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Antenna Parameters and Folded Dipole - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


Camelot
 Post subject: Antenna parameters and folded dipole
Posted: Fri Jan 19, 2007 10:42 am 
 
Captain

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 :roll: but I’m continuing studying in order to fill my gaps :wink: .
Regards,

Camelot


 
   
 
nubbage
 Post subject: Antenna parameters and folded dipole
Posted: Mon Jan 22, 2007 5:51 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
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.


 
   
 
Camelot
 Post subject:
Posted: Mon Jan 22, 2007 7:04 am 
 
Captain

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 :roll: ) 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


 
   
 
Camelot
 Post subject:
Posted: Mon Jan 22, 2007 7:06 am 
 
Captain

Joined: Fri Jan 19, 2007 10:36 am
Posts: 7
:( the scheme of antenna goes wrong...

camelot


 
   
 
nubbage
 Post subject:
Posted: Mon Jan 22, 2007 12:36 pm 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
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.


 
   
 
Camelot
 Post subject:
Posted: Tue Jan 23, 2007 11:31 am 
 
Captain

Joined: Fri Jan 19, 2007 10:36 am
Posts: 7
Thank you for your interest!

Camelot




Posted  11/12/2012

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