29 MHz – the forgotten frequency for amateur radio satellites

April 23rd, 2015, Published in Articles: EngineerIT

 

In the early days of satellites, not many years after the launch of Sputnik One in 1965, amateur satellites were designed and launched with downlinks on 29 MHz. Satellites were much larger than the present day CubeSats which only measure 10 x 10 x 10 cm and present interesting challenges for antenna  design and placement.

OSCAR-7 and one of the amateurs who worked on its development and construction, Jan King.

OSCAR-7 with Jan King, one of the amateurs who worked on its development and construction.

Universities and other scientific research institutions are using portions of the amateur spectrum for their CubeSat’s which has caused the 145 and 435 MHz amateur-satellite band segments to be very crowded, leading to an increasing number of satellite builders to explore alternatives.  For many, such as those requiring single-channel bandwidth greater than approximately 12,5 kHz, the best answer will be found in the microwave bands.

However, for those who can use it, the International Amateur Radio Union (IARU) satellite frequency coordination process has now opened another alternative: 29 MHz uplinks.

The use of amateur spectrum by universities and scientific research institution for their experiments is not the ideal solution and has only been agreed to by the IARU  for a limited period till alternative spectrum is identified and allocated by the ITU  for this purpose.

Ray Soifer W2RS, a long-time member of the International Amateur Radio Union satellite advisory panel said that the band segment 29.300-29.510 MHz has been used for amateur-satellite downlinks for more than 40 years, beginning with Australis-OSCAR 5 in 1970 and AMSAT-OSCAR 6, AMSAT’s first communications satellite, in 1972.  The band segment was heavily used for downlinks in the 1970s and 1980s.  Today, only one amateur satellite actively uses a 29 MHz downlink: AMSAT-OSCAR 7, launched in 1974. (This is quite remarkable to have a satellite built in garages of radio amateurs in the USA to operate for over 40 years, although the batteries packed up long ago and the satellite is powered by its solar panels.)

The use of the 29MHz downlink in the seventies and eighties presented some interesting propagation experiences. During the period of solar maximum, signals on 29 MHz would not pass through the ionosphere directly but travel for up to two minutes along the ionised layer and then penetrate and be received on the ground. This meant that the acquisition of signal (AOS) would be delayed and the loss of signal (LOS) would be advanced by the same amount of time. At first it was considered that the prediction programmes were incorrect but that proved not to be the case. Unfortunately not enough data was collected at the time to make any useful conclusions.

Unfortunately, a 29 MHz downlink would not be practical for most of today’s very small satellites, owing to the size of the antenna required. This raises an interesting challenge for satellite designers: to develop a downlink antenna mechanism that might, for example, release a long wire with a small weight at the end that will resonate on 29 MHz.

Why not uplink on 29 MHz?

Fortunately, uplinks at this frequency are very practical, even with small receiving antennas, because higher transmitting power at the earth station is easy to generate.

The following example from Soifer illustrates this :

  • At a slant range of 2000 km, the path loss is (only) 128 dB. Doppler shift is 0,6 kHz or less.
  • The FM sensitivity at 29 MHz of a typical amateur VHF/UHF handheld transceiver with HF receive coverage, in this case a Yaesu VX-3, is 0,35 uV (-116 dBm) for 12 dB SINAD.
  • The “gain” of a loaded 29 MHz antenna the size of one for 145 MHz will vary with the situation, but would typically be on the order of -10 dBi.
  • A typical amateur transmitter or transceiver for 29 MHz will produce 100 W output. For simplicity, assume a unity gain transmitting antenna such as a ground plane, resulting in a signal strength at the receiver of -88 dBm and a safety margin for the link budget of 28 dB. A “worst case” analysis, with a receiver sensitivity of 1 uV and a -15 dBi receiving antenna, would still leave a safety margin of 13 dB, which should be more than adequate.
  • Addition of a 500 W solid state amplifier would increase uplink S/N by 7  dB, and use of a three-element Yagi instead of the ground plane antenna would add another 6-7 dB, enabling the use of an even smaller receiving antenna at the satellite, if needed.

The IARU amateur radio satellite frequency coordination team has challenged the satellite designing and building community to seriously consider the 29 MHz opportunity for either an uplink or a downlink for their next satellite!

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