Michael,
I looked at MTMScientific web site. The tuner has a noise figure of 13dB
which is a bit high. The tuner was made for cable TV applications where
signals are strong so 13dB noise figure is good enough for the intended
application. I am going to throw out a little math here. Please put up
with me!
For radio astronomy or space communications use noise temperature is much
more useful than noise figure.
Temperature in Kelvin = T = 290 * ( 10 ^ ( NF / 10 ) - 1) where NF is
noise figure in dB.
For tuner T = 290 * ( 10 ^ ( 13dB / 10 ) - 1 ) = 5496K or call it 5500K
Note that 400 to 700MHz range and a small antenna, sky temperatures will
range from 35K to maybe 200K. These signals will be on top of the 5500k
from the tuner input so they will not be easy to see.
Now lets look at a preamp with 0.7dB noise figure and 17dB Gain. This is
what I got from Down East Microwave for my 408MHz system.
Convert the Noise Figure to temperature as before give
T = 290 * ( 10 ^ ( 0.7dB / 10 ) - 1 ) = about 51K
Convert Gain to a power ratio
GP = 10 ^ ( 17dB / 10 ) = 50 Note that power gain does not have units.
It is the ratio of two powers, so units cancel.
If we put this preamp in front of the tuner, the tuner's noise temperature
will be reduced by the factor of the preamp power gain.
5500K / 50 = 110K contribution from the tuner.
The preamp will also contribute 51K.
The noise temperature of the combination is 110K + 50K = 160K.
160K temperature is good enough to see many radio astronomy sources with the
antenna gain you are talking about.
Another source of noise temperature is the antenna side lobes. Any yagi
antenna will have some sensitive directions off the side and back. These
minor lobes see the earth, which is around 290K. ( the source of 290 in the
first formula.) I am going to take a guess that the antenna will have
between 50 and 100K due to ground noise. This will add to the 160K of the
receiver. This is still good enough.
Most preamps with good enough noise temperature will have 50 ohm input and
outputs. There will be a small loss going into the 75 ohm tuner input. For
what you are doing this small mismatch loss can be ignored.
Changes in amplification anywhere due to temperature in the system will run
you crazy. Try to protect the preamp and tuner from temperature changes.
Also the diode detector used with the tuner will be light sensitive. Put a
little black paint over the diode. I learned about this the hard way!
Dick Flag, also on the SARA board, is an expert on this sort of thing and
will hopefully throw in some more helpful hints.
Hope this has informed you and not confused you too much.
Bruce Randall