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Antenna parameters and folded dipole - RF Cafe Forums
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Camelot
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Post subject: Antenna parameters and folded dipole
Posted: Fri Jan 19, 2007 10:42 am
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Joined: Fri Jan 19, 2007
10:36 am Posts: 7 |
Hello, I'm new in antenna simulation world and
I would like to ask you some questions on some topics.
I hope someone could help me. The first question
is if it is true that
Directivity,
Gain and
Efficency
of an antenna are simply tied to the her topology
and not to the goodness of the matching network
or the source impedance, i.e, if I have to measure
these values, should I necessary verify that the
antenna is correctly matched with source? Second
(and last!) question, I have to simulate a folded
dipole with a ground connection in the center (necessary
to the supplying chip as ground reference). The
structure had hence 3 ports. My question is if a
inductive or capacitive load connected to the middle
virtual ground port(and the chip) can significantly
affect the matching and other parameter (D,G) of
the port. In few words, that third branch connected
to a virtual ground, how influence the antenna behavior?
Sorry for my trivial questions
but I'm continuing studying in order to fill my
gaps
. Regards, Camelot
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nubbage |
Post subject: Antenna parameters and folded dipole
Posted: Mon Jan 22, 2007 5:51 am
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General |
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Joined: Fri Feb 17, 2006
12:07 pm Posts: 218 Location: London UK
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Hi Camelot Here is a temporary answer, while
I think about the other points. The antenna performance
parameters you mention assume the antenna is correctly
matched to the feeder cable, and normally assume
the antenna is in free space, that is, it is far
enough away from a ground or other mirror reflector
to be able to ignore its influence. (For some
antennas, however, the ground forms an integral
part of the antenna, eg a ground plane type. Your
folded dipole is not in this class.). The
folded dipole is one of a class of antennas that
is said to be of balanced form. It needs to be fed
from a balanced transmission line or from an unbalanced
one (coaxial cable for example) using a so-called
"balun". Normally the capacitor you refer
to would have an effect on the matching. However,
I do not yet have a mental image of your circuit
arrangement. We need to have a simple graphics feature
on the site that allows us to add simple shapes.
Or maybe you could upload a graphic to a storage
site. I wil think some more.
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Camelot |
Post subject:
Posted: Mon Jan 22, 2007 7:04 am
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Joined: Fri Jan 19, 2007
10:36 am Posts: 7 |
>> The antenna performance parameters you mention assume the antenna is correctly
matched to the feeder cable Hello Nubbage,
thank you for your observation, discussion is the
better way (for me) for understanding complex thing
. About Directivity, Gain and Efficiency: from theory
(correct me if I'm wrong
) Efficiency should be the ratio of Gain out of
Directivity so if those quantity are matching-independent,
I suppose the Efficency is independent too, exact?
Why I thought those quantities (D & G) could
be independent to to matching network? Because directivity
(i.e.) should not vary if I feed the antenna with
less or more power, the same for gain I think….
What do you think about this consideration? About
the folded dipole, this is a simple scheme…
------------------------ | | | -------
| ------- | | | | | | + G - The
central branch (G) is connected to the exact middle
of top. Could any reactance attached to this branch
modify affect the matching and other parameters
(D,G) of the antenna?? Camelot
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Camelot |
Post subject:
Posted: Mon Jan 22, 2007 7:06 am
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Joined: Fri Jan 19, 2007
10:36 am Posts: 7 |
the scheme of antenna goes wrong... camelot
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nubbage |
Post subject:
Posted: Mon Jan 22, 2007 12:36 pm
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General |
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Joined: Fri Feb 17, 2006
12:07 pm Posts: 218 Location: London UK
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Hi Camelot Well, the diagram was a bit scrambled,
but I think I can see what is going on. Many
folded dipoles connect the center of the long part
to the metalwork and this is often grounded, if
only for lightning protection. The 2 short parts
of the folded shape come almost together at the
center point, and are fed with a balanced current
on a parallel line balanced feeder. The two currents
are in anti-phase. Now - if any reactance (or
resistance) is connected between what you show as
the + or the - lines to the G line, there will be
a mis-match. Regarding your other points:
the antenna gain does not depend on the input power.
It depends on the relative focussing power that
comes from the vector sum of all currents flowing
on the antenna surface. I may be wrong, but I
think the difference between directivity and gain
is to do with losses in the material and the ground.
Clearly if a metal antenna has large circulating
currents due to its low radiation resistance, then
the ohmic losses are high and the efficiency (conversion
of input power into far-field electromagnetic energy)
is low.
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Camelot |
Post subject:
Posted: Tue Jan 23, 2007 11:31 am
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Joined: Fri Jan 19, 2007
10:36 am Posts: 7 |
Thank you for your interest! Camelot
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Posted 11/12/2012
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