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Re: Modify the DEM 144-28DC Transverter



>From: "Alex Artieda HB9DRI" <hb9dri@artieda.net>
>Dear Friends:
>
>I have a question about the transverter DEM 144-28DC. I use this unit to
>convert the 144 (IF) output from my AO40 down converter MKU24 and feed my HF
>radio on 28 Mhz, the DEM 144-28 Dc in the reception stage have an mmc
>circuit MAR-6 used like preamplifier on 144 Mhz and I want to know if is
>posible to improve the performance replacing this MAR-6 circuit for a
>premplifier with 20db gain and 0.6 NF, at 144 Mhz the MAR-6 have more or
>less same gain but 1.5 db NF. If I replace this preamp with less NF. the
>response for the down converter is better??? How much better??

Alex,

I would suppose the MKU24 has a good NF < 1 dB and ample gain to overcome
the 1.5 dB NF of the MAR-6.  
Remember the formula:
NFrx = NF1 + NF2/G1 
so if the MKU24's gain, G1 is 20 dB [a ratio of 100] then the amount of
noise added by the MAR-6 is 1/100th of NF2.  To calculate properly you must
convert noise figure, nf [in dB], to noise factor, NF [ratio]:  NF =
[10^(nf/10) -1], and convert gain into a ratio, too.

In my example:
nf1 = 1.0 dB, g1 = 20 dB, nf2 = 1.5 dB:
NF1 = 0.26, G1 = 100, NF2 = 0.41, so 
NFrx = 0.26 + 0.41/100 = 0.264
nf (dB) = 10Log(NF+1) = 10 Log(1.264) = 1.02 dB

My guess is that would make very little [not detectable] difference, so
reducing the 144/28 converter NF would have no influence, either.
Ed

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