The Omegaf Antenna Noise Bridge

Noise Bridge

If you have to adjust any antenna matching sections, determine antenna resonance frequencies or cut coaxial lines % or J£ wavelength long, the new Omega-t* Antenna Noise Bridge is one of the slickest little gadgets around. When you Ye trying to prune an antenna or tune a gamma match with an ordinary antenna impedance bridge and griH-dip oscillator, von almost need five hands. Not so with the TE-7-01 Antenna Noise Bridge—it will fit in the palm of your hand and (lie only auxiliary equipment you need is a receiver.

Since the noise bridge covers the frequency range from ! to 100 MHz. and will measure impedances from about 10 to ]00 ohms, it is ideal for the 50-obm systems used by most hams. To tune up an antenna all you have to do is connect the antenna and receiver to the bridge terminals and set the dial

urdstji Tc^ns 75<>80

near 50 ohms, ( hen you tune the receiver over the frequency range for which the antenna was designed looking for a tut: i in the noise output of the noise bridge. Since noise output is more than sufficient to mask any received signals, false indications are eliminated, When there is a noise null on your receiver, either in audio level or minimum S-meter reading, the resonant frequency of the antenna may be read off the dial of the receiver.

After the resonant frequency been determined, ilie noise bridge dial is adjusted for best noise null. The antenna impedance may be read directly from the dial When the noise null is found, the potentiometer (impedance dial on the front of the bridge) may sound scratchy and noisy. This is because of fhe high resolution capability of the device, The balanced-bridge condition represents a ratio greater than 30 dB and measurement rrso!nt'on is a small fraction of an ohm.

Therefore, slight movements of the dial came a large change in the noise null level.

The Antenna Noise Bridge may also be used to determine the electrical length of coaxial line—either one-half or one-quarter wavelength long (or multiples thereof). An antenna-matching system may be adjusted to the proper impedance with the noise bridge (after the antenna is tuned to the desired resonant frequency) by alternately adjusting the match and the noise bridge. Because of the effects of coaxial-line length this should be done with the noise bridge connected directly across the antenna terminals. However, if a coaxial line must be used, it should be one-half wavelength long or a multiple thereof.

he secret to the operation of the Ome-ga-t Antenna Noise Bridge is the special quadrafilar wound toroid shown in Fig. 1, On rnie side of thos toroid is the wideband noise generator; the antenna, receiver and calibrated potentiometer are connected to the other winding. When the noise across the resistance arm equals the noise across the antenna, the bridge is balanced and minimum noise is applied to the receiver.

The wideband noise generator is a circuit designed specifically for this job. The noise source itself is a reverse biased diode—especially selected for wideband noise output. The noise is amplified and applied to the quadrafilar transformer through a three-transistor circuit- The design is such that the noise is balanced across the generator side of the transformer.

One of the big advantages of the Antenna Noise Bridge, of course, is its size and the speed with which impedance measurements can be made. This applies to any antenna that you may use—whether it is a beam, a dipole, a quad, whip, long wire, or random length wire with an antenna tuner, The only limitation is the frequency and impedance range of the unit. For most hams using 50-ohm coax this should pose no problem.

There are several other jobs that you can do very simply with the noise bridge, if, for example, you want to know what frequency range you must operate in to limit your SWR to less than 2:1, all you have to do is run some impedance points from one end of the band to the other. For an SWR less than 2:1, your upper and lower band limits will be determined by the point where the impedance indicated by the bridge is less than 25-ohms or more than 100-ohms. For an SWR

Omegfa-t TE 7-01 Specifications

Frequency range: Impedance rnnfre:

Associated equipment required ;

Circuit :

System* which may be t^sti^I :

Power supply

Price :

1 to Í00 MHz. 0 to 100 ohms (for nominal 5G-ohm coaxial systems), Receiver ^vhich tunes frequency oT inferos t* S-rneter useful but not required. Masks normal received signals to eliminate false Indication s,

Quadrafilar wound bridge transformer, 3 transistors» 1 special diode.

Antennas—quads, beams, di-poles, whips» lontf wires, random length wires with matching networks. Conxial mtftrhinp: systems — sei'ies* shunt, (Tftmmr 9-volt transistor radio battery,

of 1.5:1 or less, your points would be 33 and 75 ohms.

Another application for the Antenna Noise Bridge is checking baluns. Have you ever wondered if that balun really represented a step down of 4:1 (or 1:1)? Just connect a 200 ohm resistor across the output terminals of the balun (50 ohms for 1:1 baluns) and measure the input impedance with the bridge. You can also check the balun to see what frequency it is cut for—at frequencies very far ofi its center frequency it won t provide the desired transformation ratio, A similar cheek can be run on the broadband baluns that are currently on the market. Since the Antenna Noise Bridge may be used

Was this article helpful?

0 0


  • awate
    How to measure antenna impedance?
    9 years ago

Post a comment