Re: HIGH Q CAPS FOR VLF LOOP ANTENNA?

On Thu, 27 Oct 2005 22:38:47 -0400, TRABEM <> wrote:

>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.
>

Forget Rjeloop3 for the moment and think about what you have.

You focus on how low the resistance of the loop inductance is, and
whether or not the capacitor ESR is significant... neither is when you
jam a 2 ohms receiver in series with it all (ignoring the transmission
line).

You seem to be analysing your series circuit with part of it (the rx)
replaced with a s/c.


>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

Well, what good is it to know what the loop L and C do when not
connected to the receiver?

>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).

Read Richard's response, though it is more detailed and no doubt more
accuracy.

I think you will understand the problem when you analyse a three
component series circuit (your topology), the Loop L, the Loop C and
the Rx input z. (You can ignore radiation resistance, loop loss,
capacitor loss, they are all much less than rx input z so the loops
loss is dominated by the rx input z in your circuit.)

The place this will end up is that you will come to realise that
knowing how the L and C of the loop behave unloaded, and dwelling on
that behaviour ignoring the effect of loading is probably why you are
where you are (an assumption I know).

When you have worked that out, you may understand why others are
asking how you are going to couple to the loop. Your proposal to
insert the 2 ohms (or whatever) rx input in series with the loop
components isn't delivering what you wanted, and it won't matter how
thick the loop conductor is, or how low the ESR of the capacitor is.

Owen
--
.

Relevant Pages

  • Re: Hearing aids, Inductive loops question
    ... amplifier with a resistance of about the same value as what the ... So a resistance of roughly 8 ohms can be connected in series with the ... It allows more turns to be used in the loop without loss at ...
    (sci.electronics.design)
  • Re: HIGH Q CAPS FOR VLF LOOP ANTENNA?
    ... But your concern about Effective Series Resistance of the tuning ... All we have to go on is Trabem's value of the tuning capacitor of 0.2 ... Therefore the size of his loop is a square of sides = 5 metres. ...
    (rec.radio.amateur.antenna)
  • Re: Hearing aids, Inductive loops question
    ... The resistance has a beneficial effect - it levels the frequency ... It allows more turns to be used in the loop without loss ... The amplifier is also happy because it thinks it is driving an 8 ohm ... impedance is several ohms at the higher audio frequencies, ...
    (sci.electronics.design)
  • Re: High Q caps for 60 Khz loop antenna?
    ... The resistance would double, ... The impedance would also quagruple from about 119 ohms to 476 ohms, so the Q would roughly double when tuned with a .055 uF capacitor. ... This, of course, neglects the loading effect of the receiver input resistance and the radiation resistance of the loop, and the eddy current loading of nearby conducting objects. ... Even an expensive capacitor is probably cheaper than more wire. ...
    (rec.radio.amateur.homebrew)
  • Re: Low Noise receiving Loop antenna
    ... Antenna Handbook, ... does this not make the loop a closed loop? ... the capacitor is in series with the inner conductor of the feedline to ... In the simple case of an untuned loop loaded with 50 ohms, ...
    (rec.radio.amateur.antenna)
Loading