New England HF Noise Bridge,
from the East Coast.     VA3DIW (2024)

 
Noise Bridge is a verified idea. Used during times when there were no micro controllers and portable instruments to measure antenna impedance.
The bridge method is straight forward way to estimate modulo |Z| of complex impedance. Lightweight for the backpack and camping. Easy to assemble during muggy freezing weekend. Power supply is 9 Volts. The carbon zinc/NiMH battery will last. Noise Bridge can be used for quick antenna test, receiver performance test, and cable test.

Do it yourself, DIY.

Measurement:
Connect the RX as a null indicator. Connect the measured load. You will hear noise. Then turn the potentiometer to find null of noise on specific RX frequency. That's it. The null by 50 ohms is surprisingly sharp. With variable RC, it will fully balance the bridge for (Re +jX). You can go without the var Cap C1, C6. Traps can be tuned with this bridge.
What is quite interesting, is the precision and simplicity. There is one advantage over the SWR meter. It tells whether the antenna impedance is lower or higher than 50 ohms. The impedance shall be measured by antenna connector. With software simulation in PowerSmith (c) program, you can move along the coaxial cable from shack to antenna connector SWR. The software also enables to move from antenna towards shack transceiver connector.

There are small changes.
The 120R and 0.1uF is there to suppress Q2 to D1 feedback, oscillations. Set your bias (33k Rb?) for particular transistors. The transformer use high permeability toroid (ur=2200) 4-5 turns. For noise source try zeners, diodes, or transistors in avalanche mode. Tested for the HF bands.



Source:
[1] New England noise bridge.pdf
[2] RSGB noise bridge.pdf

It is a tool without MCU, USB2, irreplaceable BGA chips, color touch TFT display burning 0.5A for display backlight, without breaking glass, Chinese fonts, corroded tact switches, 7-bit ADC resolution (ENOB - equal number of bits), measurement of signal harmonics, temperature instability, and with DC-separated antenna measurement port. Without no coverage signal, you can leave your smart phone at home.

  Possum. Are you a possum?

Flat antenna impedance profile is usually a sign of matching loss and lower radiation efficiency.
For example, too close stacking of fan dipole wires. The wires have coupling with different band wires.
Noise Bridge is simple, elegant, easy, and working tool.
"Another successful step to independence".

MADE by VA3DIW