7/25/2019 Home Brew X Band Wavemeter

1/4

M

Olberg WISNN

Loretta Road

M A 2154

Home Brew

X Band

Waveme

ndispensable test gear

e need to know the op-

erating frequency of an

l in th e

t with an adjustable cav-

it y wavemeter and a detec-

sees this energy and con-

resonance indicator.

The wavemeter must

Photo

A.

Cavity wavemeter

for X-band.

have some sort of dial

which can be calibrated in

frequency. The accuracy of

this calibration will depend

upon many factors, such as

the temperature stability of

the cavity, the resettability

of the adjusting system, and

the exactness o f the calibra-

tion reference. There are

others, but these are the

three which wil l concern us

while constructing a suit-

able unit. The other prob-

lems will become evident

as we move along.

A cavity wavemeter

i s

a

parallel resonant circuit

(see Fig.

1 .

You have used

these devices as traps in the

output of your transmitters

on the lower frequencies

and in many other circuits

of your equipment. The op-

eration of a microwave uni t

isn't different from those.

The Q of the circuit in

Fig. 1 Parallel resonant

equivalent circuit of wrve-

meter.

for microwave mavens

which i t is used and the

Q

of

the wavemeter itself make

the difference for a cali-

brated frequency refer-

ence. A microwave cavity

such as the device we are

about to construct can

have a working

Q

of nearly

3000 The working

Q

will

depend upon the loading of

the cavity and several other

factors.

An important limitation

to recognize is that the

device we are making will

put you within the band

limits, but it is no t an abso-

lute frequency meter.

A cavity wavemeter cir-

cuit diagram might look

like the ci rcu it shown i n Fig.

2

A difference in the circui t

when using waveguide for

the transmission line will

be, for example, that the

coupling loop in Fig. 1 is a

C A V I T Y

7 5

A V E G U I D E

Fig. 2. Cavity wavemeter cir-

cu it diagram.

one-turn link to the tank

c o ~ l n the microwave con-

f~gu ra t~one wil l use, th~si s

simply a small hole In the

E-plane wall of the wave-

guide The diameter of the ,

hole determ~nes ow much

coupli ng there i s to the tank

clrcuit, wh~ ch,n our case, S

1

the cavlty The d~ameter f

the hole controls coupling

and many other factors lt

must be placed carefully so

that the m atch to the trans-

mission l ~ n e ill not be up-

set

1

Now, let us examine the

construction of a cavity

wavemeter fo r X-band. (See

Fig. 3. The cavity we will

construct is, in microwave

parlance, a right circular

cylinder cavity operating n

the

TE,,,

mode. It is mount-

ed on the E-plane of an

X-band waveguide, off cen-

ter so that the center of the

cavity will sit directly cen-

tered over a hole in the

waveguide. This hole is

called an iris hole. The re-

mainder of the cavity

bot

tom, which hangs over the

waveguide, is covered with

a brass half-circle soldered

in place. This is shown in

73 Magazine March,

1981

7/25/2019 Home Brew X Band Wavemeter

2/4

IOP V IE W, E - P L A N E

?G -52 /U WAVEGUIDE 112

X

I

( OUT S IDE D IM E NS IONS )

S E E DE T A IL

m

\ U G - 3 9 / U F L A N G ES ( 2 R E P ' D I

DE T A IL ON CY L INDE R

TOP PLATE AND

CAVITY OVER IRIS HOLE

T HRE A DE D COL L E T

B R A S S

fMENSIONS ARE IN INCHES

DO NOT SC 4L E DRAWING

1/2 COVER PLATE

FOR CAVITY BOTTOM

1/16 1 . BRASS SHEET

DRILL AND TAP

m f 1 / 4 - 3 6

3 / 8 - 2 4 T H O

TO FORM COL LET- SAW CUT PLAC ES 120 '

APART TO TOP OF PLATE (AF TER TAPPING

g

,

i

?6

A ND T HRE A DING)

D R I L L A ND T AP F OR 4 - 4 0 X 1/41 LONG SCREW

C3AE, , E$20' APAR T TO MAT CH HOL ES ON

---TIGHT F IT INSIDE 1 3 8 10 WATER P IPE (COPPER)

114 ln DIA . BRASS R OD

1 -3 HD

B

D I AL B b R i E L

3 / 8

in.

