5/21/2018 An RF Noise Bridge

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Experimenter s Corner

A

Noise Bridge for

Antenna Measurements

Introduction

by

Keith Clark, WGSIY

Have you tried a noise bridge? These in-

struments are very handy for measurements

on antennas and feedlines, providing

separate readings of the resistance and reac-

tance components of impedance. This gives a

much more complete.picture of the load

characteristics than SWR readings. While a

selection of commercial noise bridges are

available at moderate cost, this is arelatively

simple test instrument for the home con-

structor to build and calibrate. Most of the

parts are probably already in your junk box

collection.

I11 cover some background material on

bridge circuit theory and variations, then

give the detailed design information for my

home-built unit. Finally, some antenna

measurements w ll be described along with

the math equations used to interpret ;hem.

Bridge

Theory

Let's s ta rt off with the simplest example, a

DC bridge for resistance measurements. (See

figure

1.4) The bridge consists of four

resistors R1. R2. R3and RL. RL is unknown,

or load, resistance to be measured. A DC

voltage is applied to the top and bottom of

the bridge. The two sides are connected to a

DC voltmeter which can read either polarity.

R3 is adjusted for a volt reading on the

meter. This occurs when

R1 + R 3

2 RL

At this null condition on the voltmeter, no

current flows through it, so it doesn't load

the bridge circuit. The null adjustment on R3

also does not depend on the applied voltage

level. These two characteristics are what

make bridge circuits so useful. Such a DC

bridge could be used for measuring resis-

tance values above or below the normal range

df a VOM

Now, let's adapt the bridge for RF

measurements instead of DC. (See figure 1B)

A single frequency RF source replaces the

DC source, and

an

'AC null detector

replaces the voltmeter. Figure 1C shows a

typical RF voltmeter circuit suitable for a

null detector. The same equation applies at

the null condition on the meter: RlIR2

R3IRL.

Consider a practical application for such a

bridge, figure 2. Resistors R1. R2 and R3 are

each 51 ohm 2 wat t carbon. A QRP transmit-

ter is used to apply RF to the bridge, and the

unknown

i m

of the bridge is connected to a

transmatchlfeedlinelantenna. A null on the

meter will occur when the t ransmatcb is cor-

rectly adjusted to make the antenna system

look like a 51 ohm resistive load. The bridge

resistors must be non-inductive, so wire-

wound power resistors are not suitable. Also,

the physical layout of the bridge should be

symmetrical so that stray inductance and

capacitance tends to cancel out. This isn't too

difficult for HF use, but would be really

critical a t VHF.

Figures 3.4 and 3B show two more varia-

FIG

A

DC BRIDGE

FIG 8

RF BRJffiE

FIG lC AC NULL PET

RF

OUTPUT

XMTR

FEEDLINE

TEAUSMATCH

FIG Low POWER BRIDGE

FOR

TRANSMATCH ADJUSTMENT

AC NULL

FIG A RF BRIDGE WITH F IG 8 RF BRIDGE Wl

CAPACITANCEARMS INDUCTANCE ARM

tions of RF bridges. Since each side of the

bridge is a voltage divider, reactive com-

ponents can be used just as well as resistive

ones. The load has also been shown in general

form as an impedance having reactance as

well as resistance. In figures 3A,

a

capacitive

voltaee divider is shown. This circuit is used

far t

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Figure 4A is a modification of 3B with the

two inductors equal and coupled to the RF

source through a third winding. A broadband

trifilar wound transformer with a ferrite

toroid core would be appropriate. Because

the output voltages are equal in the top and

bottom halves of the hridge, our null equa-

tion becomes: Z3=ZL, again with complex

impedance.

FIG

A

TRANSFORMER

FE

RF

RlCGE

The RF source in 4A is a single frequency

output which could he set on any desired fre-

quency in the HF amateur bands. If VERY

low level RF was output, i t could also he used

for antenna measurements outside the ama-

teur bands. The AC null detector has broad-

band response, so no changes to it are

re-

quired at different frequencies.

Now consider the circuit of figure 4B. I t

has a transformer jus t like the one in 4A, hut

a RECEIVER is used as the null detector.

The same equation applies at null:

Z3=ZL,

complex impedance. We would still use a

single frequency RF source set at the

receiver frequency, hut a more useful one is a

broadband noise generator putting out RF

energy throughout the HF frequency range.

The null is observed bv listenine to the noise

in

the receiver speaker, or

hy

Lalching the

S-meter. In circuit I n we have finally arriv.

ed at the 'NOISE BRIDGE" so you can

breathe a sigh of relief.

