Earth- Moon- Earth communication on 6m and 70cms
By David, G0MRF
On any other mode, the gain from an array of 4 stacked and bayed 19 element yagis together with an output of 700 Watts from an 432MHz amplifier, would be considered QRO by anyone’s standards. But in the world of EME this hardware and the 70,000 Watts ERP is definitely considered QRP.
The 70cms station on Rodrigues has been designed to work the larger EME stations around the world. Until recently this would have represented a handful of operators and used CW only. However, thanks to the pioneering work of Joe Taylor K1JT, The ‘WSJT’ or Weak Signals by K1JT software can provide an effective method of recovering signals that are below noise level.
The windows software requires a computer with a soundcard and operates like PSK31. Simply connect the radio’s audio output to your soundcard input and connect the soundcard output to the radio’s audio input. Remember that if you use the microphone input then you will need to use an attenuator (one or two resistors) to reduce the level from the soundcard. The final requirement for WSJT is that the computer clocks should be set to the same time. This can be achieved quite easily by using a programme called Dimension 4 to dial an ‘atomic clock’ and set the computer’s clock for you. Alternatively you can set it manually using WWV or any other accurate source. You need to be within 1.5 seconds, preferably better.
The following WSJT screen shot shows a strong signal being received from the moon. In this image, taken during the Dubus EME contest, signals from the station HB9Q can be seen. HB9Q is one of the large European stations and is in QSO with ON4DPX. The Swiss station runs 1kW to a 15m (48 feet) diameter dish. The antenna used for reception to monitor this QSO was a single 19 element yagi with a long run of RG213 cable and no pre-amp. The peak shown in Red is the signal from HB9Q.
To learn more about WSJT, download the .pdf manual from the WSJT homepage.
On Rodrigues, the 6m station can be used for EME communication during the period just after moonrise on the island. During the 30 minutes or so from moonrise, the two x 5 element yagis will have a clear take off over the sea. This provides between 3 and 6dB of extra gain by using the reflections from the ‘ground’. This ‘groundgain’ can increase the signal to levels allowing the possibility of EME. On 6m the larger stations in Europe, Australia and Asia, can use JT65, one of the WSJT modes, to make QSOs with 3B9C. For QSOs with stations in the USA we will need to point the antennas towards a setting moon. At present (early March 2004) it is not certain if our location on the island will give us an appropriate open sea path. However, we will be able to assess the situation to the west within the first day of arriving on 3B9.
The 70cm station was originally designed for operating the uplink to Oscar 40. Since the battery failure on the satellite we have upgraded the station for CW and JT65 EME communication. The 70cm antenna is an array of four 19 element yagis from Tonna (F9FT) The combination have 20dB gain. A ‘Discovery’ amplifier from Linear Amp UK will produce 700 Watts from a Rugged Russian GS31B triode. The 70cm antennas will be set for horizontal polarisation and have azimuth and elevation rotators.
The best period for EME occurs between March 26th and April 5th.
Messages regarding skeds for 6m or 70cm EME can be set to our e-mail address on the island firstname.lastname@example.org. Be sure to include “EME” in the message title.