10 GHz

The slew drive for the 10GHz offset dish arrived at the start of December and over the last few days I’ve been testing it on the bench with the KN3G controller I started working on in October.

The slew drive has two-channel Hall effect encoders built into each motor housing, one for azimuth positioning and one for elevation. Each sensor generates 2 pulses per revolution (ppr) on 2 channels but the KN3G controller just needs to use one channel. The motor reducer gear ratio is 575:1 and the slew drive gear ratio is 62:1 so there should be 575 x 62 x 2 ppr = 71300 pulses for a full 360 degree rotation of the slew drive. This is equivalent to 71300 / 360 = 198 pulses per degree, or 360 / 71300 = 0.00505 degrees per pulse.

For my first experiments on 10GHz I used a homebrew W1GHZ 10GHz transverter with a 432MHz IF. I recently bought a MKU 10 G5 transverter from Kuhne electronic (now part of Alaris Holdings) and plan to install it at the dish along with the DU3T LNA and Kuhne SSPA.

The transverter is small, measuring only 100 x 80 x 14mm, and has two 2.54mm pin headers, a 2 row 4 pin header for power and a 2 row 13 pin header for PTT, monitoring and frequency selection. I’m only planning to use a few of the control connections so I soldered the sockets provided to a double sided prototype board with some screw terminals.

During the ARRL EME Contest in August and September I was struggling to keep the dish pointed at the moon using manual methods, especially for azimith positioning. I did some research and came to the conclusion that a dual axis slew drive, normally used for solar tracking, was the way to go. These slew drives have been in use for a number of years by radio amateurs across the world to move dishes much larger than 1m offset dish, so a 3 inch model should fine. I ordered one directly from the manufacturer in China and whilst I wait for it to arrive I decided to build a KN3G controller using an Arduino Mega 2560 and Nextion 5-inch display.

Since conducting the receive tests on 10GHz during the ARRL EME Contest last month I’ve been busy finalising the sequencer and control circuits and started testing the amplifiers on the bench. I’m hoping to have transmit capability for the second leg of the contest in two weeks time.

Microwave amplifiers for amateur use, especially solid state ones, are a lot more expensive than HF and VHF amplifiers, a combination of lack of demand and high device prices. The W1GHZ transverter only outputs around +10dBm (1mW) so I need two stages to get to the +40dBm (10-12W) level, considered to be the minimum for EME when using a 1m dish. After considering whether to buy or build and a lot of searching I eventually settled on a driver amplifier from Rob, PA3GIE and a second-hand Kuhne unit for the final stage, purchased from Jac, PA3DZL.

The dish used for the 10GHz EME tests during the ARRL contest has been moved to the back garden where I have visibility of the moon from 80 deg. to 235 deg. azimuth when it’s at 25 - 30 deg. elevation or more. At the moment the centre of the dish is only 1.4m off the ground so I’ll gain a bit more moon visibility when I move it higher.

Using the same dish setup from the 10GHz sun noise tests two days ago I got up early on 25 August to re-assemble the feedtray and control box. I was ready to run receive tests by 04:30 UTC but the moon was too high for the linear actuator controlling the dish elevation. I started around 05:30 UTC by trying to copy the DL0SHF beacon and stations reporting on the HB9Q logger.

Over the last couple of months I’ve managed to make some more progress with the small 10GHz station. I now have a simple dish support with elevation control using a linear actuator but azimuth control is still manual. I have also added a WR75 waveguide switch, DU3T LNA from PA0PLY along with a new linear feedhorn.

After several failed attempts I managed to copy GB3SEE on 10368.850 MHz earlier this week with the dish and 10 GHz modules set-up on the front patio. The beacon is only around 8km from my QTH and was a good signal when I used the dish with a Bullseye LNB and SDR back in February. I’ve since found out that the beacon had been turned off for a while which would explain why I couldn’t copy anything a couple of weeks ago!

Back in January I built a W2IMU 10 GHz dual-mode feedhorn using dimensions calculated from W1GHZ’s HDL_ANT program. I used a 50mm length of 22 mm od. copper pipe for the input section and 98mm length of 54 mm od. copper pipe for the output phasing section. The flare section was cut from 1.2 mm thick copper sheet which was annealed before shaping and soldering. Some simple bench testing indicated that it was working and I also managed to carry out some limited tests with the feedhorn mounted at the dish.

I need a couple of 10 GHz bandpass filters for a down-converter to test my 10 GHz station and decided to build a evanescent mode circular waveguide filter. Evanescent mode filters work below waveguide cutoff frequency and in order to famliarise myself with their properties I started by modelling 2nd and 3rd order filters in CST Studio using designs by K5TRA and G4JNT. I also found a useful paper by Sam Mandev that runs through an example of determining distances between tuning screws.