This is an old revision of the document!

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 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 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 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.

The easiest way to get a working Iridium antenna is to just buy a commercial one.

These are usually passive antennas as they are intended for bidirectional communication with the satellite. This means that, depending on your setup you might have to add some kind of amplifier right next to the antenna.

Here is a list of easily available antennas:

Name Manufacturer Connector Approximate Cost Distributors
33-2600-00-0500 Tallysman Wireless Inc Short SMA cable 70 USD Digi-Key Farnell(DE) HBE(DE)
33-3600-01-11 Tallysman Wireless Inc TNC 70 USD Digi-Key
IAA.01.121111 Taoglas 1 m SMA cable 25 USD Mouser
ADA-A1621-S ADACTUS 2 m SMA cable 50 USD Farnell(US) Farnell(DE) HBE(DE)

Most of these have a magnetic mount. They need some kind of metal plane bellow them to work properly. A small metal patch of about 20 cm in diameter (round or square) is more than adequate.

As the GPS band is very close to the Iridium band, active GPS antennas can be used as a basis for an active Iridium antenna.

This is probably the cheapest option to get an active Iridium antenna. The process is pretty straight forward and can be adapter to many different types of active GPS antennas.

Active GPS antennas contain a bandpass filter to block all signals which are not part of the GPS signal. This is done as the amplifiers will also amplify the signal of all other sources and forward this to the receiver. As the amplifiers are not perfect and the GPS signal very weak, this can have an impact on the quality of the GPS signal. For our purposes these filters are a problem though. They basically make an unmodified GPS antenna unusable to receive Iridium antennas.

We found that after removing this filter, these antennas work quite OK as Iridium antennas. They work even better if you replace the standard patch antenna with a special Iridium patch antenna.

Cheap active GPS antennas cost around 10 USD (7 Eur). Try to get one with screws so you can access the inside without damaging the protective case. Here is an example:

  • iridium/antennas.1422218508.txt.gz
  • Last modified: 2021/04/18 12:32
  • (external edit)