Anybody know where I can get some halfways reasonably priced capacitors
for making traps?

I don't need much power capability - 100 watts max.

I looked on rfparts.com and WOW, are they proud of them... can't find
a'one that's suitable for under about 15 bucks EACH ...

Rick wrote:

Anybody know where I can get some halfways reasonably priced capacitors
for making traps?

I don't need much power capability - 100 watts max.

I looked on rfparts.com and WOW, are they proud of them... can't find
a'one that's suitable for under about 15 bucks EACH ...

Hi Rick,

A stub of coaxial cable will work... approximately 30pF/foot for 50 ohm
types... approximately 20pF/foot for 75 ohm types. Aside from the low cost,
the spiffy thing about using a coax stub is that, with a pair of wire
cutters, it makes the trap easily tunable.

The breakdown voltage will be much higher for a cable with a solid
polyethylene dielectric than for the foamed PE cables:
http://www.belden.com/pdfs/Cable101/coaxrefc.pdf. In that datasheet, 8214
(RG8/U w/ FPE dielectric) is good for 300V RMS, while 8237 (RG8/U w/ solid
PE dielectric) is good for 3700V RMS. You might not think your 100W would
need much insulation (70.7V at 50 ohms) but, your trap may *not* be at a 50
ohm point. Hint: to maintain the cable's breakdown voltage, let the center
conductor & dielectric protrude beyond the shield. Otherwise, the
free-hanging end of the stub may arc over. I learned that trick when
building an 80/40m trap dipole... flashed it over a few times (running a
kilowatt) before I noticed the soot!

73,
Bryan WA7PRC

"Bryan" wrote in
:

Rick wrote:

Anybody know where I can get some halfways reasonably priced
capacitors for making traps?

I don't need much power capability - 100 watts max.

I looked on rfparts.com and WOW, are they proud of them... can't find
a'one that's suitable for under about 15 bucks EACH ...

Hi Rick,

A stub of coaxial cable will work... approximately 30pF/foot for 50
ohm types... approximately 20pF/foot for 75 ohm types. Aside from the
low cost, the spiffy thing about using a coax stub is that, with a
pair of wire cutters, it makes the trap easily tunable.
....

Something to keep in mind is that a short open circuit stub of small coax
is a relatively lossy capacitor, and a lower loss capacitor can be made
by cutting the cable in two and paralleling the two pieces.

For example, 2m of RG58C/U has a reactance of about 68 ohms at 10MHz, and
a Q of only 135. 2 parallel pieces of 1.1m length has the same reactance,
but double the Q, but still nowhere near the Q you expect of those
doorknobs.

Owen

On Wed, 04 Apr 2007 21:48:08 -0700, Bryan wrote:

A stub of coaxial cable will work... approximately 30pF/foot for 50 ohm
types... approximately 20pF/foot for 75 ohm types.

Good evening, Bryan.

Thanks, but there are a couple of problems... the antenna will be for 160
through 40 and so two of the traps will be for 80 meters... I forget what
the capacitance will be but I think it was somewhere in the 200 pf range.
Also, the antenna is meant to be portable, and so the traps need to be as
physically small as practical. Also the power will usually be low... as
low as 5 watts, usually no more than 50 and never more than 100 watts...
so I guess I need to make the antenna as efficient as I can... kind of a
contradiction in terms for trapped antennas especially where the traps
need to be physically small.

If I could find the doorknob capacitors for a reasonable price that would
be ideal, but $15 each is way too much.

On the other hand, the ones from RF Parts are all at least 4 KV, and up
from there (40-50 KV). I don't know how to accurately calculate the
voltage rating I'll need (I'll post that as another question under another
topic), but my guess is that for 100 watts max, I don't need 5 KV, and may
not need 1 KV.

Who else besides RF Parts has these things and has a decent website?

Rick wrote:
Bryan wrote:

A stub of coaxial cable will work... approximately 30pF/foot for 50 ohm
types... approximately 20pF/foot for 75 ohm types.

Good evening, Bryan.

Thanks, but there are a couple of problems... the antenna will be for 160
through 40 and so two of the traps will be for 80 meters... I forget what
the capacitance will be but I think it was somewhere in the 200 pf range.
Also, the antenna is meant to be portable, and so the traps need to be as
physically small as practical. Also the power will usually be low... as
low as 5 watts, usually no more than 50 and never more than 100 watts...
so I guess I need to make the antenna as efficient as I can... kind of a
contradiction in terms for trapped antennas especially where the traps
need to be physically small.

If I could find the doorknob capacitors for a reasonable price that would
be ideal, but $15 each is way too much.

