View Single Post
  #2   Report Post  
Old April 5th 07, 05:48 AM posted to rec.radio.amateur.antenna
Bryan Bryan is offline
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 199
Default Reasonably-priced capacitors for traps

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