wrote:
On Wednesday, October 14, 2015 at 1:34:16 PM UTC-5, rickman wrote:
So if the shield has little to do with rejecting near field electrical
noise, what does the shield do?

A shield with the usual gap promotes balance.
And a small loop with a gapped shield is no quieter than a regular solenoid
or pancake wound loop.
Much of the usual "magnetic loop" theory that is on the web is malarkey.
A small loop is a small loop is a small loop as long as all are properly
balanced.



Looking back, I now realize that there was a bit of confusion
between the Small Loops for transmitting and those for reception.
Until a short while ago Magnetic Loops was the term used for the
Small Transmitting Loops, and the lack of clarification which
exists in the ham vocabulary has occasionally been a problem. I
think it caused a bit of confusion here too, particularly in
the matter of multi turn loops!

Irv VE6BP

On 10/20/2015 3:03 AM, rickman wrote:
On 10/19/2015 7:55 PM, amdx wrote:
On 10/19/2015 2:14 PM, rickman wrote:

To be a bit simplistic, the amount of signal captured is proportional
to the loop area; the number of turns has little to no effect on that.

I'm pretty sure that is not correct. The signal strength is
proportional to the number of turns *and* the loop area. I will have to
dig out my notes on this, but some factors (like Q) even out with
various changes in antenna parameters such as number of turns, loop
size, etc. But signal strength is proportional to the area of the loop
and the number of turns.

From
http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm

E = 2pi w S µR e / λ
λ is the wavelength in meters
w - the number of ML turns;
S – is the area of the windings in m2;
μR is the effective magnetic permeability of the ferrite rod SML. μR is
always less than the permeability of the material used and depends from
the size, geometry and the way the windings are constructed. μR = 1 for
aerial loops.

The product:
� = w μR S (3)
is called effective area of the SML.


Correct me if I'm wrong,
A 1 meter square loop with 5 turns would equal 5 square meters.
A = 5 sq. meters.

A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters.
A = 5 sq. meters.

A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25
sq. meters.
A = 25 Sq. meters.

A 5 times increase in A (S) means about a 7db increase in signal
strength. (minus losses caused by series inductor)

Does that all seem right?

I forgot to include the following definitions.
Е – is the voltage between antenna terminals in uV;
е – is the intensity of electromagnetic wave in uV/m.

Not sure where you are going with this. For the purpose of calculating
the received signal strength of an antenna without factoring in
resonance, the area is just the area of one loop (S = pi r^2), not the
loop area times the number of turns. The number of turns (w) is
multiplied by the loop area in the formula along with the relative
permeability of the core material to get the effective area. Is that
what you mean? The post that Jim made explicitly stated, "the number of
turns has little to no effect on that", with "that" meaning "the amount
of signal captured", or E in the above formula. That is the point I was
correcting.

So why do you feel the need to include a series inductor with the 25 m^2
1 turn loop?


I don't know what the inductance of a 1 turn 25 m^2 loop is, but I think
it would need a very large variable capacitor to tune it.
(Gut feeling) Just want to keep it under 1200pf. Because I have that
size variable inductor.

Mikek

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.

Mikek

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.

--

Rick

On 10/20/2015 9:17 PM, amdx wrote:
On 10/20/2015 3:03 AM, rickman wrote:
On 10/19/2015 7:55 PM, amdx wrote:
On 10/19/2015 2:14 PM, rickman wrote:

To be a bit simplistic, the amount of signal captured is proportional
to the loop area; the number of turns has little to no effect on that.

I'm pretty sure that is not correct. The signal strength is
proportional to the number of turns *and* the loop area. I will
have to
dig out my notes on this, but some factors (like Q) even out with
various changes in antenna parameters such as number of turns, loop
size, etc. But signal strength is proportional to the area of the loop
and the number of turns.

From
http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm


E = 2pi w S µR e / λ
λ is the wavelength in meters
w - the number of ML turns;
S – is the area of the windings in m2;
μR is the effective magnetic permeability of the ferrite rod SML.
μR is
always less than the permeability of the material used and depends from
the size, geometry and the way the windings are constructed. μR = 1 for
aerial loops.

The product:
� = w μR S (3)
is called effective area of the SML.


Correct me if I'm wrong,
A 1 meter square loop with 5 turns would equal 5 square meters.
A = 5 sq. meters.

A 2.23 meter x 2.23 meter 1 turn loop would equal 5 square meters.
A = 5 sq. meters.

A 5 meter x 5 meter 1 turn loop with a series inductor would equal 25
sq. meters.
A = 25 Sq. meters.

A 5 times increase in A (S) means about a 7db increase in signal
strength. (minus losses caused by series inductor)

Does that all seem right?

I forgot to include the following definitions.
Е – is the voltage between antenna terminals in uV;
е – is the intensity of electromagnetic wave in uV/m.

Not sure where you are going with this. For the purpose of calculating
the received signal strength of an antenna without factoring in
resonance, the area is just the area of one loop (S = pi r^2), not the
loop area times the number of turns. The number of turns (w) is
multiplied by the loop area in the formula along with the relative
permeability of the core material to get the effective area. Is that
what you mean? The post that Jim made explicitly stated, "the number of
turns has little to no effect on that", with "that" meaning "the amount
of signal captured", or E in the above formula. That is the point I was
correcting.

So why do you feel the need to include a series inductor with the 25 m^2
1 turn loop?


I don't know what the inductance of a 1 turn 25 m^2 loop is, but I think
it would need a very large variable capacitor to tune it.
(Gut feeling) Just want to keep it under 1200pf. Because I have that
size variable inductor.

That's not the question. I'm asking why you think this antenna needs an
inductor and the other two don't. I'm guessing this is the only
configuration you are considering. I'm not sure how practical a 5 meter
tall loop will be if you are really serious about building it. If you
make it from copper pipe it will be not only large, but heavy and
require a lot of support to be used outside in winds.

The capacitance needed will depend on the frequency you wish to tune. A
round 5 meter single loop will be 29.5 uH. At 1 MHz it will require
somewhat less than 1 nF if I've done the math right. I've got this in a
spread sheet, but I've never verified it is set up correctly.

If you want to work at lower frequencies you can use a smaller antenna
radius and more turns which will increase the inductance letting you use
a smaller cap to tune it. L � r * N² Cut the radius by X, increase the
number of turns by X and the inductance increases by X. Signal strength
will only go down by a small amount related to the ln().

--

Rick

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.

Brian



--
Brian Howie

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.

--

Rick

In message , rickman
writes

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?

Not easy

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.


It seemed a good idea at the time. The original design used plastic pipe
covered with tin-foil ,but I wanted something that would survive a
Scottish winter outdoors.

PVC 4-7 Loop Antenna Al Burzynski KA5JGV ( it's on the NDB yahoo group)

it used 12 turns. I think the use of plastic pipe and external tinfoil
reduces the C.

My loop does work quite well, and has survived outdoors but I think it
could be improved

Brian



--
Brian Howie

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.
Mikek

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