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Antenna Gain of -3dB IS ACTUALLY GOOD, Here's Why!

Many customers are afraid to buy an FM antenna that has a negative gain. They seem to feel that they are losing power. Nothing could be further from the truth!

In this article, I will use our PCP1 circularly polarized FM antenna as an example.

The PCP1 has an antenna gain of -3dB. So that means that you are losing half of your power, right? This is not a true statement, here's why...

When the gain of an antenna is measured, it is only measured in one polarization plane, not both. What this means is that the gain is only measured in either the vertical OR horizontal plane. The PCP1 is a circularly polarized antenna, so there are both vertical and horizontal elements on the antenna, consequently, there are both vertical and horizontal components to the signal radiating from the antenna.

In true circular polarization, half of your signal is in the vertical plane while the other half is in the horizontal plane. So 50% of your power is in the vertical plane and 50% of your power is in the horizontal plane. ALL OF YOUR POWER (100%) IS STILL THERE! You don't lose anything!

When looking at the gain figure for ANY antenna, we have to remember that there is no magic effect that the antenna can have to either amplify or destroy your RF power. It's simply a matter of how the power is distributed or radiated from the antenna.

Likewise, when an antenna has a positive gain of +3dB, your power is not really doubled. Rather the antenna is simply re-distributing power from one area and placing it in another giving the illusion of increased power.

Let's look at it this way; imagine if you had a special pair of glasses that would allow you to "see" RF power coming off of your antenna. If you stood back far enough away from your antenna, and your antenna had a power gain of 0dB (meaning it had no gain or loss) the signal would be round, like the shape of a huge beach ball.

Now let's say that we add +3dB gain to the same antenna, it would be like you took your hands and put one on top of the beach ball and the other on the bottom of the beach ball and applied a little pressure. The ball would be "squashed" a little bit so that it would be flatter on the top and bottom, but the outer edge of the ball would "bulge" out and become larger. In this same way, an antenna with gain "pushes" the signal out around the outer edge while pulling it in from the top and bottom.

No extra power is created. Rather the exact same amount of power is coming off of the antenna, but being distributed differently.

This is why having a circular antenna such as the PCP1 is actually better than having a vertical only antenna that has a positive gain. The reason for this is because the listener's receiving antenna can come in all shapes and sizes (vertical and/or horizontal types). When you are broadcasting on FM, it is best to send your signal using an antenna that has the same format or polarization as the listener's receiving antenna. Since receiving antennas are available in vertical (like a stick antenna on older automobiles) or horizontal (like the 300-ohm twin-lead antennas on most home stereo systems and/or window antennas on cars), it is best to transmit in circular polarization (both horizontal and vertical polarization). In this way, the broadcaster has both bases covered when it comes to matching up his transmit signal with that of his potential listener.

This is why almost ALL professional FM broadcast stations are using an antenna with circular polarization and not vertical only polarization.

Since the FCC only measures the power in one plane (vertical or horizontal) you are able to actually put twice as much power into a circular antenna (such as our PCP or CIRPA antenna) than you can put into a vertical only antenna. By putting twice the power into the antenna, you are effectively putting twice the power into the air, which gives you much better coverage than any "vertical only" antenna could ever provide.

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