antenna-theory.com

[joeyg]

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]

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]

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]

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]

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]

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]