On Sat, 11 Nov 2006 21:03:20 GMT, Owen Duffy wrote:


Try the calculator at http://www.vk1od.net/tl/tllc.php to find the
input impedance of 3m of RG174 with a 100MOhm load at 7MHz... I get
1-j64, so it is a 64 ohm capacitive reactance with a D factor of 0.016
(or Q of 64). You should expect an order of magnitude or more better
performance from a quality fixed capacitor.


The selection of 7MHz for the above example should not be taken as a
recommendation that traps be resonant at one of the frequencies of
"operation"... in fact it is usually better that they are not operated
at resonance.

I also had a quick look at Reg's COAXTRAP prog. On an trial for a trap
at 6MHz using 5.2mm dia line, it came up with a resonant impedance of
187kOhms which seems very high. I make the 1.8m of coax o/c stub
bringing around 34kOhms of shunt resistance alone, so resonance
impedance should be less than that, and more likely less than 20kOhm.

Programs tend to underestimate the losses of these traps.

Owen
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