Iridium Antennas

Iridium transmits around 1.626 GHz and uses right-hand circular polarization (RHCP).

To receive Iridium signals you need an antenna which is able to receive at this frequency ideally also uses RHCP.

If the antenna uses liner polarization, you can expect a loss of about 3 dB. If the antenna is left-hand circular polarized, you can expect a loss of about 30 dB. See http://sv1bsx.50webs.com/antenna-pol/polarization.html for details on this topic.

All following antennas used RHCP unless otherwise noted.

An active antenna is a passive antenna immediately followed by an active circuit. Most of the time, this circuit is an amplifier which amplifies the received signals.
This is done to counter effects of long cables or noisy amplifiers at the input of the receiver.

Just as every other circuit, these amplifiers also add noise to the system. They are not a magical way to improve the signal quality which is available at the antenna.
They are still very useful though. If you don't want to (or can) place your SDR right next to the antenna, you can use them to counter the losses of the cable. See http://fabiobaltieri.com/2014/06/22/lna/ for some details on that.

Another use for these amplifiers arises if the first amplifier of the receiver has a large noise figure. This is usually not a problem with higher end devices like an USRP or a HackRF, but can help a little with RTL-SDR based receivers. The reason for this is that the total noise figure of the system is basically determined by the noise figure of the first amplifier. See http://www.microwaves101.com/encyclopedias/noise-figure for some details.

Active antennas have one small and one major drawback though:

• They need some from of power supply to perform the amplification.
• Without additional circuitry, a simple active antenna can not be used to transmit a signal.

The power to operate the active antenna is usually supplied via the same coaxial cable which is used to receive the signal. Have a look at [TODO] for some details on this.

The major drawback of not being able to transmit might or might not be a problem for your particular use. Just be aware of this fact.

In general: If you have a good SDR with a low noise figure and a short cable between the SDR and your antenna: Do not use an active antenna. It will not improve the signal a lot and just cause trouble if something does not work.

If you have a not so good SDR with a high (or unknown) noise figure or a long cable between your SDR and the antenna: Consider using an active antenna. It will improve the noise figure of the system and/or counter the losses inside the cable.