On 10/22/2015 6:01 AM, amdx wrote:
On 10/21/2015 9:09 PM, rickman wrote:
On 10/21/2015 8:41 PM, amdx wrote:
On 10/21/2015 2:36 PM, rickman wrote:
On 10/21/2015 6:12 AM, amdx wrote:
On 10/20/2015 10:35 PM, rickman wrote:
On 10/20/2015 9:21 PM, amdx wrote:
On 10/20/2015 1:56 PM, rickman wrote:
On 10/20/2015 10:44 AM, amdx wrote:
On 10/19/2015 10:53 PM, rickman wrote:
On 10/19/2015 3:50 PM, bilou wrote:
"rickman" wrote in message
...
On 10/19/2015 3:34 AM, Brian Howie wrote:
How does the coil affect the tuning range of the cap? A cap is
limited by
the ratio of the minimum to maximum capacitance. The ratio of
frequency
is limited to the same ratio.
In a multiturn loop you get huge capacitance between turns.
For a given variable capacitor it appears in parallel.
The Q of that big coil might be higher but as you need to add
fixed capacitors to the variable one to get useful tuning range
you loose almost what you gain.

I sort of lost the thought here. If you up the inductance of the
loop,
it lowers the required tuning capacitance, so why would fixed
capacitors
be needed? Are you saying the parasitic capacitance of the
loop is
enough to significantly reduce the tuning range of the variable
cap?
Maybe, but there are construction methods that minimize the
parasitic
capacitance of multi-turn loops. Wide spacing is important.
I've
seen
spiral loops wound on wooden frames that look like God's Eyes,
very
attractive.


I saw descriptions using a 128 pairs telephone cable and
spending
several days to wire it as a 256 turns loop.
A bad idea IMHO.

I'm not sure what problem you would be trying to solve by using a
256
turn loop. There are middle grounds...


Often a 60kHz WWVB time receiver.

So why would that be a "bad idea"?


Ahh, you ask "what problem you would be trying to solve"
I should clarify, a resonant antenna for 60kHz, and that requires a
large inductance. Or at least that is one approach.

But the context was that a 256 turn loop was a bad thing. I'm
trying to
understand what that was about. I don't need to know when it is a
good
idea... well, I guess even that is interesting. But I think the
way a
256 turn loop would be made for a WWVB receiver is around a piece of
ferrite. But who knows, maybe a large loop of telephone cable would
work well too.

It obviously works. It is not ideal because it would have a lot of
interwinding capacitance. Also the interwinding capacitance is not a
quality capacitance thus the Q is lowered.
It could be built with space between wire and layers, and 256
solder
connections is not a great idea when trying to insure high Q.
As far as "bad idea", all it has to do is receive enough signal
to keep the clock accurate, more than that is interesting, but
useless.

I haven't built a high Q antenna yet, but I am pretty sure people
greatly exaggerate the significance of solder connections in the Q
factor. Q is related to the losses. I am sure the solder connections
will not significantly impact the dissipative resistance of the wire
unless the turns are around a pencil.



On a large loop antenna, it is probably difficult to get an extremely

high Q. So, the solder connections will have less of an effect than if
it was higher.
I made a loop with 1/4" copper pipe, about 2'x 2' with a vacuum
variable.
I measured it at about Q=800.

Using a 240uh and assuming 1000kHz, That's about 1.88 ohms of loss,
split between dissipation in materials,
wire losses, connection losses and capacitor losses.
If you had 0.12 ohms additional solder connection losses, Q would drop
to 753 from 800.

Why would you assume 120 mOhms of resistance from soldered connections?
Have you measured this somewhere?


I have absolutely no idea about the resistance of a solder connection.
I used 0.12 because it made 1.88 = 2.

Ok, I'll consider that in your analysis.

