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Homebrew traps
What sort of capacitors can I use in parallel in making some 7MHz and
3.7MHz wire wound traps? What voltage should they be and is the value that important? Max power will be 200 watts. Thanks |
Homebrew traps
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 |
Homebrew traps
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 |
Homebrew traps
"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 |
Homebrew traps
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 |
Homebrew traps
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 |
Homebrew traps
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 |
Homebrew traps
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 |
Homebrew traps
"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? Some ceramic capacitors have the value change with the instantaneous applied voltage. I know somebody who tried to use one in a tuned circuit - complete disaster. Do not use the ones that are meant for power supply bypassing. Tam/WB2TT 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 |
Homebrew traps
If what you are trying to do is a dipole type antenna for 40m and 80m,
you do not need any capasitors at all. You can use choke-coils as in this design but leave the 20m and 15m dipoles out. http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=13422 73 Jouko OH5RM |
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