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Re: AO-40 spreadsheet



>From: Douglas Braun & Nadia Papakonstantinou <doug@dougbraun.com>
>I pulled out my copy of the Manual to correct the formula, but
>I got really confused after reading it.
>The way it is laid out and parenthesized, I cannot really figure out
>how to properly evaluate it.  (I think there is a good reason why
>they stopped using "x" for multiplication after 9th grade...).
>
>Also, the way I interpreted Ed's correction, it looks like the
>result would be undefined if L3 was 1, since you would be dividing by zero.
>
>Finally, I just noticed that T3 does not appear anywhere.
>
>If I needed to actually do the calculation, I would feel a lot
>more confident grabbing it from a known-good spreadsheet.
>
>Doug
>Na1DB
>
>At 10:48 AM 5/28/01 , you wrote:
>
>
>>Domenico also refers you to Bob Atkin's, KA1GT, article "Noise Temperature,
>>Antenna Temperature and Sun Noise"  published in The ARRL UHF/MICROWAVE
>>EXPERIMENTER'S MANUAL.  Bob's articles are good reading on the subject,
>>however there is an error in Bob's noise figure formula on page 7-57.  It
>>will give you fits if you are using it to evaluate your system.  This
>>article was originally published in QST in the 1980's and the error has
>>escaped correction and been copied often.  The problem lies in the third
>>coax cable loss expression T(L3-1).  It should be: T*L1*L2/G1*G2*(L3-1).  
>>
>>I have a noise figure/MDS Excel spreadsheet [cascade.xls] that computes
>>system noise temp/NF for you:
http://www.setileague.org/software/spreadsh.htm
>

Doug,

My apologies for confusion.  I changed the T3 to T since that was what Bob
had used.  I should have looked at his formula little closer to see that he
should have had T3(L3-1), but of course that formula is still wrong.

To clear up the ambiguity, the correct term is {T3*L1*L2*(L3-1)}/(G1*G2).
Does this make more sense to you.  There is no division by zero, and the
spreadsheet can be trusted since THIS is the calculation made by it.  It
has been compared with numerous authoritive but independent calculations
and there is agreement to .01 dB. 

Unlike Bob, I assume T1=T2=T3=To where To=290K.  My assumption is that
amateurs are not using cryogenics to cool equipment so its reasonable to
expect everything to be at "room" temperature.  The difference between, say
-10C to +30C [normal seasonal changes] when compared to 290K is slight
[290K = 17C].

So use the spreadsheet in confidence.  It has been presented before
professionals as a paper, with no argument.

Ed


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