--

Rick

On Thu, 22 Oct 2015 09:54:24 -0500, amdx wrote:

On 10/22/2015 9:43 AM, Brian Howie wrote:
On Wed, 21 Oct 2015 05:15:18 -0500, amdx wrote:


30pf per ft is a general number for capacitance of coax, but you know
it varies with type. I have some coax for automobile radio antennas
(AM/FM) that has 8pf per foot.
Mikek


Well its not real coax , but the diameter is about 5ft ( well its
octagonal ) say 15ft circumferance , There's 7 turns in close
proximity to the tube or each other, so the capacitance could be a few
hundred pf . I suppose could try measuring it. Self resonance with
12 turns was about 400KHz

http://www.angelfire.com/mb/amandx/loop.html


so effective minimumum capacitance works out around 100pf

Brian


I'm not sure if you're discussing 100pf of interwinding capacitance or
capacitance between the shield and the winding.

Mikek

It's pretty complicated .There's distributed capacitance between the
windings and distributed capacitance between the windings and the
shield. I know the inductance of the loop and the resonant frequency
with the variable C set to minimum, so I can work out an effective
capacitance 100pf that causes the resonance.

There's a further complication in that the coupling loop is also
capacitively coupled to the main windings and the shield, so
connecting that up changes the resonance as well.

I can't even begin to think how to model it.

The unshielded wide-spaced loop makes it easier to design, but you
get ( arguably) no electric field shielding, which is where we came
in.

I started out using the maths but ended up cut and try to get a
compromise solution.

Brian

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On 10/22/2015 2:20 PM, Brian Howie wrote:

The unshielded wide-spaced loop makes it easier to design, but you
get ( arguably) no electric field shielding, which is where we came
in.

I thought the magnetic loop was not very sensitive to near field
electrical source, hence the name?

As to the modeling, I believe there are programs available for that like
one of the many antenna simulators.

--

Rick

On Thursday, October 22, 2015 at 1:20:31 PM UTC-5, Brian Howie wrote:


It's pretty complicated .There's distributed capacitance between the
windings and distributed capacitance between the windings and the
shield. I know the inductance of the loop and the resonant frequency
with the variable C set to minimum, so I can work out an effective
capacitance 100pf that causes the resonance.

There's a further complication in that the coupling loop is also
capacitively coupled to the main windings and the shield, so
connecting that up changes the resonance as well.

If adding the coupling loop changes the resonance, it's so small
as to be ignored in the real world.


I can't even begin to think how to model it.

Modeling it can be a pain I imagine, but it's all quite easy
to calculate using Reg Edwards program rjeloop3.exe.
Which is a DOS program, but I installed "DOSbox" on my Win 7 64 box,
and it runs just fine.

Lets take it for a quick test drive.
Lets make a square loop 1000mm per side, with seven turns of 1mm wire,
with a ratio/wire spacing of 5mm between the wires.
We want to tune it to 500khz for an example.
The program proclaims that the inductance of the loop is 155.6 mh.
The inductive reactance is 489 ohms.
The HF loss resistance of the wire is 2.32 ohms.
The self resonant frequency of the loop is 4.6 mhz.
Total cap value to tune 500 khz is 651 pf, - stray capacitance of 8 pf,
leaves a setting of the cap at 643 pf.
The appx Q of the coil is 210, and the receive sensitivity is 53 db
below a 1/4 wl vertical.
Total width of winding is 31mm, and the total length of wire is 28m.
Impedance seen across loop when tuned is 102.8 K-ohms.
Impedance seen by receiver is 2.1 K-ohms via a 1 turn coupling loop.

The program can be used to play "what if" until the cows come home,
and any single specification can be changed and tested to see the
difference.


The unshielded wide-spaced loop makes it easier to design, but you
get ( arguably) no electric field shielding, which is where we came
in.

You would get an argument from me, as the "electric field shielding"
is the part I consider total malarkey, and I believe W8JI was of
pretty much the same opinion when I took a quick glance of his article.

I've built nearly every type of small receiving loop there is,
and the tested results reinforced my feeling that the concept
of "electric field shielding" is malarkey. It's all about balance,
not electric field shielding as far as I'm concerned.
The gapped shield loops were no better at reducing noise than a properly
balanced solenoid or pancake loop.
Heck, I even tried using a single turn shielded loop as the coupling
loop. Worked fine, but no better than a plain wire coupling loop,
being as I had no balance issues.