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A big part of the fun for an ham operator comes from trying challenging DX contacts or operating in extreme situations. Outdoor operation is often a mix of those two things, but it is also the most common way to make microwave contacts because lowest obstruction situation and favorable propagation can be many Km away from your home. Therefore, each microwave ham operator must be prepared to setup his own efficient and practical rover station. |
To setup a portable microwave station is not particularly difficult, but has to consider all the specific constrains that cm-bands impose. So, just after completion and bench testing of our homebuilt 10GHz stations, we had to start thinking about how to do that. Our experience is just at the beginning, so the best and the easiest way is to see how other hams have approached this "bricolage" activity, and make some refining by building-up the day-after-day experience. First help came visiting ARRL and RSGB sites, with the photo galleries and articles of microwave contests, and then we have started to learn from our own experience.
A 10GHz rover station can be composed by:
- Radio equipments (Transverter, IF transceiver, talkback
transceiver)
- Radio accessories (microphone, frequency standard, CW keyer, headphone)
- Aiming tools (compass, GPS, goniometer, paper maps/PC)
- Antennae, cables (for microwave and for VHF/UHF talkback)
- Supporting structure and aiming system
- Power supply (batteries and/or power generator)
- Basic set of tools and instruments (screwdrivers, keys, cable ties,
multimeter, coaxial inter-series adaptors, 50ohm terminations)
Few comments here about the IF transceiver. We chose the FT817
because of its unbeatable portability. It's a QRP radio, but to drive a
transverter we just need less than 1W. A further reason is that it has the 70cm band that
fits our transverter IF, plus has the 2m for the talkback channel, even if
the lack of full-duplex operation on two bands and the low TX power make
things a little troubled. Instead of using its internal batteries,
it's better to use an external main battery for it and for all other radio
equipments, so you can use its embedded supply voltage reading to
monitor battery status. An interesting function is the possibility of a
continuous beacon transmission with its internal CW keyer, as described here.
In UHF all FT817 we have seen just out-of-the-box have a significant
frequency error (even 3~4KHz). We think that this is done on purpose to
motivate people to buy and plug the TCXO option. In fact the basic stock
oscillator well suits the stability we need for the IF over normal temperatures
(1~2ppm), so the only thing we did is to set it on frequency using a
frequency counter locked to our GPS standard and make periodical checks
afterwards.
To reduce wiring, a little help comes from a simple modification we did in our
IF radio to automatically key the
transverter through the coax cable that connects them.
Among transverter accessories, the frequency standard
is an important one. Frequency accuracy of both stations is very
important, and because of the many frequency multiplications needed to
generate a 10GHz signal, a very little inaccuracy of the transverter
oscillator becomes a large frequency error that could prevent a successful
contact. 100ppb (or 0.1ppm) means a 1KHz offset at 10GHz, and is the very
maximum to consider for SSB (CW is 10 times more demanding). A good
frequency standard must be on frequency (no error) and stay stable on it (no
temperature/aging drift). Depending on the transverter you have, it can
have its internal standard (most likely an OCXO, Ovenized Crystal
Oscillator) on might need an external one. In the first case its absolute
accuracy must be periodically calibrated using an absolute frequency
standard in a well-equipped lab. We made our transverter
without an OCXO, so we bring our GPS-based frequency standard to
keep them on tune: this add a little more weight and power
consumption.
The purpose of the keyer is to send a continuous beacon transmission.
There are many project of CW keyers that store a calling message and can
send it repeatedly. Alternatively, for a voice beacon, you can
use a looped voice recording in your MP3 player connected to the mic
connector. We use, for the moment, a simpler solution described here.
| Antennae and cables would require a whole chapter, but here we just say that we use small 40~43cm offset dishes for Sat-TV, with illuminators built from LNB circular feedhorns. The gain from these small dishes is interesting, in the range of 28~30dB. A less portable arrangement is a Gibertini 60cm cassegrain dish with radome. Having the same f/D of an offset dish, we use the same illuminator. |
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| While expecting some extra gain, brief short-range tests did not show significant improvement over the little 43cm offset. Probably the obstruction of the sub-reflector cancels the extra gain from the larger aperture. Nevertheless, further long-range test should be run.... | |||
Then an important element is the supporting structure. It has to hold the dish antenna, leaving the possibility to make azimuth and elevation adjustments. And since the transverter is connected to the antenna by a very short semirigid coax (when it is not a waveguide), it must move in one with the dish. The system must remain stable against wind, but has to be light enough to be carried around. A camera tripod appears as a good option, and this is current Andrea's choice. He has solved the two main difficulties that appear: a camera tripod head is not made to hold an offset dish, and all equipments need a support to arrange the whole station in a single piece. With a discarded telephone table holder, some hardware and velcro stripes he made a single-piece 10GHz station successfully tested at Monti Lattani, close to Roccamonfina.
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| Johnny's arrangement is based on a foldable tripod for stage loudspeakers, heavier than a camera tripod, but taller and more stable with heavy loads. To increase stability, a bag containing the Pb-gel batteries is hung at bottom end of the center mast. The center mast is split in two telescopic elements, and the inner element is free to rotate inside the outer one, thus making azimuth adjustments possible. Elevation adjustment is not possible, apart the one provided by the dish antenna mount. The transverter is secured to the dish mount by a bracket, and the FT817 with the GPS frequency standard is tied by Velcro stripes to a Plexiglas cantilever: all those things being securely attached to the inner element. |  |
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| The inner element can be quickly separated from the rest, together with everything that is attached to it. So the whole station, without the tripod, can be kept assembled and wired while being carried. At destination, you just insert the inner mast into the tripod and you are ready to operate. | |||
| The radio beam should normally point to the horizon, but offset dishes normally don't tilt that much downwards. A trick is to mount the dish upside down, but in our case we have flipped only the bracket that holds the dish to the mast. While not being an absolute rule, most offset dishes have the parabola vertex at their bottom edge, thus they approximately point along the line that goes from the lowest edge of the dish to the center of the feedhorn mouth. Our experimental results on both 40cm dishes we own, from two different manufacturers, confirmed this first-approximation rule. |  |
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| To do fine adjustments, the dish mount nuts have been replaced with butterfly types, so you do not need a wrench. As a general guideline, we have extensively done this substitution to the hardware of our rover stations to be able to quickly mount and dismount everything by bare hands. |  |
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More hints and a few things we learned while operating our stations can be found in our Microwave Contacts page.
© Hotwater 2007-2009