Circuit 4A had

a

narrow hand RF source

and w~deband etector. The RF' source is set

L O the ~ P S I T P ~easurement frequency

Cir-

cuit 4B. the no w br~dge, as a wrdeband RF

source and narrow-band detector (the

receiver). We set the receiver to the frequen-

cy at which we wish to measure the load im-

pedance. This gives us two advantages over

the 4A circuit. First, we are likely to already

have

a

receiver with accurate frequency

calibration. Second, the 50 milliwatts

(roughly) RF noise output is spread over 30

MHz or more, so it won't cause interference

to anyone else, even if it puts an S9 noise

signal into our receiver. We have to be more

careful wi th single f requency

outouts-vou 're all well aware of how far a 50

mV;

W

iignal can go With such low noisr

output power,

it

is necessary

10

make sure all

antenna connections are right.

N 15E

SOURCE

FIG

8 NOISE BR l bS

RIDGE

Home-built Noise Bridne

Figure 5 is the schematic of-a practical

Noise Bridge design which I built for anten-

na measurements. A zener diode is used as

the broadband RF noise source, with three

untuned stages of amplification. Minor varia-

tions of this circuit can

be

seen in the listed

references. Power for the circuit is obtained

from a 9-volt battery.

A

hroadband

transforn~eicouples the RF noise to the

hridge with it's secondary windings forming

the left two arms of the bridge. This is like

the arrangement of figure 4B.

The transformer is trifilar wound on an

Amidon FT37-43 ferrite toroid core. Three

lengths of HZ6 wire are first twisted together,

a fes; twis ts per inch. Then eight turns of the

bundle are wound on the care. Dots on the

schematic all indicate one end of the wire

group. The center tap of t he secondary has a

wire from one end of the bundle tied to a dif-

ferent wire from the other end. The opposite

ends of these two wires connect to the right

side hridge arms. The third wire is used as

the primary, and is connected to the final

noise amplifier stage through a capacitor.

The arrangement of the two equal-

impedance arms on the right side of the

hridge needs a little explanation. The lower

arm has a coax jack for attaching the load.

but in parallel with it is a variable capacitor

having a maximum capacitance of about 400

pF. Thus, the impedance in this arm is not

jus t the Load impedance, but is the parallel

combined impedance of the load and the

capacitive reactance of the variable. The up-

per arm has a 100 ohm varihle carbon resis-

tor in parallel with a fixed capacitor. This

capacitor is selected with a value about half

the maximum value on the variable

capacitor.

I

obtained 225 pF by paralleling

150 and 75 pF disc ceramic capacitors.

Let's see how this works. First we attach a

51 ohm carhon resistor t o the

unknown

r

'load" jack using very short leads. The

receiver is attached to the "rcvr" jack via a

shor t length of coax, and power to the bridge

and receiver is turned on. Set the receiver to

a low frequency like 3.5 MHz. A loud noise

should he heard in the receiver. like S9+

FIGURE

NOISE BRIDGE

SCHEMATIC

QRN. The variable resistor and variable

capacitor on the hridge are both adjusted t o

obtain a null in the receiver noise. I t should

go down to barely audible after a few hack

and forth adjustments of the bridge vari-

ables. At this point, the variable bridge

resistor should end up at 51 ohms, and the

variable capacitor should he set mid-range a t

a value equal to the fixed one in the other

hridge arm. Each right side arm of the hridge

has 51 ohms resistance in parallel with 225

pF capacitance and the hridge is balanced for

the test load of 51 ohms resistance.

At this ooint. we should trv different fre-

~

quencies on the receiver up to the maximum

one to be used 28 MHz in my case. The null

positions on the variable resistor and

variable capacitor in the bridge should not

change. If they do, it is an indication of str ay

inductance or capacitance problems and

some re-arrangement of the bridge arms may

be needed for better symmetry. I saw

a

varia-

tion of only a couple of ohms and few pF over

the HF bands. We can calibrate the

resistance dial by using various carhon load

resistors (5 to 95 ohms) like this and finding

the null, or we could just use a VOM to

measure settings on the variable resistor

(disconnect the receiver and load resistor).

Now, how about loads which have reac-

tance as well as resistance? We will treat the

load itself as a resistance in parallel with an

inductive (=X)or capacitive (-XI reactance.

The fixed and variable capacitor arrange-

ment of the bridge arms allows both types of

reactance to be measured. A couple of ex-

amples will show how this works.

