The Hope RFM22B is an example of a low cost FSK transceiver module that was able to provide a
communications link for two low Earth orbit amateur satellites, $50SAT and T-LogoQube. The
results produced by the T-LogoQube team being particularly impressive as they demonstrated 2
way communications at up to 2,700km with only 100mW output from the satellite.
Such long distance operation of the FSK transceiver modules required the use of a 50W power
amplifier for uplink, a low noise amplifier (LNA) for the downlink and an antenna farm of accurately
tracked twin 20dB long yagi arrays to follow the satellites path across the sky. Whilst it worked well
the equipment is not simple to set up or operate and is expensive.
The $50SAT team, Howie DeFelice, Michael Kirkhart and Stuart Robinson were able to receive the
data telemetry at up to 1200km from $50SAT using hand held antennas and low noise amplifiers.
These ISM band transceivers have up to 100mW output although for license exempt ISM use the
limit is normally 10mW. For amateur band applications amplifying the power from these simple
434Mhz transmitters is not that difficult. Mitsubishi do a series of UHF power amplifiers that will
take a low input power, 25mW or less, and boost it to as much as 60W.
FSK transceiver modules do need a good LNA in front of the receiver for long distance operation,
the LNA adding around 12dB of useful signal gain, which is a range improvement of about 4. To
use the LNA and power amplifier together you need either separate antennas for TX and RX, (not
very convenient) or sequencing and switching of the single antenna between RX and TX modes.
This is all possible of course, but is not simple or cheap to set-up.
The $50SAT satellite side installation was an example of a ‘keep it simple’ system, but this was
clearly not the case for the ground station set-up. To reduce complexity and cost we need to
eliminate the LNA, power amplifier, output filters, sequencer, antenna farm, rotators and connect
our humble transceiver module direct to a single simple antenna, but how far are we going to get
at only 10mW or 100mW with FSK transceivers? Assuming ¼ wave antennas at both ends of a link an
FSK transceiver will at 1000bps go around 12km at 10mW and 40km at 100mW. 12km for 10mW or 40km for 100mW is good, but it does notallow for a low complexity set-up for satellites or high altitude balloon use where ranges of 200 to
1000km may be needed. FSK transceiver modules can be used for long range reception
however, by using a low cost LNA and or high gain antennas. In mid 2014 I launched a
Pico balloon using an RFM22B as the transmitter. With a Diamond X50N vertical antenna (
5dB low horizon gain) and £50 LNA kit I received 1000bps telemetry from the balloon tracker
with 10mW output at 174km. Since balloons don’t move around the sky much, a long yagi could b
e set-up on a mast or tripod and it would not be difficult to manually point it towards the ballo
on. However, the uplink would be troublesome, since 10mW into the yagi would exceed the effective
radiated power (ERP) limit of 10mW. Although this is a report about the LoRa transceive
rs, I do make many references to the RFM22B. I did a great deal of testing with this module for
the $50SAT project and I can therefore make directcomparisons, by using the same tests in the same lo
cations, between an FSK transceiver and a LoRa one.