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Re: LEO station design and noise figures



At 11:08 AM 8/27/2005 -0300, Bruce Robertson wrote:
>I'm designing a LEO antenna system and would like some advice on the
question of
>system noise figure, particularly how important it is to gain small
changes in
>this figure.
>
>This is what I have so far: an Orbit360 rotator which will turn a 4 element
>'cheap yagis' 145 MHz beam and a perhaps 8 element 435 beam. I have bought
>unused Andrew flex heliax for both. Since the runs are on the order of
only 35',
>I should clock in at .7 dB loss for both sides (I'm using 1/4" for the 2m run
>and 1/2" for the 70cm one). The 70cm system comprises a Microwave Modules
>transverter and a HF rig. 2m will be serviced by a FT-817 and an amp which
puts
>out 20w. 
>
>I know the next step is to add preamps. For 2m I'm buying the Ramsey kit
with an
>rf relay. This is spec'd at 1 dB noise figure and I think about 17 dB gain.
>Several sources on the web suggest that the FT-817 has a miserable 6-8 dB
noise
>figure in its first stage. Now it would be really nice, of course, if I could
>install the preamp in the amplifier brick: reduced cost of connectors,
avoiding
>the jumpers up at the beam, etc. When I run the numbers using a
spreadsheet that
>performs the 3 stage calculations in fig. 11.3 of _The Radio Amateur's
Satellite
>Handbook_, I get a system noise figure of 1.43 dB for coax -> preamp ->
>tranceiver and 1.36 dB for preamp -> coax -> tranceiver. A change of the same
>size is calculated with a Hamtronics preamp in front of my MM transverter,
which
>is said to have a NF of about 3 dB.
>
>The 2m receiver is for VO-52 and the future satellites that plan to use mode
>U/V, such as KiwiSAT, Delfi-C3 and SATEDU.
>
>I'm having trouble understanding what this system noise figure relates to.
Is it
> above the typical noise at earth? Should I care about this figure change
more
>on 70cm than 144? It seems to me that the low price of high-quality coax is
>possibly changing the situation for stations like mine, and I'd be
grateful for
>any advice.
>
>73, Bruce VE9QRP
>----

Bruce,

For mode VU Leo-Sat operation I would make the general statement that using
a preamp on 435-437 MHz is a very good idea.  This is because of two
things, in general:
1- Sky Noise is less as you go up in frequency so the limitation to hearing
has more to do with your overall receiving equipment (i.e. antenna, cable
loss, and system NF) than sky noise or antenna noise.
2- Most radios have worse NF at UHF than VHF.  If the FT-817 achieves a  NF
of 6 to 8 dB that is better than most (the FT-847 exhibits a NF of 8-dB
with internal preamp and 14-dB with internal preamp off...this has been
measured by a well-known eme station).  So do not be amazed that your nice
shiny factory radio has such a NF; it was designed for use in high-Rf
environments  where intermod and overload can be more a problem and thus
comprimise on NF.  All "weak-signal" VHF/UHF/MW enthusiasts know that a
preamp is required for best performance.

Your setup sounds fine for Leo operation (where the signals are generally
stronger than Heo sats), so antennas OK.  If I were to advise anything that
would be to have more elements for UHF.  For Heo I would recommend 10-elem
VHF and 16-element UHF min.

Your choice of coax for a 35-foot run:
VHF: 1/4-inch hardline (I looked up the loss for LDF1-50 Andrews: 1/4-inch
coax is 1.52-dB/100-foot and is equivalent to Times LMR-400) giving a
0.5-dB loss for your run.
UHF: 1/2-inch hardline (LDF4-50 Andrews is about 1.5-dB/100-foot; Times
LMR-600 is 1.7-dB/100-foot) so your loss would be about 0.5-dB for UHF.

This means in transmit you will get 88% of your power to the antenna which
is excellent.  

For receiving any loss ahead of the first-stage of your receiver directly
adds to the NF, so using the FT-817 alone without a preamp this would be
about the best you could do.  But adding a preamp with 0.5-dB NF and
placing it at the antenna you will see considerable improvement in receiver
sensitivity (your system NF will go from 8.5-dB to perhaps about 0.7-dB,
your minimum detectable signal level will change from 123.7 to -131.4 dBm,
a signal increase of 7.7 dB (almost 2 s-units on most radios).

At UHF your MM xvtr has about 3-dB NF so with 0.5 dB loss ahead of it
(sysNF=3.5 dB) the min detectable signal (MDS) is -129.2 dBm but by adding
a 1.0-dB NF preamp with 17-dB gain this improves to (sysNF=1.08 dB) and MDS
= -132 dBm; an overall improvement of about 3 dB (1 s-unit).

This shows that improvement in NF has a non-linear relationship to increase
in receiver sensitivity (why a low NF is so important).  
You will note that the part of the improved receiving on UHF is because the
sky noise is lower [my assumptions Tsky(144)=250K, Tsky(437)=170K].  I also
assumed FM bandwidth of 15-KHz in the MDS calculations.

In your analysis you compared running the preamp at the MM xvtr after the
coax run.  I get system NF=1.57 dB and MDS = -131.4 dBm, so you give up
half of your gain in sensitivity by placing your preamp after the coax run.
 Because you are running low-loss coax this is not too bad a drop in
performance.  If you were running RG-58 or RG-213 the difference would be
more striking!

By installing low-loss cable you are set up for low-loss in Tx if you chose
later to operate mode-UV, so that's wise choice.  As a general note to
other readers:  be sure your preamp is a T/R switching model if you use the
same coax for Tx and Rx, overwise you will transmit into the preamp and
smoke it.

Some links for the calculations I made:
Times Microwave Cable Loss calculator:
http://www.timesmicrowave.com/cgi-bin/calculate.pl
Andrews Cable Loss Tables (from RF Parts):
http://www.rfparts.com/heliax_LDF450A.html
and my NF calculator (Part-One of my eme link spreadsheet):
http://www.qsl.net/al7eb/emelink.xls

Bruce, the bottom line is that by using low-loss line you are ahead of the
game.  It is always best to place the preamp at the antenna to minimize
cable loss affects on overall NF (but be sure to bypass the preamp if you
will use the antenna for transmitting).  The use of a preamp is very
evident for almost all home stations (the only exception might be HT users
connecting directly to the hand-held antenna and that depends a lot on the
sensitivity (and NF) of the HT...your experience may vary!).


73's,
Ed - KL7UW 
===================================
BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
Amsat #3212
Modes: V - U - L - S
===================================
----
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