Loop vx Folded Dipole noise factor
 

Does a folded dipole antenna have the same low noise characteristics as
a loop antenna?



Ed K7AAT

On Mon, 20 Dec 2004 19:57:42 GMT, Ed
wrote:
Does a folded dipole antenna have the same low noise characteristics as
a loop antenna?

Hi Ed,

Yup, the same as a standard Dipole.

If what you are saying about "low noise" is from charge accumulation
(snow, rain, dust....) then the DC short helps in comparison to the
standard Dipole. However this is simply remedied with a large
resistor (1 KOhm) or choke across the feed point of the standard
Dipole.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

wrote:
Does a folded dipole antenna have the same low noise characteristics as
a loop antenna?

Yup, the same as a standard Dipole.

Assume you meant: "the same as a standard loop."?


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Does a folded dipole antenna have the same low noise
characteristics as
a loop antenna?


Yup, the same as a standard Dipole.


Assume you meant: "the same as a standard loop."?


Yes, I hope that he meant "loop" instead of "dipole" . I still would
like to hear from others on this, too. I am re-considering my planned
installation of a loop now, in favor of the tactically easier to install
folded dipole, if I can maintain that low noise charactistic.

Ed K7AAT



Ed

On Mon, 20 Dec 2004 22:25:43 GMT, Ed
wrote:

Yes, I hope that he meant "loop" instead of "dipole"

Hi Ed,

Aside from the charged particles introducing noise, there is no
difference. If charged particles is the noise, then I offered a
solution that reduces the issue of antennas to: no difference.

If it relates to corona discharge, then you may have to ask yourself,
what about the difference between corner fed and center fed loops
(this made a huge difference to HCJB in Quito).

Just tossing out the term "low noise" is in itself a reliance on hopes
and wishes to sort out what the noise is. This is because forcing the
discussion into more context invariably reveals the old nostrum of
magnetic antennas being immune from electrical noise. Then we get
into all sorts of equivocation which in the wash inevitably reduces
to: no difference.

73's
Richard Clark, KB7QHC

Ed wrote:
Does a folded dipole antenna have the same low noise
characteristics as
a loop antenna?

Yup, the same as a standard Dipole.

Assume you meant: "the same as a standard loop."?

Yes, I hope that he meant "loop" instead of "dipole" . I still would
like to hear from others on this, too. I am re-considering my planned
installation of a loop now, in favor of the tactically easier to install
folded dipole, if I can maintain that low noise charactistic.

You can get the same effect with a dipole by installing a toroidal
inductance (RF choke) across the feedpoint or (surprise) by using a 4:1
voltage balun. Back when I didn't know any better, I used a 4:1 voltage
balun and it was quiet but ineffective as a balun on some bands. Things
got noisier when I switched to a 1:1 choke. An RF choke solved the
problem.
--
73, Cecil http://www.qsl.net/w5dxp


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Richard Clark wrote:

wrote:
Yes, I hope that he meant "loop" instead of "dipole"

Aside from the charged particles introducing noise, there is no
difference.

Point is that a loop reduces that "noise" where a dipole allows the
"noise" to build up until it arcs across the coax connector which
causes an extreme amount of "noise". One night in AZ, my PL-239
connected to my IC-745 was arcing so loud that it woke me up. I
disconnected it and tossed it on the floor. A few minutes later,
the carpet was smoking.

Aside from the arcing waking me up and the burning of the carpet,
there was no difference.
--
73, Cecil http://www.qsl.net/w5dxp


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Ed:

You got some interesting discussion, but I wonder if any actually heard the question. Unfortunately, the question is
incomplete, and this often leads to answers to questions you never asked. What particular kind of noise do you
characterize?

Loops respond to magnetic fields, and dipoles respond to electric fields. So what, you ask? Well, if the noise you are
concerned with is locally generated RFI stuff, and you are within the near field, the shielded loop often provides
better signal to noise ratio on distant signals because of some fundamental characteristics of near electromagnetic
fields. Which Roy or Reg or a lot of others can explain a lot more lucidly than I can. Other slightly further away
noise is likely to be vertically polarized, again due to the propagation characteristics of surface waves, and any
antenna which is more sensitive to horizontal polarization and less to vertical can exhibit batter S/N on distant
signals. If, on the other hand, you are trying to receive weak local signals in the presence of strong distant noise,
well, then the reverse tends to be true. The point? No specific one answer fits all cases. If it did, there would be
only one kind of antenna.

