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More on Temp and preamps, coax and such



The medium length answer to this question harkens back a fundamental
tenant of
physics:

Johnson noise.

This is the noise voltage produced by any real resistor.  The relation
that
expresses this noise voltage has terms representing temperature (in
degrees
Kelvin), resistance and bandwidth.  Not a specific frequency but the
range of
frequencies over which you are measuring, f1 - f2.  This difference in
frequency
should be positive if we are to avoid imaginary numbers.  In our present
case
that is 4 MHz or the bandwidth of the 2m band.  Room temperature is
about 296
K.  Typically our loads and transmission lines in our VHF, UHF and
Microwave
systems are 50 ohms.

If you plug those numbers into the equation you will get about 1.88
micro volts
of noise, at room temperature, simply due to the equivalent resistance
or real
resistance of our transmission lines, termination's, filters...etc.
This does
not include other sources of noise due to active components.

At my QTH the temperature extremes are from -28 C to 37 C (-20 F to 100
F).  If
I take those extremes and add some at the top to account for the sun
beating
down on a thermally unshielded box or cable, the noise voltage changes
by 0.2
micro volts or so.  An FT-847 has a sensitivity of 0.125 micro volts in
the 2m
and 70 cm band.  This noise voltage, inherent to our transmission
systems and
antennas, should get you thinking about the theoretical limits of
omnidirectional antennas vs. gain antennas.

Since coax is a passive component there is not the same non linear
temperature
dependence on normal operation or best noise spec as you would find in a
device
like a transistor or a diode.  This keeps things simple so we don’t have
to
worry about non linear temperature behavior unlike active components.

The point here is that yes there are some temperature effects but they
can be
dominated by the inherent temperature restrictions we humans live in.
Another
factor that can hurt or hinder is the bandwidth.  The more restrictive
the
bandwidth the smaller the noise voltage.  If you want rock bottom noise
levels
you will also need to do what NASA does with the DSN (Deep Space
Network), which
is also done by radio telescope people, and that is to cool your MASER
receivers
down to within a few degrees of absolute zero (-273 deg C).

Unfortunately if you do that to coax it will crack and break into a
million
pieces when it is moved.  I’m not sure what would happen to the
dielectric
properties of the plastics in any case.  Most of the semiconductors
produced
these days, certainly all the ones we use in our systems, become
“iffyconductors” long before they reach such low temperatures.  Indeed
the
operating temperature limits often are 10 or 20 degrees above and below
the
storage temperature limits.  Storage temperature limits are often -55 C
to +150
C.  This does not include ICs.  ICs often have more restrictive but well

documented temperature limits.

At the other end of the temperature scale, most manufactures don’t
recommend
that you operate a device with the junction temperature above 150 deg C
if you
want it to last very long.  This is an issue for high power devices but
not so
much of an issue for low power devices.  Remember that the junction
temperature
of a semiconductor is often operating at a higher temperature than the
device
case.  Keep that in mind before you crank up your mast mounted power amp
on the
hottest day of the year.  The heat sink may already be at 40 or 50 deg
C, it may
not take much to push the junction close or beyond the 150 deg C limit.

I suppose if you look carefully you just might notice a difference
between the
noise levels of your station in a climate with 140 deg C swings between
the
darkest and coldest night of winter and the hottest brightest day of
summer.
The problem is that there are so many other sources of RF noise that
plague us
that it might be tough to see the difference.

--
/***************************************
Bronson Crothers
LASST
5764 Saw. Res. Center
University of Maine
Orono  ME  04469-5764

Phone: 207 581 2252
Fax:.....207 581 2255

N1ZAQ

-... ...-

Email: bronson@eece.maine.edu
**************************************/


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