DSN- Frequently Asked Questions Last update: (08/01/2009 23:43:20 -0000)
If you have a question on DSN reception / equipment please send it here
|Q: Is a single two stage LNA enough
gain for DSN reception? I've read somewhere on the UHF-Satcom site that
some people have used a single LNA at the feed and another single stage
amplifier just before the filter in the down converter. Do I really need
two of these or is one
A: Generally one preamplifier directly at the feed will suffice - as long as it has enough gain to overcome the connector losses, the losses in the coax that feeds the mixer, and the loss in the mixer. Using good quality new semi rigid like RG402 / UT141 will result in a loss of about 1.4dB per metre. If the feed has a cut-off frequency around 8GHz, then the image noise will be small so you can probably get away without the second
|Q: What is the minimum dish size
that is useable to receive space probes like the MRO?
A: It is possible to use dishes under 1m, but the signals will be extremely weak and really only visible in an FFT display program such as Spectran. If you aim for a dish of 1.2m to 1.8m as a minimum, signals will still be weak, but most likely copyable in CW bandwidth. As a rule of thumb, use the biggest dish you can lay your hands on!
|Q: How hard is it to align the
dish on a space probe?
A: This all depends on the size of the dish. For example with the 1.8m dish, you have to point within a degree or two. This is easy with a calibrated azimuth dish mount and by using an digital spirit level for the elevation. The parameters from JPL Horizons are accurate and can be relied on to be correct.
|Q: What about tuning in the
signal? How critical is it?
A: The tuning is fairly critical, but really you need a local oscillator that you can trust as being stable and on a known frequency. The most simple method is to build a 10MHz ovened oscillator (such as the G8ACE ovened reference oscillator), and lock it to GPS (with a G4JNT GPSDO), or buy a Rubidium frequency standard. Remember with an 8GHz Local oscillator, the 10MHz reference is multiplied up 800 times, this means that a 10Hz error in your 10MHz oscillator will result in a 8KHz error at 8GHz meaning that you will not receive the weak signal from the space craft!
|Q: I am interested in building a
DSN receive system. How easy is it to source information on building a
local oscillator of 7GHz, or is there a source of second hand 7Ghz
oscillators that are accessible to home brewers?
A: The easiest option is to look on eBay for the 8GHz oscillators from the likes of m/a-com, continental microwave and frequency west. These usually require a 100MHz reference signal to lock the main cavity to, the output of which is then multiplied up to 7 or 8 GHz depending on the filters in the phase locked oscillator block. It should also be possible to modify the 'multi-8' frequency multiplier from F6BVA to give a 7 or 8 GHz output. DB6NT also sells an off the shelf down converter for 8.4GHz, details here.
|Q: I need to source a low noise
amplifier for 8.4GHz, where can I get one.
A: There are several options, DB6NT of Kuhne electronics in Germany does a very very nice waveguide input version. In the UK, G3WDG makes and sells a SMA connectorised version, details here. There are also a limited number of 'at cost' LNA's available from UHF-Satcom.com being made available to people wanting to get started, details here.