# RE: Can anyone clarify dBic?

• Subject: RE: [amsat-bb] Can anyone clarify dBic?
• From: Tony Langdon <tlangdon@xxxxxxxxxxxxxxxxxx>
• Date: Wed, 13 Jun 2001 10:57:34 +1000

```> I think there is a mistake here.

I thought so when I first read it too...

> If you phase two linear yagis for circular polarization and
> each yagi has a
> stand alone linear gain of 10 dB, then the gain of the
> circular pair is
> still only 10 dB....but it is now circular. In other words,
> it takes TWO
> linearly polarized yagis to make circular polarization from
> the pair.....you
> don't get to add things in twice. The gain you would have
> gotten from a
> normal linear stack (3 dB), is actually not realized because
> of the games
> being played to get a pair of linear antennas to act as a
> circular "array"..

Agreed.  if the Yagis were polarised the same way, the vector sum of the EM
fields combines to narrow the beamwidth and increase the gain.  However, if
crossed and few 90 degrees out of phase, the vector sum of the two fields is
a circularly polarised wave, but of only 10 dBic gain.

> A linear receiving antenna results in a loss of 3 dB. So I
> agree with the
> conclusion that the net receive is only + 7 dB...but I don't

That can be easily determined.  If we assume the transmitting antenna is a
pair of 10 dBi gain Yagis appropriately fed, then we know that these Yagis
are fed by dividing the power equally between them.  Assuming our receiving
antenna is linearly polarised and is in the exact same plane as one of the
transmitting Yagis, we can deduce the folloring:

1.  The receiving antenna will be excited to the maximum extent by the field
of the Tx antenna in the same plane.
2.  The cross polarised Tx antenna will have negligible effect (remember the
-20 to -30 dB cross polarisation loss at VHF?) on the received signal.
3.  Since we're feeding only half of the original Tx power into a 10dBi
antenna, the net effect is that the power divider has a 3dB loss (hardly
surprising!), so the total gain of the linear component of the antenna
system is 7 dBi.

4.  Since it doesn't matter how a CP wave is generated, the same holds true
for any CP to linear circuit.

> There's no free lunch.

That's true.
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