Suppose this load impedance actually Iwk-

ed like 70 ohms resistance in parallel with 20

pF capacitance. We would obtain a null on

the bridge with the variable resistor at 70

ohms and the variable capacitor at 205 pF.

This capacitor setting would add in parallel

to the 20 pF load capacitance to give 225 pF.

equal to the fixed capacitance in the other

arm.

Now suppose our load was 70 ohms resis-

tance with parallel inductance such that at

the frequency of interest, the parallel induc-

tive reactance was the same magnitude as

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the capacitive reactance of a 20 pF capacitor.

X~=j2(pi)FLXC=-jl(2lpi)FCl. Then we

would get a null with the bridge variable

resistor a t 70 ohms and the variable

capacitor a t 245 pF. The extra 20 pF is used

to cancel the inductive reactance, leaving a

hridge load arm that looked like 225 pF in

parallel with 70 ohms resistance. I t sounds

complicated to describe, but works out easily

in use.

The variable capacitor dial is calibrated in

oF. with 0 a t the 225 o F ooint and one side

marked "inductive" and ;he other marked

^capacitive."Calihration is done by placing. a

47 ohm carbon resistor in the 'unknown"

jack, and placing various known capacitance

values in parallel with it. I used disc ceramics

of 30. 56. 82, 100 and 150 pF. mostly IOU.

Dial ooints are marked where the noise null

occu;s on the capacitor, which will be set to

values lower than 225 pF.

The process is repeated by placing the

same capacitors in parallel with the variable

resistor arm of the bridge to calibrate the

other half of the dial. This side is for measur-

ing inductive loads. The 47 ohm resistor is

left in the "unknown" ack during this part of

the calibration.

My hridge was built on a thick piece of PC

board material with the copper removed.

This open construction allows good access to

the circuitry for calibration and use of the

hridge.

I performed calibration while just

holding the capacitors in place, without using

solder. In a similar manner, it is possible to

extend the measurement ranee of the b r i d e

hy paralleling extra capacitance or resistance

with either arm of the bridge. Because of the

low power, they can be held in place hy hand

for quick checks.

Bridge Calculations

Suppose we wish to find the impedance of

an unknown load, let's say a dipole antenna,

a t 1040 khz. We at tach the receiver to the

bridge and set it to the frequency, then at-

tach the antenna lead-in to the "unknown"

jack. Power is turned on and we adjust the

bridge variables for a null in receiver noise.

On my Delta, it goes down from above S9 to

S3, the minimum reading on the meter. Lets

say the variable resistor setting was 60

ohms, and the variable capacitor setting was

25 pF on the capacitive side. We can

calculate the capacitive reactance XC=

-jl(2piFC)=-j904 ohms. Ow load at 7040

looks like a 60 ohm resistance in parallel with

a 904 ohm capacitive reactance. Usually, it is

more convenient to express this as an

equivalent series ckcuit instead of a parallel

one. The equations for making this transfor-

mation are:

Going thmugh the equations, we get

Rs=59.7 ohms and Xs=4. 0 ohms

(capacitive).The equivalent series circuit is a

59.7 ohm resistance in series with a 4 ohm

capacitive reactance at this particular fre-

quency.

Suppose instead that the variable

capacitor setting was 25 pF on the Tnduc-

tive" side of the dial. Now we are dealing with

July 1987

an inductive load, not a capacitive one. The

reactance is calculated with the same for-

mula as before, hut it is the amount of

capacitive reactance used to cancel the induc-

tive reactance in the load. Xc=904 ohms

also.

The same equalions apply for transforma.

t ion to the equivalent series load. so it would

be 59.7 ohms resistance series with 4 ohms

inductive reactance. These two different

cases would he represented in complex

arithmetic as:

59.7-j4.0 ohm capacitive load

59.7+j4.0 ohms inductive load

The series or complex form is needed for

feedline calculations with a Smith chart.

When we make measurements as described.

we're finding the impedance seen at the

transmit ter end of the feedline. This im-

pedance can vary quite a lot with feedline

length if the SWR is not equal to 1:l. A

Smith char t can be used to find the actual

antenna impedance if feedline length is

known.

Noise Bridge Circuit Variations

The schematic given in figure 5 is pretty

much standard for the noise source. Varia-

tions from 2 to 4 amplifier stages are

referenced with some transistor biasing dif-

ferences noticed. The article in 73 magazine

featured a I-transistor noise source which

was quite effective. However, I had no luck

achieving circuit balance over the HF fre.

quency range.