On the other hand, the ones from RF Parts are all at least 4 KV, and up
from there (40-50 KV). I don't know how to accurately calculate the
voltage rating I'll need (I'll post that as another question under another
topic), but my guess is that for 100 watts max, I don't need 5 KV, and may
not need 1 KV.

Who else besides RF Parts has these things and has a decent website?

Hi Rick,

As Owen mentioned, a single coaxial stub would likely be physically
unwieldy. IIRC, I used a short stub as a padder across a doorknob capacitor
(tuned to 40m). Certainly, coaxial cable used as a capacitor won't be as
good as a ceramic or porcelain capacitor but, my trap dipole worked fine for
me. Another common idea is to use a coaxial trap. It's worth
investigating, and inexpensive to try.

The company I work for uses a lot of porcelain dielectric capacitors (a VERY
good hi-Q/low ESR dielectric) from American Technical Ceramics
(http://www.atceramics.com/). Look at ATC's "multilayer capacitor" line.
They're physically small and, if chosen properly, will handle higher
voltages and currents.

For ceramic and mica transmitting capacitors, you might try the surplus
vendors. A few a
Fair Radio Sales (http://www.fairradio.com/)
Surplus Sales of Nebraska (http://www.surplussales.com/)
All-Tronics (http://www.alltronics.com/)
Economy Electronics (http://www.economyelectronics.com/)
Ocean State Electronics (http://www.oselectronics.com/)

Another possible resource is FaradNet (http://www.faradnet.com/).

73,
Bryan WA7PRC

On Thu, 05 Apr 2007 20:17:59 -0700, Bryan wrote:

Another common idea is to use a coaxial trap. It's worth
investigating, and inexpensive to try.

Good morning, Bryan.

I have done some reading on coax traps, and consulted with some who, as
the old saying goes, have probably forgotten more than I will ever know
about antennas, and I'm told they tend to be fairly lossy and inefficient.
They are certainly easier and cheaper to build than the coil-and-capacitor
type. The big thing that concerns me about coax traps is that they need to
be fairly small and portable, meaning I'd probably make them out of RG-58,
and I wondered if RG-58 or even RG-8X would handle the voltage.

For ceramic and mica transmitting capacitors, you might try the surplus
vendors.

Thanks. Fair Radio looks like they have a pretty good selection.

Rick wrote:
On Wed, 04 Apr 2007 21:48:08 -0700, Bryan wrote:


A stub of coaxial cable will work... approximately 30pF/foot for 50 ohm
types... approximately 20pF/foot for 75 ohm types.


Good evening, Bryan.

Thanks, but there are a couple of problems... the antenna will be for 160
through 40 and so two of the traps will be for 80 meters... I forget what
the capacitance will be but I think it was somewhere in the 200 pf range.
Also, the antenna is meant to be portable, and so the traps need to be as
physically small as practical. Also the power will usually be low... as
low as 5 watts, usually no more than 50 and never more than 100 watts...
so I guess I need to make the antenna as efficient as I can... kind of a
contradiction in terms for trapped antennas especially where the traps
need to be physically small.

If I could find the doorknob capacitors for a reasonable price that would
be ideal, but $15 each is way too much.

On the other hand, the ones from RF Parts are all at least 4 KV, and up
from there (40-50 KV). I don't know how to accurately calculate the
voltage rating I'll need (I'll post that as another question under another
topic), but my guess is that for 100 watts max, I don't need 5 KV, and may
not need 1 KV.

Who else besides RF Parts has these things and has a decent website?

I've bought a lot of HV capacitors for various applications over the
years, both surplus and new.

RF parts has fairly reasonable prices for new components. The actual
manufacturer Ceramite, is a division of Vishay. TDK also makes this
kind of capacitor. And, for surface mount, there are some high
current/high voltage RF capacitor manufacturers (ATC, Murata Erie,
Dielectric Labs). These things aren't made in huge volumes and they're
fairly expensive, brand new.

You might find surplus or used parts much cheaper (e.g. eBay)..
Something to watch out for.. there are two very different series of
"doorknob" style capacitors. One is used for DC kinds of applications
and can't handle much RF current (you used to see these as filter
capacitors in flyback type power supplies at 15.75 kHz). The other is
designed for RF current. The ones that can handle current cost a LOT more.

And, if you're buying used... The failure/wearout mode with overvoltage
is that the metalization on the ceramic gradually goes away, reducing
the capacitance. The failure/wearout mode with overcurrent is
microcracks in the ceramic dielectric from thermal stresses. The
capacitance tends to be fairly constant (these aren't high precision
devices anyway.. 20% is a close tolerance part), but they'll have a C
that varies with voltage and temperature, and the breakdown voltage will
decrease ("treeing in the dielectric").

