Very good tutorial on noise bridge theory and operation. Back in the day, I bought the Heathkit version. I wanted to build a noise bridge and I wanted the detailed manual which Heathkit was famous for. Our radio club members used these noise bridges before the MFJ-259 became THE instrument for ham antenna tuning. Presently my "go-to" instrument for antenna tuning is my Comet CAA-500. It provides very simple operation and the cross-needle display is very readable in direct sunlight.
Interesting concept, and simple basic design of an impedance bridge. Similar concept used in older wire line telephone circuits where 4 Wire voice circuits are converted to a 2 Wire conventional circuit via a 2-4 Wire hybrid. The Hybrid contains an R/L network used to balance the impedance of a 2 Wire line terminating into a 2-4 Wire Hybrid, or Term Set. By adjusting capacitance and inductance to match the 600 Ohm 4 Wire line equipment, balance is achieved through the hybrid using a white noise generator, and measured with a Return Loss meter that measures SRL Hi & SRL Low band across the voice band of frequencies. The higher SRL/ERL losses indicate a "Best" match that indicates that the 4 Wire Hybrid is balanced to the 2 Wire section relative to the impedance of the 2 Wire line. A near perfect match results in very low reflected power being returned to the 4 Wire line. The concept is the same with RF, where the best impedance match will result in the least amount of reflected energy being returned. In the RF world, the objective is a low SWR. In telephony its the suppression of talker echo, and high pitched squealing, AKA A Singing circuit. A condition that is relatively rare in today's conventional telephone equipment as the copper loop disappears. Now, if a concise, and clear explanation of the relationship between the two adjustment knobs on the noise bridge were given, the noise bridge can be an attractive old school bargain VNA.
You could set up LRC in series and observe their voltages on an oscilloscope. Since all currents are synchronous, you can observe the relative phases directly and you can add V_L+V_C to see why we set Z=R+j(X_L-X_C). There is a caveat that we cannot wind inductors out of copper without resistance, but you could show the trend toward perfection by reducing a variable series resistance. By varying the frequency, you can vary the relative amplitudes of V_L and V_C.
If you have an ideal resistor as your DUT the adjustable capacitor should null at 150pf. If your DUT is predominantly inductive this will accentuate the capacitance on the test side and your adjustable capacitance will have to be increased toward the inductance side. If you DUT id predominantly capacitive this capacitance will be in series with teh 150pf capacitor and thereby reduce the capacitance on the test side (as you showed, putting capacitors in series reduces the overall capacitance) and you adjustable capacitor will have to be decreased below 150 pf toward the capacitance side. These are normally used to test antennas. If the antenna is too long it's inductance is dominant and it will read some where on the inductance side, and if it is too short its capacitance will be dominant and it will read on teh capacitance side.
I have had the Palomar RX Noise Bridge since 1994. Have used it several times to adjust CB and 2M antennas. This was before I had access to the MFJ-259 analyzers.
I have the 202. I also have the 201 'grid'-dip meter. The thing about this old-school gear is that you have to understand how they work. You have inspired me to experiment with them using my new-school toys, the NanoVNA and Tiny SA. So thanks.
Good video.. I have an old Palamor-Engineers noise bridge and it works great within its limitations. Some of them have died an early death by being left inline on a xmtr tune up. Luckily mine has not suffered that fate.
Would have been interesting to put this on your VNA on the 40 meter band, then you could show how the effective capacitance and inductance change as you change the capacitor. I have a noise bridge kit I have never built. Now I have a reason to build it!
I need to get me one of these, I have always said the one thing missing form my home station is the noise of a bridge. All those cars, buses and taxi's going over it, boats going under it, sharks, fish, seagulls....the works. Must have that noise.
Very well explained demonstration of how to see what the noise bridge is doing. However, how would you use it in the real world? Especially if one is repairing radios. Can it be used to more easily reduce noise in an antique radio? Lets see a repair application of the MFJ-202 box!
