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.

Brian
--
Brian Howie

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.

--

Rick

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.

Brian

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.

The number of turns greatly effects the inductance.

Multiturn loops are used at VLF frequencies to get the inductance large
enough so the loop resonants with a practical capacitor.

Unless you are trying to operate on the 2200 meter band, forget multiple
turn loops.


--
Jim Pennino

rickman wrote:

I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

I have read that the electric field sensitivity is non-directional, and
therefore interferes with directivity even though the sensitivity is
low. I have no idea if this makes sense when worked out quantitatively.

--
Roger Hayter

rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

The single-turn tuned magnetic loop as used for transmitting is a
different animal than the aperiodic loop of usually a couple of turns
that is used for receive-only applications.

The tuned loop cannot be shielded because of the parasitic capacitance
that would add, it would limit the high end of the tuning range.

Of course a shielded loop also will resonate at some frequency due to
parasitic capacitance.

On 10/14/2015 1:34 PM, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.


I bought a "Pixel" shielded magnetic loop from Pixel. It included a 30db
LNA. It works better than my dipoles for receive on the 40 meter band on
up. I guess I should be clear. I don't have 6 meters, so I am talking
about 40, 20, 17, 15, and 10. The SNR is better than my dipoles on all
these bands. It is significantly worse on 75 and 160.

It was well worth the money. It is probably the best 400 bucks I have
ever spent on ham radio.

I just bought a used FTDX-3000. It has a special coax connector just for
a receiving antenna. I can switch receive antennas on the front of the
radio. A nice feature.

On 10/14/2015 02:34 PM, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

Hello, and that seems to be ham radio jargon. Hams seem to think the
adjectives "magnetic" and "electric" are needed when referring to loop
and dipole antennas, respectively. Textbooks on electromagnetics and
antennas don't use those terms except in the case when discussing
theoretically small radiators, i.e. "magnetic dipoles" and "electric
dipoles".

My hypothesis on the ham terminology is that a loop is viewed as an
inductor. That's OK for close-in (non-radiative) mutual coupling to
some source but when you're several wavelengths away (in the far field)
then the loop (or dipole antenna for that matter) responds to the
electromagnetic field (the electric and magnetic far fields can't be
considered separately). The fact that an axis of either antenna lines
up with the electric or magnetic field vector in the far field is moot.
Does this mean that the loop doesn't have inductance? Of course not
and it plays a role in establishing the feedpoint impedance of the loop
at the operating frequency. Now if folks would just stop using that
word "literally" so damn much...

Sincerely, and 73s from N4GG0,





--
J. B. Wood e-mail:

"J.B. Wood" wrote in message
...
Hello, and that seems to be ham radio jargon. Hams seem to think the
adjectives "magnetic" and "electric" are needed when referring to loop and
dipole antennas, respectively. Textbooks on electromagnetics and antennas
don't use those terms except in the case when discussing theoretically
small radiators, i.e. "magnetic dipoles" and "electric dipoles".

My hypothesis on the ham terminology is that a loop is viewed as an
inductor. That's OK for close-in (non-radiative) mutual coupling to some
source but when you're several wavelengths away (in the far field) then
the loop (or dipole antenna for that matter) responds to the
electromagnetic field (the electric and magnetic far fields can't be
considered separately). The fact that an axis of either antenna lines up
with the electric or magnetic field vector in the far field is moot. Does
this mean that the loop doesn't have inductance? Of course not and it
plays a role in establishing the feedpoint impedance of the loop at the
operating frequency. Now if folks would just stop using that word
"literally" so damn much...

Sincerely, and 73s from N4GG0,
Hi
I totally agree with you.
You only get a feeling of an antenna behaviour a few wavelength
from it.
This is very hard to do at HF for amateurs.
Specially in the vertical plane.
I made a few tests of small loops in the broadcast FM band.
What surprised me was their ,almost perfect,omnidirectional behaviour
in horizontal polarisation.
A too small vertical dipole needs to be loaded by a coil.
The loop ,for me, is a too small slot aerial and it needs to
be loaded by a capacitor.
On receive both have a small efficiency due to their small size
On HF this is hiden by the high level of noise.

On 10/15/2015 5:42 AM, J.B. Wood wrote:
On 10/14/2015 02:34 PM, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

Hello, and that seems to be ham radio jargon. Hams seem to think the
adjectives "magnetic" and "electric" are needed when referring to loop
and dipole antennas, respectively. Textbooks on electromagnetics and
antennas don't use those terms except in the case when discussing
theoretically small radiators, i.e. "magnetic dipoles" and "electric
dipoles".

My hypothesis on the ham terminology is that a loop is viewed as an
inductor. That's OK for close-in (non-radiative) mutual coupling to
some source but when you're several wavelengths away (in the far field)
then the loop (or dipole antenna for that matter) responds to the
electromagnetic field (the electric and magnetic far fields can't be
considered separately). The fact that an axis of either antenna lines
up with the electric or magnetic field vector in the far field is moot.
Does this mean that the loop doesn't have inductance? Of course not
and it plays a role in establishing the feedpoint impedance of the loop
at the operating frequency. Now if folks would just stop using that
word "literally" so damn much...

Sincerely, and 73s from N4GG0

I agree. If the jargon is either magnetic or electric, how do we define
a folded dipole antenna? It is a loop. Is it electric or magnetic?

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.