--- "Jim B." <[EMAIL PROTECTED]> wrote:
> > mch wrote: > > > Actually, 3 dB is the point at which you can first > notice a change in > > the signal. As far as whether it is worth it is up > to the individual. > > > > If a signal is 'fine', lowering it 3dB should not > make it unusable. The > > change should be noticable, but that's about all. > Of course, that all > > depends on the baseline of 'fine'. > > > > Joe M. > > And remember-dB is relative. A 3dB change in power > output will seldom > result in a 3dB change in signal strength at the > other end. For these > kind of measurments, you need to do it with a sig > gen in a 'closed' > circuit, ie, plug the coax directly in to the rx to > eliminate fades, > 'multi-path', etc. > Also, don't forget that 3dB is a factor of two ONLY > if you're talking > about power. For voltage it's 6dB for a factor of > two. so if you're > measuring rx sensitivity in uV, going from .5 uV > down to .25 uV is 6dB, > not 3. > -- > Jim Barbour > WD8CHL Two things: First, I would think that if you changed the power out (not antenna) by 3 dB, then at the RX site you would see a 3 dB change during an instant A/B comparison, where fading, multipath, what have you, would be the same on both signals. Same thing if you averaged the signal strength, you should see 3dB of change. There is nothing in the air that works in a non-linear way with RF power. Second, I see this 3 dB thing bantered about as the smallest change that can be heard. I think this is wrong, due to personal experience, and talking to others. Most CW ops will tell you that when someone changes power by 3 dB, that it will make a good difference on the RX end, when the signal is in the noise. One dB of change down would be roughly 90% of the original voltage. This means that a .5 microvolt signal would now be .45 microvolt. If this is where your squelch threshold is set, then 1 dB could make the difference of no signal going through a repeater, or "readable" signal going through the repeater. Joe __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
