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Re: Converter gain and noise..



Jens etal,

Attenuation, whether produced by cable, resistors, or lossy atmosphere, all
are sources of noise.  Transmission thru any media above absolute zero
introduces noise in proportion to its temperature.  So it matters not what
is used.  Impedance mismatch reduces the efficiency of power transfer, and
if the transmission medis is lossy the effect is magnified.

So two things are at work here:  1) signal power is reduced, and 2) noise
is introduced.  The effect on overall NF is determined by the NF and gain
and loss of preceeding amplifier stages and cables.  You need to do the
math to really know how much attenutation will affect your systems's NF
[see the recent post of K2RIW's comments].

So there is no "natural" or "organic" or any other such thing :-)  Lossy
coax is just often cheap since you may have it vs. buying an attenuator.  I
once made a 432 dummy load with 100 foot of RG-58 with a 1w resistor
terminating it.  

Ed   

>At 14:06 26-04-2001, you wrote:
>
>>Hi Jens,
>>
>>I run an attenuator between my Drake and FT-290R MKII IF. I insert 10 dB.
>>This gets the S meter near 0. When I first installed my Drake, I ran the 2m
>>IF down the tower in a spare run of 1/2" hardline. I had way too much gain!
>>So I now run the IF in RG-58 to get a little bit of attenuation. I should
>>mention that my FT-290 has been modified with a muTek board. This could be a
>>partial reason I have to run the attenuation I do . The muTek board really
>>makes the 290 hear well.
>
>Ok, Mike, sounds like the right thing to do. RG58 is a "natural" attenuator 
>in that frequency range and cheap too, making the high an advantage, rather 
>than inserting an extra attenuator that might make noise or introduce 
>impedance mismatch.
>
>73 de OZ1LRG
>
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