Butch:

Thanks for the great noise bridge explanation!

I feel I have learned something today!

Let me know when you get yours, and how it works for you.

I may be next on the list to purchase one.

73 from Tom Behler: KB8TYJ

----- Original Message ----- 
From: "Butch Bussen" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, August 16, 2012 8:07 AM
Subject: Re: Tuning aids, noise bridges, etc.


> 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.
>