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Old October 28th 05, 03:38 AM
TRABEM
 
Posts: n/a
Default HIGH Q CAPS FOR VLF LOOP ANTENNA?

Hi Owen,

Hi Owen,

I am using large copper cable and relatively low loss capacitors. I
expect the 9 ohms of inductive reactance to cancel out the -9 ohms of
capacitive reactance leaving only the sum of the AC (or RF) resistance
of the copper and the ac resistance of caps to give me my net
impedance. Am I correct up to this point??

I'm trying to take this step by step so I can understand where I've
gone wrong.....clearly I must have made an error somewhere.

I don't know what the actual ac resistance of the caps is, but I do
know the ac resistance of 20 meters of #2/0 copper welding cable is
pretty damn low.

Yes, the loop is series tuned, so the output is taken on the
unconnected capacitor terminal and the unconnected wire end.

Rjeloop3 gives estimates my Q at 221 even though it thinks I'm
building a parallel tuned loop. But, I think Q is Q, and the Q of both
types of loops is the same provided the same materials have been used
in both loops.

I am (for now) not considering the effects of hooking it to a receiver
and/or the transmission line. I confess I have not tried to quantify
the actual values of the ac resistance of the copper and have only
rough estimates of what the esr of the caps is. I'd be pretty
surprised if the dc resistance of the cable is much more than .1 ohms
though, so the ac resistance should be a little higher at 60 KHz.

Can you estimate what the unloaded Q of the loop is (in round
numbers), and if so, can you agree that it might be around 221 (as
Reg's software predicts)? Can you estimate what the impedance of the
loop is (in round numbers)? Again, do not factor in the receiver input
impedance as we aren't sure whether I'll keep it as is or match it's
impedance with a preamp and/or toroidal transformer. For the moment,
assume the receiver is mounted at the loop (which is a very real
possibility since it's fairly small).

Thanks for jumping in.

T