10 GHz Where, Why and How!
By David Minchin VK5KK.
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After many years in obscurity, 10 GHz has quickly become the new frontier in Tropospheric DXing. What has happened in VK is interesting, in as much as most active stations on 10 GHz, currently about 16 to 18 VKs, have jumped from 1296 straight to 10 GHz. Some have dabbled with 2.3 GHz, but 3.4 GHz and 5.7 GHz have been tackled by only a few after getting active on 10 GHz. Internationally it is no different where 10 GHz is the most active band above 1296. In the early nineties, 95% of global activity was from portable locations. Now the swing both here and overseas is towards higher power home stations. So why all the interest?
WHERE IS 10 GHz?
Lets look closer at the band in question. We have a secondary allocation between 10,000 and 10,500 MHz, where we can use all licensed modes available to us, in a relatively interference free environment. The term "Microwave" is broadly used to describe this part of the spectrum. The only definition I can find for Microwave is something between UHF and "Millimetre" Wave (> 30 GHz) regions. At 10 GHz, a wavelength is 30mm, a dipole therefore about 14mm long. Its the same frequency as early Microwave door openers and security detectors.
Once upon a time, this frequency was the domain of relatively crude and mechanically complex equipment, however the availability of "off the shelf" Gasfets and SMD components has reduced the effort and expense required, while enabling better performing narrowband equipment to be built.
TRUTH and some MYTHS About 10 GHz....
10 GHz Doesnt go through Trees? VERY TRUE, but there are ways of dealing with tree's.. if you're portable, move!
10 GHz doesn't bend around corners? TRUE, but 10 GHz bounces off more things and will work its way around some things.
10 GHz doesn't go far? MYTH, 10 GHz over a near line of site path will travel with similar signal strengths as lower frequencies. Over some 100-300 Km Tropo paths, it may be stronger.
10 GHz Equipment is expensive and hard to get? In real terms, it would probably cost $250- $500 to set up a Transverter in 1996, everything is available off the shelf in Australia.
10 GHz Equipment is harder to get going? TRUE it is harder, mainly due to the lack of test equipment, but see further...
WHY go to 10 GHz?
At the risk of repeating clichés, "because it is there"! 10 GHz is a band best described as a band at a spectrum crossroad. It is conveniently below the Satellite DBS (Direct Broadcast Service) band of 12.25 - 12.75 GHz. This means there is a large selection of devices and circuit ideas easily translated to 10 GHz. 10 GHz is also the highest Amateur band that Test equipment (Surplus or otherwise) is commonly available in some quantity. Indeed, locally, we have as good a supply, thanks to DSTO; Telstra and RAAF surplus per capita compared with most parts of the world. More on test equipment later.
Building techniques are specialized, but still within the capabilities of a home workshop. Like anything, you tend to crawl before you walk. Yes SMD components are small and require some patience. Often, more time is involved in getting set up to build equipment than in the actual building! The Printed Circuit Boards for a typical 10 GHz Transverter can be completed in a couple of nights. The effort is about the same as building a HF linear, just you may need a magnifying glass more!
As far as propagation goes, 10 GHz is our highest allocation that doesnt suffer significant atmospheric absorption problems and still has useful, predictable propagation. As you approach 23 GHz water vapour absorption becomes a severe path attenuator. Just how this comes to play is demonstrated by the comparison of the current terrestrial world record on 10 GHz at 1911 Km, to 24 GHz at 396 Km and 47 GHz at only 184 Km.
Indeed propagation at 10 GHz is still subject to professional study. At this point in time the amateur community has, perhaps, collected more data on Tropospheric ducting than any other single group. EME is achievable with 20 - 50 Watts and a 3-metre dish. This will probably be the upper frequency technological limit for EME for many years, for factors already mentioned.
Hence this "crossroad" frequency gives us perhaps, the last frontier of Tropo DX a quantum jump above current Tropo DX frequencies.
While amateur communications where first achieved on 10 GHz, 50 years ago it is only in the last 25 or so years that technology has enabled more general usage.
The first significant step towards easier 10 GHz operation, was the introduction of the Gunn Diode in the seventies. This enabled around 5 - 20 mW of RF to be generated, stable enough to provide Wideband FM communication. Very simple full duplex transceivers could be made using a 30 MHz or even a 100 MHz (e.g. FM tuner) IF. In Europe, Wideband operation was the dominant mode until the late 80s with the introduction of narrowband equipment. Gunn diodes and newer DRO tuned oscillators are still more than suitable for Wideband applications like 2 - 10 Mbit Data links and FM ATV. Distances covered with such equipment ranged from 120 Km LOS up to 600 Km with Tropospheric enhancement.
