loop antennas
 

On 6/15/2012 6:03 PM, W5DXP wrote:
On Friday, June 15, 2012 4:06:25 PM UTC-5, NM5K wrote:
Only in the cases of corona buildup, etc, on the
elements would that be the case.

He may be talking about precipitation static which was considerably reduced for me in the AZ desert when I converted my dipole to a folded dipole. My problem was dust storms even on a sunny day.

http://www.its.bldrdoc.gov/fs-1037/dir-028/_4096.htm

P-static is unrelated to the reception of EM RF far-field signals. It is a static charge transferred from charged particles in the air directly to an antenna. It was so bad in AZ that arcing occurred at my coax connector. I have actually seen the p-static envelope on my IC-756PRO's display.

The problem with a dipole is that one element of the dipole normally does not have a DC path to ground. When the p-static discharge takes place, it may be through the series capacitor in the receiver. With a loop antenna, including a folded dipole, the p-static usually has a path directly to ground from any point in the antenna system. It's not the only path but it certainly reduces the p-static noise although not completely eliminating it.

Of course, it is possible to reduce the p-static problems on a dipole with a parallel resistor/choke, a 4:1 voltage balun, or heavily insulated wires. Hams who live in low p-static areas of the country will invariably say that there is no such thing because they have never seen it. But hams who have lived in the AZ desert know better.
--
73, Cecil, w5dxp.com

True, but I mentioned that first thing..
"
Only in the cases of corona buildup, etc, on the
elements would that be the case. "

The "etc" including what you are referring to.. I was
just too lazy to include that in the list. :/

But say here in Houston, with the high humidity, we rarely
ever see the type static you had in AZ.
You can also see that in some snow storms from what I hear.

But I mainly want to vote against the idea that a loop, with
no static problems, has the ability to receive less noise as
far as radiated RF from other sources.
It's one of those myths "as far as I'm concerned" that needs
to be put out of it's misery.
For instance, some will claim a small shielded loop that is
indoors, will receive less noise than an equal size unshielded
loop in the same location. Have never seen that to be the
case here, when comparing them. In fact, both the loops I kept
for permanent use were regular old unshielded diamond loops.
The biggest being the PVC frame loop that is 44 inches per side
as I recall.. I have another one that is a circular loop about
16 inches dia. The large one is 7 turns. The small one I think 12
or so.
I tried using shielded loops, but the performance was the
same. I also tried using shielded loops as the coupling loop
inside the larger unshielded loop. Worked fine, but no better
than an unshielded coupling loop with no balance problems.
I kept the shielded coupling loop for the small loop, but
used plain wire on the large one. It was easier to thread
through the spreaders, and also lighter.

loop antennas
 

On 6/15/2012 4:06 PM, NM5K wrote:
On 6/15/2012 2:23 PM, Boomer wrote:


I have used a 75 meter loop antenna here where I live for the past 5
years. It works very well. I live right in town on a lot surrounded by
other homes. I started with a dipole but was advised that a loop would
hear less noise. It turned out to be quite true. I am now a convert to
the loop antenna. Have no idea of the physics of how it works, but it
sure does work well on bands between 75 and 20 meters. It actually seems
to work best on 40 meters.

It is not true. Only in the cases of corona buildup, etc, on the
elements would that be the case.
If you hear less noise with the loop, vs the dipole, it's due to
the change in pattern. Not due to any qualities of the loop itself.
Noise is RF the same as any other signal, and follows all the same
rules. It's no different than an actual signal.
If what you/they say is true, and the loop received less noise, it would
also receive less "desired" signals. Or in other words, everything
would be down vs the dipole.
The most likely explanation is the change in pattern less favored
the direction the noise is coming from. Either that, or the noise is
local to your shack, and for some reason the loop's feed line is better
decoupled than the one feeding the dipole.
If I had to bet, I'd say it's the change in pattern.
There are no magical anti noise properties with loops.



I understand that my experience contravenes your theories about how
antennas should work.

I had several local hams suggest that I use a loop after I kept
complaining about noise. I finally put it up. It is more difficult to
erect than a simple dipole. It is at the same height as was my dipole 35
feet.

The receiver noise level dropped dramatically. I was so glad I went to
the trouble to do it. I finally took down my dipole after switching back
and forth for a year just to be sure the dipole was not better in some
circumstance.