Ok, we understood how does it work, but what are the applications? I think the capacitor to measure inductor was cleaver. *** Thinking out loud: *** Also, we might connect an antenna as the unknown, and try to measure its impedance at some frequency (Maybe?) at given atmospheric condition. As an alternative circuit design we could use two similar antenna that are directed toward the same target as the two bottom resistors, get ride of the noise source but keep the transformer. Now the output of the transformer (where it was connected to noise source before) , gives us the net noise of these antenna elements, because the signals cancel out. We can add the signals of these two antenna, they act as array antenna now, we get the signal, and subtract the noise we found from the transformer, we can eliminate the elemental noise of the antenna (May be) This does not eliminate the path or channel noise, just the noise of antenna itself. we can have antennas and their LNAs in the above circuit, this way we can cancel out noise of the antenna and the LNA (Maybe)
Yes, but it's the nature of the reactance that is being uncovered here. Whether it's being caused by excessive capacitance (antenna too short) or excessive inductance (antenna too long).
@@uni-byte Yes of course, but my point was it’s specified in ohms XC (capacitive reactance), or ohms XL (inductive reactance); so although I haven’t measured it to verify, I think it’s not an arbitrary scale on the device, rather it shows the ohms of reactance, one side for XC and the other side for XL.
@@ruhnet Okay. I see what you mean. You could very well be right. They still sell a version of that noise bridge. I wonder if the user manual is on-line.
@@uni-byte Ah, great idea---I just looked and found a PDF of the manual. It says the righthand dial is showing the *value of the variable capacitor in pF.* That can of course be used with a formula to find the reactance value in ohms. (The manual also conveniently provides the formulas needed.) So that clarifies what the scale actually is. 😀
@@ruhnet Yeah, it can't be Xc or Xl in ohms because what we're varying is a capacitor -- the scale would have to change for different frequencies. What *doesn't* change is the actual capacitance of the component, so we have to convert to reactance in ohms when we know the frequency.
Yep. But there are people with a TinySA who can't afford even a NanoVNA yet, and they can build a noise bridge for next to nothing out of junk-box parts, or get one like this at a ham swap for $5, and merrily tune their antennas and filters all day long.
Very good tutorial on noise bridge theory and operation. Back in the day, I bought the Heathkit version. I wanted to build a noise bridge and I wanted the detailed manual which Heathkit was famous for. Our radio club members used these noise bridges before the MFJ-259 became THE instrument for ham antenna tuning. Presently my "go-to" instrument for antenna tuning is my Comet CAA-500. It provides very simple operation and the cross-needle display is very readable in direct sunlight.
Interesting concept, and simple basic design of an impedance bridge. Similar concept used in older wire line telephone circuits where 4 Wire voice circuits are converted to a 2 Wire conventional circuit via a 2-4 Wire hybrid. The Hybrid contains an R/L network used to balance the impedance of a 2 Wire line terminating into a 2-4 Wire Hybrid, or Term Set. By adjusting capacitance and inductance to match the 600 Ohm 4 Wire line equipment, balance is achieved through the hybrid using a white noise generator, and measured with a Return Loss meter that measures SRL Hi & SRL Low band across the voice band of frequencies. The higher SRL/ERL losses indicate a "Best" match that indicates that the 4 Wire Hybrid is balanced to the 2 Wire section relative to the impedance of the 2 Wire line. A near perfect match results in very low reflected power being returned to the 4 Wire line. The concept is the same with RF, where the best impedance match will result in the least amount of reflected energy being returned. In the RF world, the objective is a low SWR. In telephony its the suppression of talker echo, and high pitched squealing, AKA A Singing circuit. A condition that is relatively rare in today's conventional telephone equipment as the copper loop disappears. Now, if a concise, and clear explanation of the relationship between the two adjustment knobs on the noise bridge were given, the noise bridge can be an attractive old school bargain VNA.
I picked up a noise bridge for $2 at a swap. Sprayed the pots, put in a battery and it works fine.
You could set up LRC in series and observe their voltages on an oscilloscope. Since all currents are synchronous, you can observe the relative phases directly and you can add V_L+V_C to see why we set Z=R+j(X_L-X_C). There is a caveat that we cannot wind inductors out of copper without resistance, but you could show the trend toward perfection by reducing a variable series resistance. By varying the frequency, you can vary the relative amplitudes of V_L and V_C.
If you have an ideal resistor as your DUT the adjustable capacitor should null at 150pf. If your DUT is predominantly inductive this will accentuate the capacitance on the test side and your adjustable capacitance will have to be increased toward the inductance side. If you DUT id predominantly capacitive this capacitance will be in series with teh 150pf capacitor and thereby reduce the capacitance on the test side (as you showed, putting capacitors in series reduces the overall capacitance) and you adjustable capacitor will have to be decreased below 150 pf toward the capacitance side. These are normally used to test antennas. If the antenna is too long it's inductance is dominant and it will read some where on the inductance side, and if it is too short its capacitance will be dominant and it will read on teh capacitance side.
