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

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 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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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

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