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 10/21/2015 4:42 AM, Brian Howie wrote:
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

Yes, well, they can *all* be improved. I find it interesting to make
stuff like this from discarded materials like rain gutter. I'm trying
to understand why one poster in a yahoo group says thin stock is no good
for transmitting loop antennas. The skin effect limits the signal to
the outer few mils of copper or aluminum. I think thin stock will do
just fine if the circumference is large, overcoming the limitations of
the skin effect. At least you are using all the material rather than
just the outer few mils.

--

Rick

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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