The other effect was an immediate increase in the signal received by my
friends who live within about 200 miles. We operate at 3913. I could not
figure this out until I consulted a pdf file about antennas. The 75
meter loop has a 9 dbi gain at the optimum height of 25 feet. Mine is a
bit high but still gets some gain. All my friends reported on this
increase in my output signal. This information can be found at
http://www.hamuniverse.com/n4jaantennabook.html

This configuration is basically a NVIS antenna. It works so much better
than did my dipole. It gets pretty good dx on 40 and 20. I use an Imax
2000 for 15 and 10.

So, I and my friends know that this antenna works better than a dipole
by actually using it for several years. One of the hams in our group has
been using a loop hung from 2 200 foot towers in a vertical position. He
has done extensive experimentation over the last 45 years.

And yes, a 75 meter loop at 35 feet above ground has a different pattern
than a dipole. I don't care why it works better at all. I and others
just know that it works better and with less noise than a dipole.

And no, a loop antenna is not magic. It is different than a dipole.
Different types of antennas behave differently. The worst antenna I hear
on the air comes from people using G5RVs. Their signal is just totally
lame when they use the recommended 80 foot dipole on 75 meters. If they
would just extend that same antenna to 120 feet they would do so much
better.

Sorry my loop works so well.

Michael

loop antennas
 

On 6/15/2012 4:06 PM, NM5K wrote:
On 6/15/2012 2:23 PM, Boomer wrote:


I have used a 75 meter loop antenna here where I live for the past 5
years. It works very well. I live right in town on a lot surrounded by
other homes. I started with a dipole but was advised that a loop would
hear less noise. It turned out to be quite true. I am now a convert to
the loop antenna. Have no idea of the physics of how it works, but it
sure does work well on bands between 75 and 20 meters. It actually seems
to work best on 40 meters.

It is not true. Only in the cases of corona buildup, etc, on the
elements would that be the case.
If you hear less noise with the loop, vs the dipole, it's due to
the change in pattern. Not due to any qualities of the loop itself.
Noise is RF the same as any other signal, and follows all the same
rules. It's no different than an actual signal.
If what you/they say is true, and the loop received less noise, it would
also receive less "desired" signals. Or in other words, everything
would be down vs the dipole.
The most likely explanation is the change in pattern less favored
the direction the noise is coming from. Either that, or the noise is
local to your shack, and for some reason the loop's feed line is better
decoupled than the one feeding the dipole.
If I had to bet, I'd say it's the change in pattern.
There are no magical anti noise properties with loops.




Here is quote from N4JA's informative antenna book.

......XIV. ONE-WAVELENGTH SINGLE LOOP ANTENNAS


1. The Horizontally Oriented Loop


To calculate the length in feet of any one-wavelength loop, divide 1005
by the frequency in MHz. Horizontally oriented one-wavelength loop
antennas have become very popular on 160, 80, and 40 meters and it is
one type of NVIS antenna. (NVIS stands for "near vertical incidence
skywave" because of its high angle radiation pattern.) It is claimed by
its users that the loop antenna is quieter than other antennas. This is
because it doesnt pick up the noise from power lines, thunderstorms,
etc., coming in at low angles. These antennas radiate on their
fundamental frequencies with a broad pattern straight up to put a strong
signal for nearby contacts. Recently published articles on this type of
antenna have called them "cloud warmers." There are other types of
antennas called NVIS antennas other than loops. They are dipoles at low
heights or dipoles with parasitic reflectors placed under them to cause
the signal to radiate mostly straight up. The NVIS antennas have an
advantage in working nearby stations because you dont get the static
noise and interference from far distances. They are definitely not DX
antennas. An article on NVIS antennas appears in the December 2005 QST......

loop antennas
 

On 6/15/2012 7:23 PM, Boomer wrote:

If I had to bet, I'd say it's the change in pattern.

An article on NVIS antennas appears in the December 2005
QST......



I guess my hunch on the difference being pattern related
panned out.. :)
I've used horizontal loops on the low bands. But I came to the
conclusion they were generally not worth the extra trouble, vs
a dipole. I could barely see any difference here from the usual
dipoles I ran. And if the signal is stronger at one angle or direction,
vs another antenna, it's weaker in another. So it's all a compromise.