Another area of considerable variation i s in

the coupling transformer between the noise

source and the hridge. A toroid is generally

used with 3 or 4 windings. The 4 th winding

may he left unterminated at one end. The

"Ham Radio" article described very detailed

adjustment and calibration of a precision

hridge using a 4-wire transformer with the

wires twisted together hut not wound on a

core.

Bridge elements can be arranged in several

ways. Some circuits feature the reactance in

series with the resistance lor "unknown") in

both arms. This eliminates the need for the

parallel lo aeries imp dan ce cdcularions.

Ilowrver. ir might hc less convenient for

i n

serrlnr additional components for range

ex

tmsiun. The parallel equivalent is nicer for

calrulating Dower to rhe load i RMS voltage

is measured. Some designs place the variaGe

resistor and variable capacitor in the same

bridge arm, with the fixed capacitor across

the "unknown" load. Since these variahle

components are rather large and require

space for dial calibration, it is easier to

achieve bridge layout symmetry if they are

placed in different arms.

My noise bridge has a capacitor in the

outut to the receiver. This was not needed

with test loads on short leads. However,

antennalfeedline measurements would not

give a good null until the capacitor was add-

ed. I'm not sure why this was needed, but it

was a simple trial and error cure for the pro-

blem.

Antenna Measurements

Antenna measurements can he accomplish-

ed very quickly using the noise hridge. With

everything hooked up and turned on, the

hridge dials are adjusted for a noise null.

Resistance and capacitance values are

recorded, notingif t he capacitor dial is on the

"inductive" side. The impedance calculatons

take a little longer, as does the analysis of

feedline effects using a Smith chart . See the

ARRL

ntenna Book for information

of the Smith char t it's not difficu

there isn't room here for a detailed ex

tion. I've made several measurements

antennas a t frequencies X I O S 3 each

HF bands. However. Illon ly show resu

a couple of cases - the dipole at 7050 k

the Miniquad at 14050.

Dipole

My 40120 meter dipole has separat

elements for the two bands. They all

downward, inverted vee fashion, exc

the 40 meter leg which goes level ov

house roof. The center is just over 1

high and driven directly with some

RGRAU and RG58AU coax totalling

feet length. This coax should have a ve

factor of .66 for an electrical length o

feet. A waveleneth at 7050 khz is 139

so we would ex-& the feedline to loo

0.315 wavelength.

This can he checked by temporarily d

necting the antenna from the feedlin

replacing it with a resistor such as 82

Pick a value away from 50 ohms line

dance, but not too close to the 0 or 10

range limits of the hridge. A noise

reading is made at the transmiter

parallel reactance calculated and rea

transformed to seriesimpedance. This i

entered on a Smith chart and the

wavelength read out. Mine measured

wavelength a t 1050 k z the old coax

velocity factor of 0.61 and this was d

checked at 14 MHz.

With the antenna re-attached t

feedline, noise bridge readings were tak

the transmitter end. They were 70

resistive and 35 pF on theinductive sid

35 pF represents 650 ohms inductive

tance in parallel with the 70 ohms resis

The transform from parallel to

eauivalent imnedance eives the

.

69s j7. 5 ohms at the rransmll rer end

line. This is the anrenna sys tem impeda

7050 khz and it represents an SM R o

read from the Smith chart.

The Smith chart was then used to tra

back up the feedline 0.406 wavelen

what the impedance of the antenna

was. This assumed loss-less coax, givi

same SWR at the antenna end. Impeda

the antenna itself came out 60-j16 oh

this frequency, a capacitive value.

Mini-Quad

My HQ-1 Miniquad is mounted a t 23

and fed with 51.2 feet of 3-vear old

Shack RG58U. A balun is installed be

the feedline and antenna. The feedline

was measured and found to have a ve

factor of 6.5, much closer to the expect

value. The line came out t o 1.118 wave

at 14050 k z for an electrical length o

feet.

The antennalfeedline impedance

measured on the noise bridge as 18

resistive in parallel with 240 pF capac

at 14050. Extra parallel capacitance

need to extend the instrument range. T

18 ohms resistance in oarallel with 47

capacitive reactance. Transforming t

scrl rs equivalent given 16 ohms reskta

series with 6.0 ohms capacitive reactan

16-j6 ohms for an SWR of 3 2 1 on the

chart. This can be transferred back u

feedline to an an tennabalun alone impe

of 36-j50 ohms. In this case i t wa

ee

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