On Sat, 11 Nov 2006 14:24:04 -0500, "C. J. Clegg"
wrote:


Good afternoon...

Anybody know where I can find some formulas or design information for
designing coax traps for a dipole?

I assume that to mean a parallel LC circuit where the L is mainly
formed by the inductance of the outer of a coax cable, and the
capacitance is mostly formed by the inner to outer of the coax cable.

The programs that I have seen assume that the impedance of a short o/c
transmission line stub is purely capacitive and has a constant
capacitance per unit length.

Both assumptions are wrong, which is why these traps have higher loss
and are wider band (both inextricably linked) than predicted by those
programs.


I've seen some articles on coax-trap antennas but the traps they use are
too big. I need to make some traps for 40 meters that are narrower (but,
longer is OK) than the ones I've seen in the articles.

(A thought I had was perhaps using RG-174 instead of RG-58 ... these will
be used at low power only, 10 watts max)

At the further expense of efficiency, if that matters.


High Q and narrow bandwidth is OK. These traps only have to resonate at
one specific frequency.


You could wind a traditional solenoid with thick copper, and use a
quality fixed capacitor. Short o/c stubs of lossy coax make for lossy
reactors.

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.

But they "work". The benefit of wider trap bandwidth and less critical
tuning are at the expense of radiated power.

Any thoughts or advice?

Thanks...

Owen
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