Antenna - Theory .com: The Antenna Site

Antenna Basics Antenna Types Smith Charts Antenna Measurements

antenna-theory.com :: View topic - Antenna Gain dBiC and dBi
antenna-theory.com Forum Index antenna-theory.com
Antenna Theory
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Antenna Gain dBiC and dBi

 
Post new topic   Reply to topic    antenna-theory.com Forum Index -> Questions Related to Antenna Theory
View previous topic :: View next topic  
Author Message
joeyg
Antenna-Theory.com Newbie


Joined: 03 Mar 2011
Posts: 3
Location: Australia

PostPosted: Thu Mar 03, 2011 1:21 am    Post subject: Antenna Gain dBiC and dBi Reply with quote

Hi All,

Thanks for the website and forum..... I have long since learned that "antenna is almost everything in RF". Since by CB and HAM radio days.

I have a question about two circular polarized antennas.

How can I compare these two antennas if one is gain rated in dBiC and the other is dBi.

Regards
Joseph
Back to top
View user's profile Send private message
bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Thu Mar 03, 2011 2:44 am    Post subject: Reply with quote

Do you have a link to some spec sheets that have this so I can take a look?

I believe dBi and dBic should be the same thing if the antennas are specified as circularly polarized....

I think the i is redundant, as a directivity in dB is pretty clear already. Sometimes people overspecify things and make them more complicated than they should be
Back to top
View user's profile Send private message
joeyg
Antenna-Theory.com Newbie


Joined: 03 Mar 2011
Posts: 3
Location: Australia

PostPosted: Fri Mar 04, 2011 7:14 am    Post subject: Reply with quote

Hi bigSteve,

I have the spec sheets in the office.... but definately one says dBi and the other is dBiC. Both circular polarized antennas.

I did see on TI document that said dbi = dBiC - 2.15 dB


Regards
Joe
Back to top
View user's profile Send private message
bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Fri Mar 04, 2011 11:21 pm    Post subject: Reply with quote

They do state that:
dBiC = dBi - 2.15

In my professional opinion, that equation is nonsense and has no basis.

Now, the directivity of a half-wave dipole is 2.15 dB:
http://www.antenna-theory.com/antennas/halfwave.php

I have seen the equation dBd = dBi - 2.15, where dBd is decibels "relative to a dipole antenna", which makes sense.

The 2.15 doesn't arise anywhere though for circularly polarized antennas. I have no idea where they got it and believe it to be incorrect.

Further, the document states "In your calculations you will need to compensate for the lower output of a circular polarized antenna".

There's no need for a circularly polarized antenna to have lower output power....circular polarization has no "loss" or reason that the output power should be lower than a linearly polarized antenna. There certainly are high efficiency circularly polarized antennas for which the above equation (subtracting 2.15dB) would definitely not apply (example, helix antenna: http://www.antenna-theory.com/antennas/travelling/helix.php)

Anyway, if you find an explanation for the equation, I'd like to hear it.

Thanks
Back to top
View user's profile Send private message
joeyg
Antenna-Theory.com Newbie


Joined: 03 Mar 2011
Posts: 3
Location: Australia

PostPosted: Sun Mar 06, 2011 10:13 am    Post subject: Reply with quote

Hi bigSteve,

Please check out the specification of the Laird CushCraft RFID circular polarized antenna. They state a 8dBiC and 6dBi




What I am looking at ......
I believe the antennas have similar design, but one is rated at 9dBiC and the other at 6dBi..... and I would like to technically compare the two before purchase based on their data sheet.

Regards
Joe
Back to top
View user's profile Send private message
bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Sun Mar 06, 2011 8:06 pm    Post subject: Reply with quote

I took a look at the spec sheet, and it says the antenna provides "reception of linear and circularly polarized signals".

The gains are quoted as 8dBic and 6 dBi.

Here's what I think the deal is. If the antenna was perfectly circularly polarized, then the received energy from a linearly polarized wave would be 3 dB less. That is, for a perfectly circularly polarized antenna of gain 8 dBic, the linearly polarized gain would be 5 dBic.

This is simply because circular polarization is the sum of two orthogonal linear polarizations, each 90 degrees out of phase: http://www.antenna-theory.com/basics/polarization.php

Of course, no antenna is perfectly circularly polarized. I think in this case, they measured the gain pattern, and calculated what the gain would be for a waves of both polarizations (linear and circular).

The spec sheet goes on "VSWR and axial ratio are both excellent and allow the user to achieve the maximum performance for an antenna of this type."

Axial ratio is the ratio of the magnitude of the orthogonal E-field components. For circularly polarized antenna is should be close to 1, but they don't specify it. The "maximum performance" is a pretty ambiguous statement to make.
Back to top
View user's profile Send private message
R. Fry
Antenna Theory Regular


Joined: 06 Jun 2011
Posts: 43
Location: Illinois USA

PostPosted: Mon Jun 06, 2011 12:21 pm    Post subject: Reply with quote

bigSteve wrote:
Axial ratio is the ratio of the magnitude of the orthogonal E-field components. For circularly polarized antenna is should be close to 1, but they don't specify it. The "maximum performance" is a pretty ambiguous statement to make.


An axial ratio of 1 for a circularly polarized radiator means that E-fields of every rotation angle are equal, not just those with an orthogonal relationship.

For example, a linear dipole antenna in free space has equal orthogonal E-field components for two rotation angles with respect to its radiation center, but clearly a linear dipole is not a circularly polarized radiator.
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    antenna-theory.com Forum Index -> Questions Related to Antenna Theory All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group