DL7AFB

Earth-Moon-Earth (EME) Communication

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During the winter break I'm using the field-day contest equipment for EME operation: four 6l DJ9BV-(I) optimized yagis, a K2RIW amplifier with 2 x 4CX400's and a MGF1601 LNA. Most of the equipment was spent by my good old father, DL7AKL, who had to disassemble his station some years ago. The equipment has been placed provisionally in the garage on a workbench. Here you will find a small set of photographs showing some details of my station...

The 70-cm EME equipment consists of ICOMs IC402 transceiver, external self-made frequency counter, self-made driver PA  and finally a rebuilt K2RIW Power Amplifier. A cascade of Mitsubishi RF Power Modules M57745 and M67728 providing linear drive level of 25 W at 6dB backoff. In JT65b-mode the high power amplifier delivers 750W RF output power, which is attenuated just 0.6dB by 10m (30ft) LMR-600 Low Loss Coax Line from Times Microwave Systems.

In February 2007 the K2RIW-Amplifier has got some new modern tubes. The old 4CX250b's were replaced by a pair of Svetlana's GS-36b (4CX400). Just a few changes were necessary for that upgrade. Most of the datasheet parameters are identical, e.g. anode-, and screen voltages, as well as socket type and pin-out. However, the heater (filament) has much bigger power consumption and draws 3.2A at 6,3V instead of 2.6A at 5,8V. In addition to that the negative grid voltage must be reduced from -45V to -34V for Class AB operation.

More over GS-36b's anode radiator has a significant larger diameter (51mm instead of 41mm), as well as higher output capacitance. To cope with that  the plate line was rebuild from solid copper. With respect to the increased anode capacitance the line length was shortened by about 8mm (1/3") and measures now 127mm x 220mm. Therefore the flappers as well as the complete input matching network can remain unchanged. One can use conical chimneys made of PTFE-foil, to keep the original top cover unchanged, too.

Initial tuning was quite easy. Before high voltages were applied both tubes were pre-conditioned at nominal filament current for at least 32 hours under full ventilation. Then heater current was reduced by 5% to prevent rapid aging. Afterwards tuning was started at 10dB reduced input drive level. The plate capacitance was needed to increase reasonably while load capacitance showed minor effect. In addition input tuning needed significant readjustment. Then input power was increased continuously while checking permanently all currents as well as exhausted air temperature until maximum input power was applied. At least nearly 2dB more output power was achieved  and the amplifier showed to be very stable, no retuning due to drift were necessary any more, as known from the old 4CX250's. At 17W input level the amplifier delivered about 875W RF output power. Therefore the anode efficiency was about 57% at 1850V and 0,85A anode supply.  Finally a maximum output power of 1,2 kW was achieved.

A big advantage is the considerably increase of airflow making the PA much more stable. Here you see a simple test using a table tennis ball within the exhausted airstream. On the left photograph the PA was equipped with 4CX250's where the ball has been laying lazy on the exhauster grill, while in comparison to GS-36b the ball hovers on the airstream, as seen on the right.

On the weak signal side the first stage 432-MHz pre-amplifier is equipped with Mitsubishi MGF1601 medium Power GaAs-Fet in JA6CZD/DL7YC-Design. The output matching network was changed to a broadband ferrite transformer, as proposed by DJ9BV. This beautiful design was manufactured by DL7AKL using brass and galvanic silver plating. NF was measured in a professional environment to be about 0.35dB, as shown in the diagram below.

Since I'm living in the forest, my moon window is quite small. Due to tree blockage by 100 year old pines EME operation is possible only from AZ 135° to 160° for EL > 45° and 160° to 260° for EL > 25°, otherwise one has to overcome some more 5 to 10dB attenuation each tree top.

WSJT - JT65-Interface ;-)

JT65b screen dump supported by Bernd, DL7APV, showing my CQ-CALL in the lower trace as well as my first answer on Bernd's call in the upper trace. In each trace you'll find both signals: on the right hand side (earlier) those received via tropo at -280Hz and on the left hand side the EME-Signals at -460Hz. In addition to that, some air-plane back-scatter is seen on the lower trace at +220Hz.

JT65b screen dump of qso between DL7APV and PA3DZL on Dec. 2, 2006. The upper trace showing both, tropo and EME signals of DL7APV. PA3DZL's trajectory  beneath; he was using just 2 x 21 el. F9FT and 1kW.

( > Bernd and Bodo in front of DL7APV's Monster Antennas < )

Sked proposals are very welcome, either in CW or in JT65. Please contact me via e-mail or chat.

     last change 02.05.2007 22:58    Haftungsausschluß

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