Some one sent me an email and said these will do the job. Ceramic capacitor
disk 10KV/1000pF. Do you think these would work?

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.


Thanks


"Cecil Moore" wrote in message
t...
Barrett wrote:
What sort of capacitors can I use in parallel in making some 7MHz and
3.7MHz wire wound traps?

I've had good luck with door-knob caps.
--
73, Cecil http://www.w5dxp.com

"Barrett" wrote in message
. uk...
Some one sent me an email and said these will do the job. Ceramic
capacitor disk 10KV/1000pF. Do you think these would work?

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.

No. For an 80/40 meter trapped dipole, the traps will need a capacitor close
to 60 pF. 1000 pF will not be suitable.

You might look at the traps made of coaxial cable where the inductance is
tuned to resonance with the capacity of the cable length itself. No easier
to construct, and heavier than the wire wound/capacitor trap, but eminently
easier to obtain in the materials department.

Info available from Google.

W4ZCB

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

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

Barrett wrote:
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?

Articles on capacitors for traps specify "transmitting"
type caps meaning high voltage ratings. I don't know
exactly the voltage rating required but it is in the
thousands of volts. That's why I used door-knob caps.

You can put capacitors in series to increase the
voltage rating but you also decrease the capacitance
and the voltages across each capacitor needs to be
equalized using resistors.
--
73, Cecil http://www.w5dxp.com

On Fri, 28 Mar 2008 13:47:11 GMT, "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?

Hi Barrett,

A lot of this flies on the wings of presumptions. One, is that you
have an 80M dipole being trapped for 40M. If so, then two of them in
series for each trap will work just fine with an inductance of roughly
20 microhenries.

No, you don't need expensive 6KV caps unless you put the wrong
frequency to the antenna - and by wrong, I mean wrong band, not just
slightly up or down band.

The voltage across these traps (with all presumptions being observed)
will vary from 400V to 550V across the 40M band for an ideal antenna
in free space. You probably live on earth, so presumptions are
already beginning to shift.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
. . .
No, you don't need expensive 6KV caps unless you put the wrong
frequency to the antenna - and by wrong, I mean wrong band, not just
slightly up or down band.

The voltage across these traps (with all presumptions being observed)
will vary from 400V to 550V across the 40M band for an ideal antenna
in free space. You probably live on earth, so presumptions are
already beginning to shift.

There are quite a few variables involved in determining how much voltage
the traps will see, not the least of which is the L/C ratio of the trap.
I dug out a model of a 40/20 meter trapped dipole using traps using
RG-58 I did some time ago, and found the trap voltage to be 568 volts
RMS with 100 watts at 14.0 MHz. That's 800 volts peak. A model of a
40/20 meter trapped dipole with trap component X = 1000 ohms at
resonance and moderate inductor Q showed highest voltage of 648 RMS
(over 900 volts peak) at 7 MHz. So I'd want to use capacitors with a 2
kV rating to provide some margin.

Roy Lewallen, W7EL

Roy Lewallen wrote in news:13uqriie50k822
@corp.supernews.com:

....
I dug out a model of a 40/20 meter trapped dipole using traps using
RG-58 I did some time ago, and found the trap voltage to be 568 volts
RMS with 100 watts at 14.0 MHz. That's 800 volts peak. A model of a
40/20 meter trapped dipole with trap component X = 1000 ohms at
resonance and moderate inductor Q showed highest voltage of 648 RMS
(over 900 volts peak) at 7 MHz. So I'd want to use capacitors with a 2
kV rating to provide some margin.

I have done some mathematical modelling of so called coax traps with the
bootstrap connection (or the Hi Z connection as used in ARRL pubs), and
the work has halted needing some reconciliation with quality measurements
of inductor Q and trap impedance. The problem relates to estimating the
equivalent resistance in an inductor made from coax braid and covered in
the PVC jacket.

I see W8JI reports measurement of a coax trap at 7Mhz with about 18k ohms
at resonance, I have some data from DG1MFT who measured a trap around
7MHz on a R&S ZVRE VNA at somewhere around 22k ohms at resonance. I have
measured some prototypes at over 15k ohms using the TAPR VNA, but it is
not in the same class as quality instruments. In conclusion, I suspect
that it is likely that coax traps have impedance well over 10k ohms at
resonance.

One way to reduce the voltage impressed on the trap is to design the
system so that the trap is not close to resonance at any operating
frequency.

Such a design means moving beyond the simplistic explanation that such
antennas depend on traps acting like an on/off switch at the resonant
frequency, and that when they are resonant, the outboard conductors do
not exist.

Owen

Owen Duffy wrote:
Such a design means moving beyond the simplistic explanation that such
antennas depend on traps acting like an on/off switch at the resonant
frequency, and that when they are resonant, the outboard conductors do
not exist.

The ARRL Antenna Book says the above is the "amateur"
version of how traps work. :-) They say "commercial"
versions use non-resonant traps.
--
73, Cecil http://www.w5dxp.com

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