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From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of
[EMAIL PROTECTED]
Sent: Sunday, February 11, 2007 10:23 PM
To: [email protected]
Subject: RE: [Repeater-Builder] Advanced Receiver Research Preamp 144-148
At 2/11/2007 16:02, you wrote:
> > If you really want to isolate the preamp from the duplexer,
> > you need to put
> > an isolator ahead of it.
> >
> > Bob NO6B
>
> From a matching standpoint, yeah, but now the NF goes up due to the added
>losses of the isolator. You might come out ahead by modifying/retuning the
>preamp's input network for a better input match even if means sacrificing a
>few tenths of a dB of NF.
>
>Ponder this Bob (and anyone else). Assume the preamp input is connected
>directly to the output port of the isolator. For the sake of argument, say
>that the preamp has a NF of 0.5 dB, and 6 dB input return loss. The power
>transfer from isolator output to preamp input would be 75% for 6 dB RL; the
>other 25% of reflected power was lost/dissipated in the reject load.
>Wouldn't it be mathematically correct to say that there was effectively
1.25
>dB of additional loss ahead of the gain stage, and therefore the noise
>figure would likewise then be 1.25 dB worse (i.e. 1.75 dB)? Yes? No?
>Maybe?
No. The reason is that the noise figure is specified for a 50 ohm input
feed to the preamp, IOW the noise source used to test the preamp is 50
ohms, so any power lost due to mismatch at the preamp input is already
taken into account in the measurement.
Bob NO6B
True that the noise figure is specified for a 50 ohm input but that is for a
non reactive 50 ohm input. Once you hook it to a cavity or antenna etc. you
no longer have a pure 50 ohm input so the noise figure may not be the same.
73
Gary K4FMX
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