On 10/15/2015 6:13 AM, Brian Howie wrote:
In message , Jeff Liebermann
writes
On Wed, 14 Oct 2015 14:34:10 -0400, 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 shielded loop reduces local noise pickup by eliminating much of
the electric component of that noise in the near field. Since the
ability of a small loop antenna to hear properly is primarily an
exercise in improving the SNR, any reduction in noise levle, without a
corresponding reduction in signal level, is a very good thing. More
detail:
http://electronics.stackexchange.com...-if-anything-m
akes-shielded-loop-antennas-so-great-at-rejecting-local-nois

I've built small loops that were not shielded and measure the SNR of
some stable signal, such as WWV. I then wrapped the loop in aluminum
duct tape, leaving a gap to prevent a shorted turn problem, retuned,
and found that the baseline noise level had decreased and the SNR had
improved. It works.


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.

I assume there is nothing to space the wires from the pipe other than
the insulation. Maybe you could use wire with thicker insulation? Or
if you are using straight pipe, could you use a fabricated spacer at the
corners? I guess that might be hard to assemble with soldering the
joints.

--

Rick

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.

In message , rickman
writes

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.

I assume there is nothing to space the wires from the pipe other than
the insulation. Maybe you could use wire with thicker insulation? Or
if you are using straight pipe, could you use a fabricated spacer at
the corners? I guess that might be hard to assemble with soldering the
joints.

No just the insulation. It was hard enough to thread it without spacers
..

I should have stuck to the original design that used plastic pipe with
aluminium foil stuck to the outside

Brian GM4DIJ
--
Brian Howie

On 10/16/2015 6:53 AM, Brian Howie wrote:
In message , rickman writes

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.

I assume there is nothing to space the wires from the pipe other than
the insulation. Maybe you could use wire with thicker insulation? Or
if you are using straight pipe, could you use a fabricated spacer at
the corners? I guess that might be hard to assemble with soldering the
joints.

No just the insulation. It was hard enough to thread it without spacers .

I should have stuck to the original design that used plastic pipe with
aluminium foil stuck to the outside

I saw one receiving antenna made from a bicycle rim. Easy to thread. I
assume you only use this for receiving?

--

Rick

On 16/10/15 17:07, rickman wrote:
On 10/16/2015 6:53 AM, Brian Howie wrote:
In message , rickman
writes

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.

I assume there is nothing to space the wires from the pipe other than
the insulation. Maybe you could use wire with thicker insulation? Or
if you are using straight pipe, could you use a fabricated spacer at
the corners? I guess that might be hard to assemble with soldering the
joints.

No just the insulation. It was hard enough to thread it without spacers .

I should have stuck to the original design that used plastic pipe with
aluminium foil stuck to the outside

I saw one receiving antenna made from a bicycle rim. Easy to thread. I
assume you only use this for receiving?

At an AR convention in the Netherlands ,last year , there was a 14 MHz
bicycle rim loop (aluminium) with motorised variable capacitor ,very
well made. Recently I got a bicycle rim , made of stainless steel
,hence probably not very effective as tx antenna . But the rim can also
serve as a guide for bending a copper loop .....shall try that after
testing the stainless steel rim.

Frank GM0CSZ / KN6WH

In message , rickman
writes
On 10/16/2015 6:53 AM, Brian Howie wrote:
In message , rickman writes

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.

I assume there is nothing to space the wires from the pipe other than
the insulation. Maybe you could use wire with thicker insulation? Or
if you are using straight pipe, could you use a fabricated spacer at
the corners? I guess that might be hard to assemble with soldering the
joints.

No just the insulation. It was hard enough to thread it without spacers .

I should have stuck to the original design that used plastic pipe with
aluminium foil stuck to the outside

I saw one receiving antenna made from a bicycle rim. Easy to thread.
I assume you only use this for receiving?


Neat idea . I used a hula hoop for a previous version. Yes receive only.
I wanted to cover 136kHz and 472Khz . In theory it should have done it ,
but for the capacitance. I had to take a lot of turns off, which also
meant the coupling winding loaded the loop a lot more, reducing the Q
factor.

Brian
--
Brian Howie

"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.

"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.

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