We have realized all of the original design goals for our station except one – operation with Low Earth Orbit (LEO) HAM Satellites. Our next series of posts will be about realizing this operating goal. The team at AMSAT has been working diligently to get a new generation of small satellites called Cube Sats into space to provide additional “birds” for HAM satellite operators to use so now seems to be a good time to add satellite capability to our station.
Most recent LEO satellites use a combination of 2m and 70cm signals for their uplink and downlink frequencies so we’ve decided to build an antenna system for satellite operation on these bands. Satellites which use these frequencies would be either U/V Mode (70 cm uplink, 2m downlink) or V/U Mode (2m Uplink, 20 cm Downlink) birds. We installed an Icom IC-9100 Transceiver some time ago for both VHF/UHF weak signal work as well as for future satellite operations. The IC-9100 provides 100w of Transmit power on 2m and 75W on 70cm. This is more than adequate for LEO Satellite operation. The IC-9100 also has some nice VFO tracking features to make operation with satellites that use linear transponders easier (more on this in a future post).
I have been working with the folks at M2 Antenna Systems to come up with a simple antenna system to allow us to work LEO satellites. LEO Satellites do not require a great deal of radiated power to work but do require a reasonably sensitive receive system. The design that I have settled on will use M2’s Eggbeater Satellite Antenna System. This setup combines a set of omnidirectional 2m and 70cm antennas on a cross-boom to support U/V and V/U mode satellites.
The “eggbeater” antennas uses a reflector (radial elements below the “eggbeater” elements shown above) to direct the antenna’s pattern upward. The resulting pattern is circularly polarized and is omnidirectional. This is ideal for LEO satellite operation and does not require a complex rotator system to “point” the antenna.
While it does not take a great deal of radiated power to work LEO Sats, a fairly sensitive receive system is required due to the low transmit power used for the downlink on these birds. We are planning to mount the antennas for our satellite system at around the 90 ft level on our tower. This distance plus the run from the tower to our shack will result a total feedline length of about 190 ft. To ensure that the feedline losses do not compromise the performance of our satellite system, we have decided to install tower-mounted Low Noise Preamplifiers (LNAs) on our tower near the antennas. We are using a 2m LNA Preamplifier from M2 Systems and 70cm LNA Preamplifier for Advance Receiver Research.
Mounting electronics on the tower is always a reliability concern. There are two issues to be addressed here. The first is proper sequencing control to ensure that the RF power going to the antennas during transmit does not destroy the preamplifiers and the second is to provide adequate protection from the weather. We chose to use M2 Antenna Systems S3 Sequencers to safely switch the LNAs to bypass during transmit (more on this below). We also decided to mount the LNAs and associated control interconnect points inside a NEMA enclosure. The picture above shows all of the parts and components which make up the LNA preamp system. The components include (from top left to lower right) the LNA preamps, a NEMA enclosure purchased at our local electrical store along with stainless steel clamps to attach it to our tower, N-type feed-through connectors to pass the feedline connections in and out of the enclosure, a terminal strip for control cable and power connections, hardware to mount the preamps in the enclosure, LMR-240 coax for RF connections inside the enclosure, the M2 Antenna Systems S2 Sequencers, and N-connectors for the LMR-240 coax jumpers.
The picture above shows the completed preamp system ready to go onto the tower. The unit is designed to allow easy access to the electronics for testing and service on the tower should this be needed.
The picture above shows the M2 Antenna Systems S3 Sequencers that we are using to control the preamps (the S2 sequencers at the bottom control the tower mounted electronics for our 2m and 70cm weak signal yagi antennas which were installed previously). We connected the S3 sequencers to our Icon IC-9100 Transceiver and (temporarily) to our new Satellite preamp system so that we could test the package before installing the electronics on the tower. We checked the SWR of the system in transmit mode using a dummy load as well the operation of the preamps in receive mode.
The final design decision for the hardware side of our LEO Satellite system is the choice of coax for our feedlines. The total length of our feedlines including the connections between the tower mounted preamplifiers and the antennas as well as the connection from our shack entry point to the radio is about 190 ft. I set a design goal of 1.5 dB of total loss for this path. To meet this goal at 450 MHz (70 cm), I have decided to use 7/8″ Hardline Coax (LDF5-50A) plus LMR-400UF Coax for the jumpers to connect the antennas at the tower end and the IC-9100 Transceiver at the shack end. We will also be using N-type connectors throughout the feedline system.
We have been doing quite a bit of reading to learn about how design and build our Satellite System. One excellent source of information on this topic is the ARRL Satellite Handbook. I can recommend this book as a great source of information for anyone considering the construction of a satellite system and for those interested in learning more about satellite operations.
Our next steps will be to order the hardline and connectors and assemble our antennas. Once the materials arrive, we will install the antenna system on our tower. This will be the topic of the next post in this series.
Other articles in the series include:
- LEO Satellite System Part 2 – Antenna Assembly and Ground Test
- LEO Satellite System Part 3 – Final Installation and First Contacts
You might also be interested in the series on our Portable Satellite Station. You can read about that here.
– Fred (AB1OC)