The second significant step has been the introduction of Narrowband equipment. Narrowband equipment has enabled a direct 30- 35 db link system improvement when compared with Wideband FM systems. The first narrowband equipment appeared in Germany in 1980, at that time running less than a milliwatt. The original design work used waveguide multipliers and large amounts of plumbing borrowed from earlier Gunn diode equipment.
The appearance of Gasfets soon enabled the eventual migration of circuitry to Teflon PCB, paralleling the development in Satellite receiver technology. The first all PCB, 200 mW transverter designs appeared in 1986, with further designs refining packaging from DC0DA (1987), G3WDG (1991), DB6NT (1992), KH6CP (1993), JE1AAH (1992) and Qualcomm Surplus sat gear (1996) . The increase in availability of components led to a virtual explosion of 10 GHz operation in Europe/USA. The first 10 GHz Narrowband equipment imported "readymade" into VK in 1991 with the first "homebuilt" transverters appearing by 1992, in VK2 and VK5. The most popular design has been the DB6NT design although several G3WDG and Qualcomm systems are in use.
WHERE ARE WE NOW
The current state of technology allows 1-Watt power output without too much expenditure. Solid State amplifiers can be built up to about 10 Watts, although the cost per watt is still a bit prohibitive. Noise figures of under 2 db are easily achievable, really no different to 2 Metres! With such small wavelengths, antenna gain is easy to achieve with a modest dish. 31 dbd is available from a 600mm (2-foot) dish. DBS 650mm Offset dishes are now becoming more common with an improvement in gain over prime focus types due to the better feed illumination and lack of feed blockage. In reality, anything bigger than that becomes a bit hard to aim and keep aimed while portable, unless you have a solid pole cemented in the ground!
Components and/or Kits are available from several sources. The WIA Equipment Supplies has all the basic components, including dishes and can make up Kits upon request. Antenna feeds are available from Des Clift VK5ZO. A large number of circuits have evolved using the MGF1302, the BC108 of Microwaves! HEMT's like ATF36077's and MGF4317's are all similarly priced and work up to 24 GHz. You can buy, for around $5-00, MMICs (ERA series) that have 10 db gain on 10 GHz!
WHERE DO YOU START ...
Some one once said, " Its not what you know ... but whom you know". The best way to get off the ground is to join or start an activity group and pool resources. I keep mentioning test equipment as being one requirement. Well the most basic requirement is for a power-measuring instrument, e.g. a 12 GHz Bolometer. They dont pop up at buy and sells that often but look out for HP 430/1/2 or General Microwave 454s. They are about or can be borrowed to calibrate a homemade power meter. Other than an accurate frequency meter and the usual voltmeter, that is it. You don't really need a spectrum analyser or a signal generator. In Adelaide the VK5VF beacon on 10368.450 MHz gives more than enough signal to tune up on!
Perhaps your interest is not in Microwave DXing, maybe you want to send Data or FM TV pictures across town. Or link repeater sites with mW transmitters. The same technology (and simpler) can be used and re-used for different applications. Information is available from several overseas magazines as well as publications from the RSGB and the ARRL.
SO NOW I AM SET-UP.....
Next summer will be interesting for New and Experienced Microwave DXers alike. Given good Tropo, distances of 200 - 600 Kms will be spanned from more locations and more clues will come from the Albany to Adelaide path as to its particular characteristics on 10 GHz. The race is on for the First 2000 km Tropospheric Contact on 10 GHz. We are seeing the first 10 - 40 Watt portable and home stations appearing in VK3 & VK5.
Even without propagation, 200 -300 Kms can be achieved from selected hilltops, and closer in even mobile operation is possible. Example, it is possible to work from Mt William to Mt Dandenong on 10 Ghz anytime (270km). And from VK5 it is possible to work from Pt Lincoln (Winters Hill) to Adelaide (275km) in the middle of the day in Winter! Home QTH operation is possible, if you can get above the tree line in the direction of interest. As we are talking about ERPs of 200 to 1000 Watts, the range under near line of site conditions is similar to 25 Watts into a beam at 2 Metres. The curious effects of propagation at sea level (and I mean tripod in the sand stuff!) is still largely unexplored.. how many other bands can you use 10mW effectively? Real QRP!
Openings have already been noted where signals have been far stronger on 10 GHz than lower frequencies, due to the peculiar effects of layers less than 30 metres above water.
I hope this brief paper gives you a little insight into this "last frontier". It is now within the reach of many experimenters, giving greater scope for operation.
(C) VK5KK 10 GHz Presentation to SA VHF Group / WIA 08/07/96 22:21
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