I have had the Palomar RX Noise Bridge since 1994. Have used it several times to adjust CB and 2M antennas. This was before I had access to the MFJ-259 analyzers.
I have the 202. I also have the 201 'grid'-dip meter. The thing about this old-school gear is that you have to understand how they work. You have inspired me to experiment with them using my new-school toys, the NanoVNA and Tiny SA. So thanks.
Good video.. I have an old Palamor-Engineers noise bridge and it works great within its limitations. Some of them have died an early death by being left inline on a xmtr tune up. Luckily mine has not suffered that fate.
Enjoyed vid thanks for posting. Concept summaries easy to comprehend (@ 10:45 'transformer serves to inject signal').
Would have been interesting to put this on your VNA on the 40 meter band, then you could show how the effective capacitance and inductance change as you change the capacitor. I have a noise bridge kit I have never built. Now I have a reason to build it!
I need to get me one of these, I have always said the one thing missing form my home station is the noise of a bridge. All those cars, buses and taxi's going over it, boats going under it, sharks, fish, seagulls....the works. Must have that noise.
The right knob is 20 ohm of capacitance reactance
I have the same noise bridge. It was accurate.
Very well explained demonstration of how to see what the noise bridge is doing. However, how would you use it in the real world? Especially if one is repairing radios. Can it be used to more easily reduce noise in an antique radio? Lets see a repair application of the MFJ-202 box!
Should be able to use it in some cases to impedance match different elements, though there will be some devil in the many details!
Ok, we understood how does it work, but what are the applications?
I think the capacitor to measure inductor was cleaver.
*** Thinking out loud: ***
Also, we might connect an antenna as the unknown, and try to measure its impedance at some frequency (Maybe?) at given atmospheric condition.
As an alternative circuit design we could use two similar antenna that are directed toward the same target as the two bottom resistors, get ride of the noise source but keep the transformer.
Now the output of the transformer (where it was connected to noise source before) , gives us the net noise of these antenna elements, because the signals cancel out.
We can add the signals of these two antenna, they act as array antenna now, we get the signal, and subtract the noise we found from the transformer, we can eliminate the elemental noise of the antenna (May be) This does not eliminate the path or channel noise, just the noise of antenna itself.
we can have antennas and their LNAs in the above circuit, this way we can cancel out noise of the antenna and the LNA (Maybe)
Is this basically a manual antenna tuner?
same idea
At 4:38 isn't reactance measured in ohms? (Frequency dependent resistance to AC current.)
Yes, but it's the nature of the reactance that is being uncovered here. Whether it's being caused by excessive capacitance (antenna too short) or excessive inductance (antenna too long).
@@uni-byte Yes of course, but my point was it’s specified in ohms XC (capacitive reactance), or ohms XL (inductive reactance); so although I haven’t measured it to verify, I think it’s not an arbitrary scale on the device, rather it shows the ohms of reactance, one side for XC and the other side for XL.
@@ruhnet Okay. I see what you mean. You could very well be right. They still sell a version of that noise bridge. I wonder if the user manual is on-line.
@@uni-byte Ah, great idea---I just looked and found a PDF of the manual. It says the righthand dial is showing the *value of the variable capacitor in pF.* That can of course be used with a formula to find the reactance value in ohms. (The manual also conveniently provides the formulas needed.) So that clarifies what the scale actually is. 😀
@@ruhnet Yeah, it can't be Xc or Xl in ohms because what we're varying is a capacitor -- the scale would have to change for different frequencies. What *doesn't* change is the actual capacitance of the component, so we have to convert to reactance in ohms when we know the frequency.
VNA
Yep. But there are people with a TinySA who can't afford even a NanoVNA yet, and they can build a noise bridge for next to nothing out of junk-box parts, or get one like this at a ham swap for $5, and merrily tune their antennas and filters all day long.
@@johnwest7993 I intended to suggest that the device be examined with his VNA.
@@johnwest7993 I got one at a ham swap for free. I didn't know what it was. I got it because I thought I could use for a project. 73
@@johnwest7993 I wish I had a junk box with decent 300pF air variable capacitors in it... This box is basically worth $5 for the capacitor alone!
Get to the point........
Chill out. Maybe some more cannabis would help
@@IMSAIGuy I felt that burn, boss.
I felt that burn.
😊