Re: A comment about Receiving

Posted by Dale Parfitt-3 on Mar 29, 2011; 1:55pm
URL: http://elecraft.85.s1.nabble.com/A-comment-about-Receiving-tp6216105p6219272.html


I believe I understood all of your comment Ron.
But as our noise floor at HF is limited by external noise (atmospheric,
manmade and galactic) I don't understand how efficiency, within reason,
comes into play for a receive antenna. Certainly, for signals above the
noise floor, adding in 3 or 10 of attenuation makes no difference in the S/N
ratio, and since the preselector cannot discriminate between signal and
noise, I still don't understand the mechanism of how a preselector improves
S/N unless its BW is better than the BW of the receiver- and I can't imagine
that situation either.
One of my best low band antennas is a rotatable half sized Flag whose gain
runs around 30dB below my sloper.

Dale W4OP

> The antenna tuner acts as a pre-selector so to speak, by making the sytem
> of
> the antenna, the transmission line, and the radio, resonant, and matched
> impedence.
>
> All of this adds up to minimize losses in a couple of ways. The impedence
> matching means that the xmission line and antenna are as  close to the
> input
> impedence of the radio (nominally 50 Ohms these days). If it is not at 50
> Ohms then you do not get maximum power in to the rig based on simple
> simple
> Ohms law power.
>
> A highly simplified way of explaining resonance part of "a resonant
> antenna"
> is the act of making sure all of the current is in phase with the voltage
> so
> that when the power does get to the reciever, that it is in a useable form
> to drive things. It is somewhat the reverse or inverse of a good spring
> and
> shock absorber system on a car....if you have things wrong, and drive
> along
> a rough road with bumps spaced jsut right relative to how fast, then all
> heck breaks loose; maximim power transferred, and that is a resonant
> system.
> If things are not resonant, well, you get a smooth ride in your car, or in
> the case of your radio, poor reception (and transmission).
>
> -ron WT5RZ
>
> --
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