On the low bands, I came to the conclusion my favorite antenna for
mostly NVIS and medium path work was the turnstile.
Which are crossed dipoles. You can feed them in or out of phase
for either dipole patterns, or with them 90 degrees out of phase,
for a fairly round omnidirectional pattern.
I like to do well to the close in stations, but also the ones
farther off too.. So even if I used a loop, I would generally
try to get it as high as possible.
Which I also do using dipoles, or turnstiles.

loop antennas
 

"Wimpie" wrote in message
el.net...
El 15-06-12 16:09, garyr escribió:
I agree with your second link (by VK1OD).

The time varying magnetic field generates an electric field and that is
received by the loop. When you screen it completely, it doesn't work,
you need the gap.

By placing the gap opposite to the feed point, you get a balanced loop
without the need of ferrite or other constructions. If you can get
balance
via other means, you don't need the screen. Balancing the loop reduces
noise due to common mode issues. This isn't different from using a balun
between a coaxial cable and a symmetrical dipole.

Your option three may behave competently different, as the coaxial
cable,
power supply cable, switch mode power supply, etc may contribute to
reception of signal and noise due to common mode to differential mode
conversion.

From my experience (reception) with electrically small well-balanced
indoor loops and indoor dipoles, I found some advantage of the loop over
the electric dipole at low frequencies (say below 3 MHz). I contribute
this mainly because of the nulling capability. Whether is applies to
your
location depends on the field distribution of the noise at your
location.

At higher frequencies there is difference in S/N ratio, but not in favor
of one antenna. Sharp nulling wasn't possible. So to know what option
is
best for you, you need to try it. Maybe install both options and select
the antenna that gives best results as this will depend on frequency and
the angle of arrival of the radiation you want to receive.

Other thing that may really help is to find your local source(s) of
noise,
use lots of ferrites and try to find a sweet spot for best S/N ratio.

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM

So you are saying that cases 1& 2 above are essentially equvalent if the
loops are balanced. In terms of noise rejection, there is no analogy to
be
drawn between a shielded loop and a shielded cable.





Hello Gary,

You are right, it is what I am saying.

There is no analogy between the shielding function of the braid in a
coaxial transmission line and the shield in your loop.

For the "shielded" loop, the received voltage is across the gap in the
shield. The shield is the actual single turn loop. The inner conductor is
just there to transport the received signal to the opposite side of the
gap where you can go down (with coaxial cable) to your receiver,
maintaining balance.

Theoretically a coaxial transmission line system is completely closed.
Water from the outside can't reach the inner conductor, source or load.
When cutting a gap in the screen, the coaxial transmission line system
will leak.

Shielded loop with more turns
If you run more turns within the shield of the loop, you pass the gap many
times. When you pass it 3 times, you will get three times the voltage,
hence the impedance and loop inductance increase.

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM

Wim,

Thanks very much for your lucid explanation of how shielded loops function.
I've been using one with a receiver I made to monitor VLF signals. It works
quite well and I had thought it was because of all the local noise I was
avoiding. But then I stumbled across VK1OD's article...

Best regards,
Gary Richardson, AA7VM

PS
Thanks also to all the other respondents. This has been a very interesting
thread.

loop antennas
 

On Wed, 13 Jun 2012 15:11:06 -0700, "garyr" wrote:

This site http://www.frontiernet.net/~jadale/Loop.htm states that: "A
properly designed Loop primarily responds to the magnetic component of the
radio wave. Note that noise resides primarily in the electrical
component..."

Whereas this site shows that that is not the case:
http://vk1od.net/antenna/shieldedloop.

So what is the advantage, if any, of a shielded loop antanna?

Consider three receivers:

1) Shielded loop antenna, receiver with differential input (center-tapped
transformer or instrumentation amp). The two ends of the inner conductor
of the antenna connected to the differential inputs and the shield
connected to ground.

2) Same as above but without the shield.

3) Unshielded loop antenna, receiver with single-ended input.
One end of the loop connected to the receiver input and the other to
ground.

Assuming equal gains and bandwidths, would there be any
difference in the sensitivity or noise level at the output of the three
receivers?

