acts somewhat like an open circuit at the resonant
frequency blocking the flow of current to the outer
parts of the dipole. At the resonant frequency, the
inductive reactance and capacitive reactance are equal.
A Grid Dip Meter will indicate the resonant frequency.
At 1/2 that resonant frequency, the inductive reactance
goes down and the capacitive reactance goes up so the
"trap" acts like a loading coil instead of an open
circuit on the lower frequency.
Traps can be made out of coax if you are interested.
The coiled outer shield provides the inductance and
the capacitance is provided by the center-wire-to-
shield capacitance.
Kraus talks about self-resonant traps with no physical
capacitors. The inter-winding capacitance of the coil
wire supplies the necessary capacitance.
--
73, Cecil http://www.w5dxp.com
NOT to mention, that at the HIGHER frequency, the trap is at
the HIGH VOLTAGE point of the antenna at that resonant frequency
! At 50 ohms, impedence, at the feedpoint , the Maximum Voltage for a
100 watt transmitterwould be 50 volts at 2 Amps, but at the HIGH volatge
points (1/4 wave each side of a dipole), there can be literally
THOUSANDS of volts, at (admittedly) very low current . And, caps are
rated at Breakdown Voltage! This is why (unless you use very low
power),that you need these high voltage Capacitors . Hope this helps
explain.
Jim NN7K