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Shortwave Listener Site -ZS1JHGFRIDAY, FEBRUARY 11, 2011

Noise Bridges - How to use themOk so you have been making a lot of noise about NoiseBridges for testing for impedance and reactance insetting up simple antennas like a dipole. So what else can it do ?

Palomar Noise Bridge note clip for 9v battery mounted on the outsideTrap dipoles : The noise bridge will give a null on each band that the trap dipoleresonates.Start with the highest frequency band and measure impedance and reactance as for anormal dipole. Adjust your antenna cent re section ( or lower for a vertical) resonate thenrepeat on the next lower band and so on. Th is method is used on both horizontal andvertical trap antennas.

Beam antenna : Connect the noise bridge to the driven element. Tune your receiver to

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the operating frequency and read the impedance and reactance. Adjust the elementlength to bring to reso nance.

Testing Baluns : How do you know if your homebrew balun is working ? You cannot testwith a multimeter as this will show a direct short at DC. So connect your noise bridge to

the coax input of the balun. If its a 1:1 balun place two 25 ohm carbo n resistors in seriesacross the output connectors. Now turn on the noise bridge and adjust for a noise null(or S meter reading). You should see an impedance of close to 50 ohms and reactance of almost 0.

Can one get an indication of the makeup of the reactance i.e. capacitive or inductive.With some noise bridges (example : Palomar) a reactance chart is supplied standardisedto 1 Mhz frequency , to find higher frequency values the values per the chart are dividedby the required frequency in Mhz.

There are also computer programmes that can do this for you such as TLA.ex by N6BVDean Straw which has a special input for noise bridge readings. ( More details on howthis all works with examples later )

Lastly you can also use a noise bridge to give course measurements of InductanceandCapacitance. To do this a standard 100 pf silver mica capacitor and a standards 5microhenry inductor are used in series with the component being measured. There isalso a programme that will work out the L inductance or C capacitance from the noisebridge measurements obtained. If you like maths ....... L = 25.330/ f2C (f2 is f squared)

C= 25.330/f2Lf=resonant frequency , L= inductance in microhenrys, C= capacitance in p icofarads.

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