A loop is a loop is a loop, meaning that one side conductor goes
upwards while the other side goes downwards in contrast to a
linear wire antenna or a dipole which is extending in one direction
only. The up-and down (left and right) parts of the wire cancel out
each other at least partially when collecting energy from an electric
field, while the energy uptake from a magnetic field is proportional
to the loop area.
This electric noise cancellation is not perfect, due to unsymmetries
in the field, therefore it pays to shield the loop wires and decrease
the electric influences further.

I hope my explanation is simple enough.

w.

loop antennas
 

"W5DXP" napisal w wiadomosci
...
On Friday, June 15, 2012 2:44:39 AM UTC-5, Szczepan Bialek wrote:
Radio waves and light are the oscillatory flow of electrons (L. Lorenz
1869).

Lorenz (and all other physicists and mathematicians) were obviously
ignorant of photons in those days. Here is what a more knowledgeable
physicist has said more than a century later:

Quoted from: "The Strange Theory of Light and Matter", (c)1985, by Richard
P. Feynman

Feynman has been called the "Great Explainer".[22] He gained a reputation
for taking great care when giving explanations to his students and for
making it a moral duty to make the topic accessible. His guiding principle
was that if a topic could not be explained in a freshman lecture, it was not
yet fully understood. Feynman gained great pleasure[23] from coming up with
such a "freshman-level" explanation". The below is the example:

"So now, I present to you the three basic actions, from which all the
phenomena of light and electrons arise:

-Action #1: A photon goes from place to place.
-Action #2: An electron goes from place to place.
-Action #3: An electron emits or absorbs a photon."

When Feynman says "light", he is including RF. Photons travel at the speed
of light in the medium which is impossible for electrons which possess rest
mass. There are no electrons in a pure vacuum, yet light and radio waves
pass through it at the speed of light with no problem.

Feynman was a teacher. The scientists were the opposite opinion:

"
In 1900, Max Planck was working on black-body radiation and suggested that
the energy in electromagnetic waves could only be released in "packets" of
energy. In his 1901 article [4] in Annalen der Physik he called these
packets "energy elements". The word quanta (singular quantum) was used even
before 1900 to mean particles or amounts of different quantities, including
electricity. Later, in 1905 Albert Einstein went further by suggesting that
electromagnetic waves could only exist in these discrete wave-packets.[5] He
called such a wave-packet the light quantum (German: das Lichtquant). The
name photon derives from the Greek word for light, ??? (transliterated
phôs), and was coined[Note 1] in 1926 by the physical chemist Gilbert Lewis,
who published a speculative theory in which photons were "uncreatable and
indestructible".[6] Although Lewis' theory was never accepted as it was
contradicted by many experiments, his new name, photon, was adopted
immediately by most physicists. Isaac Asimov credits Arthur Compton with
defining quanta of energy as photons in 1923."[7][8]

For scientists the photon is like the tone in sound.

In the pure vacuum are electrons. Do not you know about the Dirac electron
sea?

S*


Following your "logic", why go back to 1869? Why not question the periodic
table of elements because a few millennia ago, men of science asserted that
there are four elements: earth, air, fire, and water. So why not adopt the
four element argument as well?
--
73, Cecil, w5dxp.com

loop antennas
 

Szczepan Bialek wrote:
Feynman was a teacher. The scientists were the opposite opinion:

Were. In 1900. But now it is 2012.

loop antennas
 

"Szczepan Bialek" wrote in message
...

In the pure vacuum are electrons. Do not you know about the Dirac
electron
sea?

S*

Hello Szczepan.
Can you explain the Dirac electron sea in simple terms, please?

Regards, Ian.

loop antennas
 

On Saturday, June 16, 2012 3:12:44 AM UTC-5, Szczepan Bialek wrote:
In the pure vacuum are electrons. Do not you know about the Dirac electron
sea?

Yes, I know about it but Dirac nor anyone else who matters has claimed that electrons can travel at the speed of light. The thing that travels at the speed of light has been proven to be quantized, i.e. to consist of discrete packets or particles. It has been 84 years since Dirac came up with his theory. It is now generally accepted that Dirac's "sea of electrons" is made up of something other than electrons, i.e. dark matter/energy, in a quantum structure which results in the Casimir effect.

Here's the problem with a "sea of electrons" or virtual particles to explain the quantum structu

http://science.nasa.gov/astrophysics...s-dark-energy/

"But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues."