On 10/21/2015 2:06 AM, rickman wrote:
On 10/21/2015 2:18 AM, Brian Howie wrote:
In message , rickman
writes
On 10/19/2015 3:34 AM, Brian Howie wrote:
In message , bilou
writes

"Brian Howie" wrote in message
...

I've a 5 foot Octagonal loop for MF. The shield is copper water pipe,
with
a gap , 7 turns inside plus a coupling winding. It does a good job
eliminating local noise (mostly ASDL hash from the phone lines)
compared
with a vertical. However the capacitance between the shield and
turns
seems to load it quite a bit meaning I can't get the tuning range I'd
like.

Brian GM4DIJ
--
Brian Howie
Hi
My own experience is that ,at least for receive, multi turn loops are
useless.
Instead you can use a single turn one with a good coil in serial.
The tuning range for a given variable capacitor is much greater
especially if ,at low frequency, the coil is using ferrite .
Switching the coil can increase the tuning range easily.
The coil, with a secondary winding,is also very useful to
adjust the coupling to the receiver.

I'd have thought I'd get a better signal from more turns, but maybe
better coupling and a higher Q from your suggestion would do the same.

I can't imagine why more turns won't help a receiving loop. I guess
it depends on what is limiting reception. Adding a coil may improve
the Q or it make make it worse depending on the Q of the coil. More
turns won't help the Q of a receiving loop, other than reducing the
significance of the resistance of connections and other components.
More turns *will* increase the signal strength.

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.


The capacitance of the loop to the screen meant that at the minimum
variable C setting ,I couldn't get the maximum frequency of about
500KHz I wanted, so I had to take turns off. I now need more parallel C
to tune the look down to 136KHz.

Wow, that loop must have a *lot* of capacitance. Is there a way to
space the conductors away from the copper tubing in the run?

I'm curious why you would use copper pipe for the shield. Because it
provides both shield and support? I guess there are a million ways to
build a shielded loop. I like the idea of using coax, but I don't know
if that also has serious limitations from the capacitance between loop
conductor and shield.

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

On 10/21/2015 6:15 AM, amdx wrote:
On 10/21/2015 2:06 AM, rickman wrote:
On 10/21/2015 2:18 AM, Brian Howie wrote:
In message , rickman
writes
On 10/19/2015 3:34 AM, Brian Howie wrote:
In message , bilou
writes

"Brian Howie" wrote in message
...

I've a 5 foot Octagonal loop for MF. The shield is copper water
pipe,
with
a gap , 7 turns inside plus a coupling winding. It does a good job
eliminating local noise (mostly ASDL hash from the phone lines)
compared
with a vertical. However the capacitance between the shield and
turns
seems to load it quite a bit meaning I can't get the tuning range
I'd
like.

Brian GM4DIJ
--
Brian Howie
Hi
My own experience is that ,at least for receive, multi turn loops are
useless.
Instead you can use a single turn one with a good coil in serial.
The tuning range for a given variable capacitor is much greater
especially if ,at low frequency, the coil is using ferrite .
Switching the coil can increase the tuning range easily.
The coil, with a secondary winding,is also very useful to
adjust the coupling to the receiver.

I'd have thought I'd get a better signal from more turns, but maybe
better coupling and a higher Q from your suggestion would do the
same.

I can't imagine why more turns won't help a receiving loop. I guess
it depends on what is limiting reception. Adding a coil may improve
the Q or it make make it worse depending on the Q of the coil. More
turns won't help the Q of a receiving loop, other than reducing the
significance of the resistance of connections and other components.
More turns *will* increase the signal strength.

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.


The capacitance of the loop to the screen meant that at the minimum
variable C setting ,I couldn't get the maximum frequency of about
500KHz I wanted, so I had to take turns off. I now need more parallel C
to tune the look down to 136KHz.

Wow, that loop must have a *lot* of capacitance. Is there a way to
space the conductors away from the copper tubing in the run?

I'm curious why you would use copper pipe for the shield. Because it
provides both shield and support? I guess there are a million ways to
build a shielded loop. I like the idea of using coax, but I don't know
if that also has serious limitations from the capacitance between loop
conductor and shield.

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.

Doesn't the capacitance vary mostly with diameter? The RG-6 I have is
16 pf/ft about a quarter inch diameter.

--

Rick

On 10/21/2015 2:47 PM, rickman wrote:
On 10/21/2015 6:15 AM, amdx wrote:
On 10/21/2015 2:06 AM, rickman wrote:
On 10/21/2015 2:18 AM, Brian Howie wrote:
In message , rickman
writes
On 10/19/2015 3:34 AM, Brian Howie wrote:
In message , bilou
writes

"Brian Howie" wrote in message
...

I've a 5 foot Octagonal loop for MF. The shield is copper water
pipe,
with
a gap , 7 turns inside plus a coupling winding. It does a good job
eliminating local noise (mostly ASDL hash from the phone lines)
compared
with a vertical. However the capacitance between the shield and
turns
seems to load it quite a bit meaning I can't get the tuning range
I'd
like.

Brian GM4DIJ
--
Brian Howie
Hi
My own experience is that ,at least for receive, multi turn loops
are
useless.
Instead you can use a single turn one with a good coil in serial.
The tuning range for a given variable capacitor is much greater
especially if ,at low frequency, the coil is using ferrite .
Switching the coil can increase the tuning range easily.
The coil, with a secondary winding,is also very useful to
adjust the coupling to the receiver.

I'd have thought I'd get a better signal from more turns, but maybe
better coupling and a higher Q from your suggestion would do the
same.

I can't imagine why more turns won't help a receiving loop. I guess
it depends on what is limiting reception. Adding a coil may improve
the Q or it make make it worse depending on the Q of the coil. More
turns won't help the Q of a receiving loop, other than reducing the
significance of the resistance of connections and other components.
More turns *will* increase the signal strength.

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.


The capacitance of the loop to the screen meant that at the minimum
variable C setting ,I couldn't get the maximum frequency of about
500KHz I wanted, so I had to take turns off. I now need more parallel C
to tune the look down to 136KHz.

Wow, that loop must have a *lot* of capacitance. Is there a way to
space the conductors away from the copper tubing in the run?

I'm curious why you would use copper pipe for the shield. Because it
provides both shield and support? I guess there are a million ways to
build a shielded loop. I like the idea of using coax, but I don't know
if that also has serious limitations from the capacitance between loop
conductor and shield.

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.

Doesn't the capacitance vary mostly with diameter?

Distance between conductors and material between.


The RG-6 I have is 16 pf/ft about a quarter inch diameter.

Foamed PE! But in general, it looks like my 30pf is a little high.

Mikek

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


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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

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.