# Choose antennas for RF (915 MHz)

I am designing a PCB and I will use a 2 dBi antenna like this image. But the point where my radio should connect has a 9 dBi antenna like this picture here.

I know everything will work perfectly, but the question is: this antenna 9 dBi is very expensive and it really will make a difference in the whole set? I say this because I think that to take advantage of all pontencial this antenna I would need to have two identical antennas.

I think my antenna 2dBi (placed on the radio) is that will dictate the maximum distance of communication and not 9dBi antenna. I am very wrong? Would not the same just use two equal 2dBi antennas?

• I know this sounds like a strange question, but do you understand what dBi means? Aug 24, 2016 at 13:57
• Marcus, Not really :( I read wikipedia about it, but it was only Aug 24, 2016 at 14:00
• well, then you should probably ask a different question – what's the difference between a 2dBi and a 9dBi antenna? Aug 24, 2016 at 14:01

An antenna with a gain of 9 dBi means it has a radiated power per square metre in its "best" (or optimum) direction that is 9 dB greater than an isotropic antenna in any direction. An isotropic antenna emits the same power density in all directions and although it sounds really useful is an impossible dream BUT useful as a concept.

This means that the 9 dBi antenna MUST not be as good as an isotropic antenna in some directions and this could be a big problem to some applications.

On the other hand, a 2 dBi antenna is like a half wave dipole and it will produce a doughnut shaped power density that has 2 dB gain in X and Y directions: -

The down side is that it produces very little power (theoretically zero) in the Z direction. So, if you add all the power densities up and calculated an average you would find that the average power density produced is the same as an isotropic antenna.

What you gain in one respect you lose in another because a high gain antenna becomes very directional.

• +1 in general, but isotropic antennas are possible. One downside is that the polarization comes out as it comes out. I used a isotropic antenna in a movable product once where the antenna was very carefully folded up to radiate reasonably equally in all directions. It took many hours with high-end simulation software (not done by me). The receivers were stationary, so they had center-fed helix antennas. Those have a dipole radiation pattern, but work equally at all polarizations. Think of them as a blend between loop and dipole. Sep 23, 2016 at 15:36
• @OlinLathrop, doesn't isotropic mean a spherical radiation pattern?
– user98663
Sep 23, 2016 at 17:58
• @Woss: Yes. --- Sep 23, 2016 at 18:23
• @Woss: Of course you're never going to get a perfect isotropic pattern due to physical realities. In our case, the transmitter was battery operated and sortof inside the antenna, so no external feed. As I said, you don't get to choose polarization when the radiation pattern is spherical. We used spherical radiators, then fixed-position receivers with a dipole pattern but that accepted any polarization equally well. Sep 24, 2016 at 22:18
• @Woss: It was too long ago that I don't have units available to me anymore. The receiver used center-fed helix antennas. Think of a helix as a blend between a loop antenna and a dipole. Picture the loop in a horizontal plane and the dipole vertical. Both have the same radiation pattern, but opposite polarization. With just the right shape helix, it works half like a loop and half like a dipole. It has the same radiation pattern as either, but can pick up waves with any polarization equally. Sep 25, 2016 at 21:40