I made my first 10GHz EME QSO in February 2005 with
WA7CJO (who else!?) I first started down the path back in about
1982, but work, inappropriate houses and other interests got in the
way. I started putting together a station just before we moved to Wales
in 2002, and finally managed to get everything in place at the end of
2004.
At the time I worked Jim, my station was very modest.
Just +40.2dBm (~10.5W) at the feed of a 2.4m offset dish. The power
was generated by a Mikom SSPA. This was the only part of the
microwave system that wasn't self-designed and built. I've since moved
on to a TWT amplifier giving around 35W.
Unlike many earlier 10GHz EME systems, my system is entirely built in coax. At power levels below about 50W, and given the presence of Moon noise, there is no real justification for using waveguide. I use a NAIS 26.5GHz rated coaxial changeover relay. This has about 0.2dB insertion loss, and something in excess of 60dB isolation at 10GHz.
The preamp uses a FHX05 HEMT giving 1.1 - 1.2 dB NF. This drives a two stage HEMT amplifier (using a FHX05 driving an FHX06 - and I still have a few of these devices left!!) and finally a second two-stage amplifier designed in about 1991 using plastic packaged Avantek GaAsFETs. I drive a 25m length of LDF250 coax (IL~18dB) back to the transverter, which sits in the shack. That should explain the large amount of front-end gain! I've never liked the concept of the masthead-mounted transverter. The total noise temperature of the preamp, coax antenna relay, and feedhorn driving my transverter via the long cable is in the region of 120 - 150K measured using cold sky to ground. I currently see about 1.0 - 1.1dB of moon noise at apogee, which is consistent with the antenna gain (~45dBi) and system noise temperature. I can, and will, improve on this in time, but unlike the lower frequency bands, potential improvments to the received signal to noise ratio will quickly run into a limit caused by the presence of Moon noise.
My transverter still employs aspects of 1982 technology!
It has a
waveguide image recovery mixer using post-coupled filters converting
10368 to 432MHz. A waveguide SRD multiplier driven with a couple of
watts at 368MHz provides the LO source. Slightly more modern (1991
vintage) 2-stage GaAsFET amplifiers form an RF stage, and also
generate the +17dBm on transmit which is fed-up a second length of
LDF250 to the dish. The LO is not locked to my Rb standard, but uses
a (very well aged!) free-running FET Driscoll oscillator. It can
usually be trusted to stay within 100Hz or so during a typical 30min
QSO. I'm currently designing a modern tecnology 'direct to baseband'
front-end, which will drive a lap-top directly, to replace the
historical kit.
I use a 1MHz bandwidth 30MHz receiver - built originally as part of a noise-figure measuring system - to track the Moon, and to look at astronomical noise sources. There's something very reassuring about seeing Moon noise, and being absolutely certain that the dish is pointing in the right direction...
My 2.4m ex-VSAT offset dish came with a fixed mount which I've slightly modified. I use TV-type jackscrew actuators to steer the antenna in both azimuth and elevation. Positional feedback uses incremental shaft-encoders. The azimuth encoder is coupled directly to the azimuth motion, and the elevation encoder has a pendulum (actually an M16 x 150 bolt!) attached to make a simple inclinometer. I use IN3HER's 'Rotosys32' software. Raimund set-out to employ the shaft encoders and electronics from an old serial computer mouse to make a dual-axis position indictator with a resolution and accuracy of about ±2º. I currently use a pair of incremental shaft encoders found cheaply on Ebay together with a PIC programmed to emulate the (obsolete) mouse employed by Raimund. Positional data from the dish is sent back to the shack over an approximation to a RS485 link.
As it currently stands, the tracking gives a resolution of 0.7º in both planes, limited by the shaft encoders (and a minor software bug) . With a beamwidth of ~1º, that's good enough to find the Moon, but is too coarse to allow consistent tracking, so once I've located the Moon, I nudge the antenna around with the up/down and left-right buttons. I need to get better encoders and to sort-out a minor SW bug so as to get the resolution below 0.1degree. In the longer term I intend to migrate to absolute shaft encoders.
If anyone is interested, I have Gerber files for both my ersatz 'mouse' and interface/motor switching PCBs available. I'd be happy to share the PIC code, as well, if anyone is either foolish or clever enough to want to play with it.
As
my dish has 0.935 f/D, I can't use any of the standard feed antennas.
W2IMU's
1.8WL aperture dual-mode feed is the nearest design readily available
in the 'amateur' literature. W1GHZ's e-book
suggests that the efficiency of my dish with that horn should still
be OK.
The
current feed is OK but as a next step I want to try to make something a
bit better. I had been playing with the idea of combining a
W2IMU dual-mode launch structure with a Potter-like flared horn, but
W1GHZ, on hearing of my need for a better feed offered to design
something suitable. He has come up with a Potter-type dual-mode conical
horn and I hope to build and characterise it in the next month or two.
I currently have a limited window, and can only
run skeds when the Moon is at elevations >35° at my local
transit. Another task is to tackle local beaurocracy and to try to get
permission to site the dish somewhere which will enable me to get
closer to the horizon.
