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joeyg
Antenna-Theory.com Newbie

Joined: 03 Mar 2011
Posts: 3
Location: Australia

 Posted: Thu Mar 03, 2011 1:21 am    Post subject: Antenna Gain dBiC and dBi 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
bigSteve
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Thu Mar 03, 2011 2:44 am    Post subject: 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
joeyg
Antenna-Theory.com Newbie

Joined: 03 Mar 2011
Posts: 3
Location: Australia

 Posted: Fri Mar 04, 2011 7:14 am    Post subject: 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
bigSteve
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Fri Mar 04, 2011 11:21 pm    Post subject: 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
joeyg
Antenna-Theory.com Newbie

Joined: 03 Mar 2011
Posts: 3
Location: Australia

 Posted: Sun Mar 06, 2011 10:13 am    Post subject: 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
bigSteve
Antenna Wizard

Joined: 14 Mar 2009
Posts: 265

 Posted: Sun Mar 06, 2011 8:06 pm    Post subject: 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.
R. Fry
Antenna Theory Regular

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

Posted: Mon Jun 06, 2011 12:21 pm    Post subject:

 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.
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