[Moon-net] additional cable loss in front of LNA

[Chris Bartram]

Bodo

> Lets take following case:
> My LNA is situated a few meters behind my
> perfectly (nearly) matched antenna. The SWR of
> the LNA is about 7:1 (return loss ca. 2-3dB) which
> is highly common with MGF1302 preamps.
> The loss of the cable between antenna and LNA
> is 0,6dB.
> According to a.m. table my additional cable loss would be
> ca. 1,3dB.
> Since any loss in front of the LNA must be added
> to the noise figure, the NF will be finally instead of
> lets say 0,5dB
> then 0,5dB plus 1,3dB = 1,8dB.. Is this right???

No! This is a situation that's frequently misunderstood. 

Think of it this way...

Your antenna produces a _source_ impedance of close to 50ohms. Your preamp 
requires a source impedance of 50ohms for optimum noise figure.

Connecting a lossless matched 50ohm cable to the antenna transfers the 50ohm 
source to the end of the cable remote from the antenna. Therefore with a 
perfect cable, the preamp will produce the same noise figure. 

If the cable has resistive losses, the impedance seen by the preamp will not 
change, so its noise figure won't change, but the resistive losses of the 
cable will add to the noise figure of the preamp.

That's just the same as putting a matched attenuator between the antenna and 
preamp; something most of us do when making noise figure measurements!

Mismatch losses occur when you are trying to transfer power along a 
transmission line. In the case of a preamp you're not trying to do that. 
Rather, you want the preamp to see the source impedance which gives best 
noise figure.

Vy 73

Chris
GW4DGU

**Celebrating 30 years since my first 432MHz EME QSOs in 2008. Currently QRV 
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