Even the RF devices aren't intended for use in a tuned circuit.. they're
used for DC blocks and RF bypassing, where the capacitance isn't
critical. They often have a fairly large temperature coefficient, as well.

What you might want to look into is capacitors with very low ESR made
for power converters. The Cornell Dubilier 942P or Wima FKP1 series,
for instance. They use polypropylene dielectric with extended foil
construction and 0.15 uF 2kV parts run a couple bucks. They're widely
used in 1MHz power applications (and can be run at 2 or 3x their
voltage ratings, if you want to be bold and thrifty). The key specs to
look at are RMS rf current ratings (which are thermally limited, by the way)

If they aren't low loss enough (or you can't get them in small enough
capacitance), you might look at paralleling or stacking SMT ceramic caps
from the vendors above. 1kV parts are readily available. I have a 1kW
13 MHz power amp here that uses 11 pieces of a 56 pF cap in parallel in
series with the RF and a half dozen 100pF 2.5kV units in shunt in the
output network.


Jim, W6RMK

Rick wrote in
news
On Wed, 04 Apr 2007 21:48:08 -0700, Bryan wrote:

A stub of coaxial cable will work... approximately 30pF/foot for 50
ohm types... approximately 20pF/foot for 75 ohm types.

Good evening, Bryan.

Thanks, but there are a couple of problems... the antenna will be for
160 through 40 and so two of the traps will be for 80 meters... I
forget what the capacitance will be but I think it was somewhere in
the 200 pf range. Also, the antenna is meant to be portable, and so
the traps need to be as physically small as practical. Also the power
will usually be low... as low as 5 watts, usually no more than 50 and
never more than 100 watts... so I guess I need to make the antenna as
efficient as I can... kind of a contradiction in terms for trapped
antennas especially where the traps need to be physically small.

If I could find the doorknob capacitors for a reasonable price that
would be ideal, but $15 each is way too much.

On the other hand, the ones from RF Parts are all at least 4 KV, and
up from there (40-50 KV). I don't know how to accurately calculate
the voltage rating I'll need (I'll post that as another question under
another topic), but my guess is that for 100 watts max, I don't need 5
KV, and may not need 1 KV.

Who else besides RF Parts has these things and has a decent website?


I would just use normal 500v silver mica capacitors in series if
necessary. At the 100 watt level the capacitors in a trap don’t have to
work too hard. Silver micas have good RF performance and easy to come by.
If you use EZNEC to model your antenna it can tell you the voltage across
your traps at your max power level making it easy to pick out your
capacitors. You can down load a trial version that can easily model your
antenna.

73
John Passaneau W3JXP

John Passaneau wrote:

I would just use normal 500v silver mica capacitors in series if
necessary. At the 100 watt level the capacitors in a trap don’t have to
work too hard. Silver micas have good RF performance and easy to come by.
If you use EZNEC to model your antenna it can tell you the voltage across
your traps at your max power level making it easy to pick out your
capacitors. You can down load a trial version that can easily model your
antenna.

Keep in mind that EZNEC reports the RMS voltage, while capacitors are
rated for peak voltage. So multiply the EZNEC values by 1.41 to get the
required capacitor rating. Then add whatever additional safety margin
you think is appropriate.

Roy Lewallen, W7EL

On Mon, 09 Apr 2007 09:12:01 -0700, Roy Lewallen
wrote:

John Passaneau wrote:

I would just use normal 500v silver mica capacitors in series if
necessary. At the 100 watt level the capacitors in a trap don’t have to
work too hard. Silver micas have good RF performance and easy to come by.
If you use EZNEC to model your antenna it can tell you the voltage across
your traps at your max power level making it easy to pick out your
capacitors. You can down load a trial version that can easily model your
antenna.

Keep in mind that EZNEC reports the RMS voltage, while capacitors are
rated for peak voltage. So multiply the EZNEC values by 1.41 to get the
required capacitor rating. Then add whatever additional safety margin
you think is appropriate.

Roy Lewallen, W7EL

Hi All,

I would add that even if this safety factor is ignored (even at the
additional burden of ignoring the RMS to Peak conversion); that some
may feel their set up works fine, but is now off frequency. Mica caps
are said to be "self healing." That is accounted for the removal of
plate area (by the arc through) near the puncture. Net result of
overloading is neither a short nor an open.

You could test them with a Capacitance meter and find they still
exhibit capacitance, and perhaps not much D (but how many meters
actually measure D?).

73's
Richard Clark, KB7QHC