I should have reread this one more time,  I said left and right is input 
and output, actually, think of this as the input to the bridge, in my 
example, ten volts across these two points.  Sorry for the goof.
73
Butch
WA0VJR
Node 3148
Wallace, ks.


On Thu, 
16 Aug 2012, Butch Bussen wrote:

> Here is how I understand  how a noise bridge works.  The noise bridge
> works on what we call a wheat stone bridge.  It is the same principal as
> the old Simpson s f b meters with the pointer use to work.  Picture a
> diamond, 4 resistors.  On the left and right of the diamond is the input
> and output.  For the moment, we assume all resistors are the same value.
> Now the top and bottom of the diamond of these 4 resistors is where we're
> measuring, the test points.  If the resistors are all equal, there is no
> voltage difference between these two points, top and bottom test points.
> No voltage drop, no current flow.  You have same voltage drop across each
> of the 4 resisters .  So let us say we apply 10 volts to left and right of
> the diamond, what I called input.  If we assume, just to make things easy
> to figure, say each resistor is 1 ohm, we'd have one ohm plus one ohm on
> top leg, 2 ohms, one ohm plus one ohm on bottom leg, again 2 ohms.  We
> have two two ohm resisters in parallel.  Hope this all making sense.  Now
> if you think about it, across each resister there would be a 5 volt drop.
> Across the two middle tie points, top and bottom, there is no voltage, no
> current flow, and we call this balanced.  This is how the old audible
> meters from s f b worked.  When you balanced the bridge, the tone was
> nulled out.  They replaced the meter circuit with a wheatstone bridge.
> Now is we change some values and make the antenna, a 50 ohm antenna one of
> the legs, we can make a noise bridge.  The old palamars and , the first
> one I had was Omega t as I recall, had a pot on it.  It had input and
> output connectors.  Input went to your antenna, output was the receiver.
> Now, go back to our diamond and replace the top left resister with the
> antenna, and the bottom right with a pot.  The way these worked is you
> tuned for minimum noise.  These bridges  had a noise generator, very broad
> band, although they wouldn't go hardly to six.  This noise is applied to
> the left and right input points of our bridge.  The receiver is hooked to
> the  top and bottom test points.  You turned on the bridge and tuned
> across the band until you found a dip in the noise.  Remember we replaced
> one resister with the antenna and other with a variable pot.  Once you
> found the dip, say a 50 ohm match or say a 70 ohm, whatever.  You then
> tuned the pot for minimum noise fed to the receiver, which you may recall
> is across the output of the bridge, top and bottom test points.  When the
> pot reaches its lowest point, its resistance matches the impedance of the
> antenna at that frequency and if you read the pot, that is the impedance
> as well as the resonant point.  The bridge is again balanced.  With the
> old palamars, you had to turn the rf gain way back or turn off a g c if
> you could so you could hear the dip oint as you tuned across.
>
> I don't have mine yet, but here is how the vectronics works as best as I
> understand.  Same as above except it puts out a pulsing noise, so it easy
> to hear.  The leg where the pot was above is 50 ohms, the antenna or tuner
> is the unknown.  You turn on the noise bridge, and start adjusting the
> tuner.  When it is 50 ohms, the bridge is balanced and noise is minimum.
> I'll report when I get mine.  The main problem with the older bridges is
> if you bumped the mike button or transmitted into them puffff!!!!, gone
> forever.  As I read it, the vectronics has built in protection if you
> forget and leave it on, so you can leave it in line all the time.  I had a
> friend who use to say, I just asked you what time it was, not how to build
> a watch.  <Smile>  But, you did ask how it worked.  Hope this makes sense.
>
>
>
> 73
> Butch
> WA0VJR
> Node 3148
> Wallace, ks.
>
>