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

Not knowing where to get the right capacitors from and at a good price I am
thinking more on the lines of making some coaxial traps for the time being
and see how I get on. What put me off of the coaxial traps is the size. I
did just find this article on another type of trap.
http://www.arrl.org/tis/info/pdf/9207035.pdf

It doesn't make it clear how you join the inner and outer coil together and
if they are wound in the same or opposite direction. What do you think is
the correct way to wind and join them are?

Is it possible to make all the traps on 40mm PVC pipe or do they have to be
a certain size diameter for each band?

Thanks


"JN" wrote in message
...
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

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

"Jim-NN7K" . wrote in message
...
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


25.4 pf per ft - That's the capacitance of Belden 9907 RG-58a coax. I got
the info he
http://www.trinetcn.com/images/spec_...en/bel9907.pdf
All cable manufacturers publish the capacitance per ft of their coax.
With that said, I've heard of folks using that property of coax along with
a form to wind their own traps using RG-58.
I would think that since we use coax all the time for transmitting, it may
be able to withstand the electrical stress of working as a cap.
Oh - here's an article on the idea: http://degood.org/coaxtrap/
and this one has color illustrations and a calculator :
http://www.seed-solutions.com/gregor...n/CoaxTrap.htm

I don't see the original post, so I am replying here.

Here are some links where antennas are using coax traps.

http://degood.org/coaxtrap/

http://www.arrl.org/qex/Mueller.pdf

http://www.cebik.com/wire/trapqq.html

http://www.smeter.net/antennas/coaxtrap.php


--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

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

"Bryan" wrote in
:

....
cutters. Most coaxial cables exhibit a capacitance of about 30pF/ft,

50 ohm cables with solid PE dielectric ALL exhibit a capacitance of about
30pF/ft. It is not a coincidence, C/unit length, Zo and velocity factor are
all related.

That method of estimating the capacitive reactance of an o/c stub (for that
is what you actually have) is only accurate for very short stubs.

Capacitors fabricated from coaxial transmission line are often much lossier
that quality fixed capacitors, so they should be used with care.

Owen

Barrett wrote:
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

Hi Barrett,

I once fabricated just such a trap dipole. I started by cutting a dipole
for 40m w/o traps.

You'll need to adjust the resonant frequency of the traps to the 40 meter
band, so that the antenna wire on the outer ends of the traps is
electrically "cut off". Physically, this is not very easy to do with fixed
value components. I used stubs of coaxial cable as capacitors across the
inductors, and tuned them by trimming the ends with wire cutters. Most
coaxial cables exhibit a capacitance of about 30pF/ft, and I believe mine
were around 18" long... about 45pF (498 ohms reactance at 7 MHz). At 7.1
MHz, an inductance of 11.15uH resonates with 45pF. Any combination of LC
that yields resonance on 40m is acceptable but, the more L you have, the
shorter the overall length will need to be in order to resonate on 80m. To
a point, larger wire size and turn spacing for the inductor will yield
better Q.

Unless you're running very high power, it really doesn't matter much what
type of coax to use. The concern is voltage rating as, when operating on 40
meters, there will be higher voltage across the "capacitor". Larger
diameter coax will have a greater voltage rating. Solid polyethylene coax
will have a significantly higher rating than foamed polyethylene. From
Belden...

Type: RG8/U
Part No: 8214
Dielectric: FPE (foamed polyethylene)
Max RMS: 300V
Capacitance: 26.0/ft

Type: RG8/U
Part No: 8237
Dielectric: PE (solid polyethylene)
Max RMS: 3700V
Capacitance: 29.5/ft

Type: RG58A/U
Part No: 8219
Dielectric: FPE (foamed polyethylene)
Max RMS: 300V
Capacitance: 26.5/ft

Type: RG58A/U
Part No: 8259
Dielectric: PE (solid polyethylene)
Max RMS: 1400V
Capacitance: 30.8/ft

Even using solid PE coax, I had issues with arcing over the ends of the
stubs when running a kilowatt (it was OK with only 100W.) I alleviated that
problem by trimming only the outer jacket and shield, leaving a short stub
of center conductor and dielectric protruding. When done, seal the coax
stub with non-corrosive RTV, etc.

I noticed that others made reference to the voltage rating of various
capacitors. When AC is applied across a capacitor, a certain amount of
current will flow thru it. Capacitors also have a rating in terms of
current as well as voltage. While a TV doorknob capacitor has a voltage
rating in tens of kilovolts, it won't handle much current. This is why you
don't see them used (for example) as plate blocking capacitors in RF
amplifiers, even though the DC voltage across them is much less than their
rating. Where I work, we use lots of multilayer porcelain capacitors in
industrial RF power sources (for CO2 lasers), mostly from American Technical
Ceramics (http://www.atceramics.com/). Their datasheets list the current
rating vs capacitance vs frequency.

73,
Bryan WA7PRC

One last question. Is the diameter of the traps at different frequencies
that important and why?

They tend to be larger towards top band.

Thanks


"Owen Duffy" wrote in message
...
"Bryan" wrote in
:

...
cutters. Most coaxial cables exhibit a capacitance of about 30pF/ft,

50 ohm cables with solid PE dielectric ALL exhibit a capacitance of about
30pF/ft. It is not a coincidence, C/unit length, Zo and velocity factor
are
all related.

That method of estimating the capacitive reactance of an o/c stub (for
that
is what you actually have) is only accurate for very short stubs.

Capacitors fabricated from coaxial transmission line are often much
lossier
that quality fixed capacitors, so they should be used with care.

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