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