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發表於 31-10-2010 00:06:50
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1.7GHz Amplifier
Mine was a commercial ready made Pseudomorphic High Electron Mobility Transistor (or P-Hemt for short) purchased second hand from a fellow GEO member for the bargain price of £25, ) (fig 5&7) I also received one similar amp designed by Sam Elsdon thrown in for free when I purchased the HRPT receiver, very handy if one amp goes down due to lightning strikes, which are quite common here in Bulgaria. The amplifier is placed directly behind the helical feed in the centre of the satellite dish for maximum signal gain. The P-Hemt field effect transistor tends to give higher gain than the normal FET’s due to the way it’s made, but for the moment let’s keep things simple.
Downconverter
fig 6 fig 7
Remember the days of WEFAX? Remember that old downconverter you had that became obsolete? (figs 6&7)Well, it’s not any more. You can give it a new lease of life as part of your HRPT system. Used in conjunction with the NE receiver it can cover ALL the HRPT frequencies! That’s going to save you a lot of money. How can it do the job I here you ask? Well, let me explain. The downconverter is broadband so it will cover from 1690-1710 MHz and the NE receiver covers from 135-155MHz and that’s how it’s done! All the frequencies you’ll ever need.
Dish antenna
The most common and suitable antenna it seems for HRPT is a prime focus dish antenna. You don’t want anything too heavy for fear of overloading the rotator but yet you need something large enough to capture the signal. I decided to try and find something midway and settled on a 1.5 m prime focus dish made from Aluminium to reduce the weight and stop rust, although offset dishes have been used for such purposes, see: http://members.inode.at/576265/index.html which is the homepage of Gerald Ihninger of which I find very informative (well done Gerald) I know there are many mesh dishes about but I read that they are not as efficient as solid, I can’t confirm this as I haven’t done any tests, maybe that’s a chance for some GEO member to study and write about the results to enlighten us all.
Dish Feed
The Dish feed of course must match the HRPT receiving frequency of around 1.7GHz. The signal sent is circular polarized so we need an antenna to receive such signals, which usually involves a helical feed. This again can be home made. Again, Gerald Ihninger has an excellent site with antenna feed specifications. I am not yet experienced enough yet in this subject but by reading this article along with the web pages of Rob Alblas and Gerald Ihninger you will have a pretty good idea of what is involved. I myself can only relate my own experiences.
Rotator/Controller
Believe it or not but this is the most expensive part of the set-up, if purchased new anyway. You can of course build your own and save money but I felt this was a bit out of my league so to speak. It seems to me that the most common antenna controller/rotator is the Yaesu 5500 which retails about £450-£500 GBP although I have had no experience whatsoever in using one. At the moment my antenna is still turned by hand! Remember, you need a an Az/El rotator for azimuth and elevation i.e. left and right as well as up and down in order to track the moving Polar satellite. (The antenna must always be pointing at the satellite) There is a way round this expensive problem. I was recently in communication with Ed Murashie from the USA and he has found a perfect solution at a lot less cost. Instead of using the Ham Radio rotator why not use a cctv one? And so it was. I decided to follow Ed’s advice and settle on the Pelco brand, which is very much cheaper (starting around £100 second-hand) and also much quieter. Ed wrote an article on this for GEO so I won’t cover it here.
I’m still in the process of setting up my HRPT station but due to ill health and rather than wait until all this info is out of date I thought I would send it in now. It’s not nearly as informative as I wanted it to be but I hope to follow up this article with the results of my final installation once I obtain all the equipment so watch out for this in a future edition of the GEO quarterly.
So, to sum up, if you were to ask me the way to go, I would definitely recommend asking in the GEO group for second-hand equipment, and don’t be afraid of having a go at making some of it yourself, there are always people around willing to help. Of course if you have the money there are new commercial systems available but at a cost. I’m afraid that path is out for me!
Thanks to Rob Alblas for the donation of IC’s and postage, and his help, inspiration, expertise and getting me started in this, To Far Circuits USA for the donated PCB board, Tim Holdsworth for the Amp, Brian Sherriff for the HRPT receiver, down-converter, and donated amp with decoder board. JRW Electronics UK for supplying the components via Farnell Electronics UK and their own source, and anyone else I forgot. |
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樓主: VR2GY