--
Crazy George
Remove N O and S P A M imbedded in return address
"Ed" wrote in message . 93.175...


Does a folded dipole antenna have the same low noise characteristics as
a loop antenna?



Ed K7AAT

Crazy George wrote:
. . .
Loops respond to magnetic fields, and dipoles respond to electric fields. . .

That's not at all true. Both kinds of antennas respond to both electric
and magnetic fields.

An *electrically small* loop responds more strongly to a magnetic than
an electric field only if the source of the field is much closer than a
wavelength. Likewise, an *electrically small* dipole responds more
strongly to an electric than a magnetic field only if the source of the
field is much closer than a wavelength. At some distance from the source
still less than a wavelength, they actually reverse -- the short dipole
responds more strongly to a magnetic field than a small loop, and the
small loop responds more strongly to an electric field than a short dipole.

The response of electrically large (on the order of a half wavelength
and larger) dipoles and loops to electric and magnetic fields depends on
the direction and distance to the source. No single rule of thumb can be
used when the source is very close to anything but an electrically small
antenna. The relative responses of *all* antennas to electric and
magnetic fields are essentially the same as each other if the source is
a fair fraction of a wavelength away (i.e., the antenna is in the far
field of the source).

Roy Lewallen, W7EL

You got some interesting discussion, but I wonder if any actually
heard the question. Unfortunately, the question is incomplete, and
this often leads to answers to questions you never asked. What
particular kind of noise do you characterize?


My thanks to all for responses to my quetion. And my somewhat belated
response to the comment (above): I guess I must be somewhat ambiguous in
reference to the low noise of a loop. I have always heard that proper
loop antennas had a lower noise characteristic than other open ended
antennas. The type of noise in question, I don't know. I suspect at my
locations I have a combination of man made and atmospheric noises being
received at my presesnt dipole.

Since I intend to build a new antenna, and since I am now forced to
consider the construction of a folded dipole in lieu of the more
difficult to erect loop, that was the reason for my original question.

I have two nice 100'+ trees between which it would be quite easy to
install a dipole..... whether it be folded or standard depends on the
comments I receive from you guys on the low noise characteristic I am
questioning.

Ed K7AAT

"Ed" wrote

I have two nice 100'+ trees between which it would be quite easy to
install a dipole..... whether it be folded or standard depends on the
comments I receive from you guys on the low noise characteristic I am
questioning.

Ed K7AAT

Ed. if you let the ends of the dipole be the high points and avoid the
inverted-vee (skip the temptation for a little wider bandwidth of the vee,
it comes at some expense of optimum dipole performance and low noise of a
real dipole) you should be very happy with new antenna. I too use nearly
100' trees and achieve between 1/4~ and 1/2~ elevation for the 60-70 meter
bandwidth. I stopped at that height only because it was a point at which the
tree trunks exhibited very little movement. With a 125' span between them,
that was important! Higher would be better, but probably not survive high
winds either. 3/16" milspec dacron holds a balun (supported only from both
ends), the antenna copper, and all 60-odd feet of RG-8X feedline under it
with no problem. I use kevlar standing rigging to hold the blocks of the
running rigging for this antenna. This allows more flexibility than if the
main horizontal support line made a bight over each tree limb at its two
ends. It has survived two hurricanes, many Nor'Easters and a lot of ice as
well. Not too many locales have all those extremes ;-)

73,
Jack Painter
Virginia Beach, Virginia

Roy, W7EL wrote:
"The response of electrically large (on the order of a half wavelength
and larger) dipoles and loops to electric and magnetic fields depends on
the direction and distance to the source."

Yes. The dierectional response of a 1/2-wave folded dipole is the same
as that of a 1/2-wave open-circuit dipole. The main difference may be
the difference in impedance presented to the transmission line.

Here is my experience. Lightning is an extremely large noise. My
corporation used VHF radios to interrogate remote data stations. For
decades we used Andrew Corporation folded 1/4-wave unipoles atop high
towers around the world in base stations to communicate with mobiles in
any direction. These had proved indestructable. Both the stainless steel
antenna and the radio used, with no lightning protection on the
feedline, other than the Heliax used to connect the antenna with the
radio. This was before we started the data radio operation. The Heliax
is a common-mode rejecter due to its equivalent circuit.

For the data radios, the operation was point-to-point. Directional
antennas were useful in this service. The data radios immediately
started to be destroyed by lightning strikes. Problem was the yagi
antenna. The driven element was an open circuit. We quickly fixed that
with an short-circuit 1/4-wave stub shunted across the antenna at its
feedpoint. No more lightning damage. The short-circuit removed enough of
the off-frequency noise (lightning) to save the radios. So the operation
continues decades later with Motorola transistorized mobile radios as
the data base and remote stations.

Best regards, Richard Harrison, KB5WZI

Ed wrote:


I have two nice 100'+ trees between which it would be quite easy
to
install a dipole..... whether it be folded or standard depends on the

comments I receive from you guys on the low noise characteristic I am

questioning.

Ed K7AAT

Myself, I would just build the standard dipole. As far as any far field

noise, I consider the two types the same in performance. The main
difference being the impedance step up on the folded version. As
Richard
mentioned, there may be lightning advantages to the folded version, but
for general HF noise, I don't think it would matter. You have to
consider
what noise is. It's RF. Your antenna is *supposed* to be able to
receive
noise. If it doesn't , it's defective as far as I'm concerned. Many
build
antennas that they claim to be low, or lower noise, but myself, I'm of
the
opinion they are usually building lower efficiency antennas which
naturally pick up less noise. To decrease the pickup of far field
noise,
you can change polarization, directional pattern, or efficiency. Being
as
your proposed dipoles will have the same polarization, and pattern, the
only thing left to reduce noise pickup is efficiency. Do you *really*
want to lose efficiency in order to reduce far field noise pickup? I
sure wouldn't...There may be an advantage as far as static buildup with
the folded dipole, but do you
actually have a static buildup problem? I don't have that problem here
in Houston. It's hardly ever dry enough around here.. Any common mode
noise problems are dealt with using baluns, chokes, etc...
To make the best decision, you need to know what noise? you are trying
to cut.
If it's general far field noise, you are chasing a dream that doesn't
really exist, unless you want to degrade your antenna system, or make
it directional, etc. I *want* my antennas to pick up noise, if it's
there to be picked up.
Lets me know they are working...:/ MK

There are three common sources of HF noise:

1. (Propagated) atmospheric noise. There's no magic antenna that can
tell the difference between atmospheric noise and signals coming from
the same direction as the noise. An antenna that attenuates one will
attenuate the other by an equal amount. A directional antenna will
improve the signal to atmospheric noise ratio only if the two are coming
from different directions and the antenna can be oriented to favor the
signal and reject the noise.

2. Static discharge. Rain and snow can carry static charges, creating
noise at the antenna itself. Also, a gathering storm can cause a large
static potential to build up on an ungrounded antenna, even to the point
of corona at the antenna ends and arcing across the feedpoint. I saw
this many times in Denver, but never here in western Oregon. This can be
prevented by making a DC short or near short across the feedline with a
resistor, RF choke, or shorted stub, or by using an antenna that
inherently has a DC short across the feedpoint, and by avoiding any
sharp points like wire ends. A loop of any size has both these features,
although there are other ways to get them.

3. Local noise. One antenna can be much better or worse than another at
rejecting noise that originates close to the antenna. Often, horizontal
polarization is better than vertical for rejecting local noise, or any
noise that's within surface wave range. If the noise source is very
close -- within a fraction of a wavelength -- a small loop might help,
since it's relatively insensitive to electric fields from sources which
are very close by. However, it's difficult to make a small loop that's
efficient, so it's usually a better choice for receiving, with some
other antenna being used for transmitting.

Roy Lewallen, W7EL

Ed wrote:

My thanks to all for responses to my quetion. And my somewhat belated
response to the comment (above): I guess I must be somewhat ambiguous in
reference to the low noise of a loop. I have always heard that proper
loop antennas had a lower noise characteristic than other open ended
antennas. The type of noise in question, I don't know. I suspect at my
locations I have a combination of man made and atmospheric noises being
received at my presesnt dipole.

Since I intend to build a new antenna, and since I am now forced to
consider the construction of a folded dipole in lieu of the more
difficult to erect loop, that was the reason for my original question.

I have two nice 100'+ trees between which it would be quite easy to
install a dipole..... whether it be folded or standard depends on the
comments I receive from you guys on the low noise characteristic I am
questioning.

Ed K7AAT