0.0

WATER

CAVITY

DRIL L 3 P L A CE S P L A S T IC T UB E

I 2 O 0 A P A R T FO R 4 - 4 0 F L A T

HE A D S CRE W S , COUNT E RS INK

i i /H

F ,

t

1/16 A A:4IA

TUNING PLATE

PLASTIC DISC

DE T A IL S HOW ING CA V IT Y

S OL DE R E D T O W A V E GUIDE DR IL L 1 14 D IA E X A CT

E ND V IE W

v

Fig 3. Construction details for cavity wavemeter.

-the photos and drawings ac-

companying this article.

Tuning of the cavi ty is ac-

length. This i s done by mov-

ing a circular plate longi-

d t o the circular plate

es as the device t o

t into the cavi ty. The

g wi ll

as a collet- threaded

is particula r part of

t care since i t di-

i s

used to

In the design shown, a

s used. I t i s larger,

more easily-and for me to

read without my glasses.

The disk is constructed

from a piece of flat brass at

least 1/16 thick. Dimen-

sions of the disk shown in

the drawings should be fol-

lowed carefully . As you wil l

note, the dimensions allow

only .03-inch spacing be-

tween the wall and the disk

edge. This means that the

drive mechanism must also

be placed in the center of

the drive holder. In each

case, i t is best to have your

friendly machinist fabricate

these pieces on a lathe. This

means that the top cover,

the disk, and disk holder all

must be centered exactly or

the disk w il l rub on the cav-

ity walls.

The disk must have some

absorbing material added

to the back o f it. This is nec-

essary because the back of

the cavity

hat is behind

the disk-otherwise can

support resonances which

wll be coupled weakly from

the waveguide and wil l pro-

duce unwanted responses

which can mislead the user.

The absorbers may be con-

structed from a disk made

of a piece of attenuator

card. If you have access to

powdered iron material,

paper can be shaped

around the periphery of the

disk to make a form. Mix

the powdered iron particles

with some coil dope and

pour in enough to make a

level surface about 1/16

thick. Be sure that the sur-

face is held level while the

mix is hardening. (Pow-

dered iron can be obtained

from cup core rnanufactur-

ers or, if you are really am-

bitious, you can make your

own simply by crushing a

cup core with a steel rol ler.

Make sure that the particles

are very small.) After the

mix has hardened, be sure

to remove all of the paper

and glue from the. edge of

the disk.

Next, construct the wave-

guide section by mounting

the two flanges, one on

either end of the guide. This

is done by cutt ing the wave-

guide ends square and then

removing all burrs from

them with a smooth file.

Each end wil l have a flange

soldered in place. This is

done simply by sliding the

flanges on each end of the

guide, after first applying a

small amount of soldering

paste (flux) to each end.

Now heat the entire assem-

bly until the flux runs and

turns the copper waveguide

a deeper color. Apply just

enough solder so that i t wil l

run around the joint. When

this operation is completed,

lap the ends of the flanges

against a piece of very fine

emery paper. Now inspect

the joint to be sure that

there are no gaps in the

solder around the face of

the joint.

Next, measure the dis-

tance between the rear

butts of the flange, and in

the center of this distance,

using a square, draw a line

across the E-plane of the

waveguide.

On one side of the guide

lying on this line, place a

centerpunch mark at the

distance from the outside

wall shown in the mechani-

cal assembly drawing, Fig.

3.

Now drill the hole also

described in this detail.

Deburr this hole inside and

out of the guide so that i t is

smooth. This is the iris

7

Magazine March, 1981 6

7/25/2019 Home Brew X Band Wavemeter

3/4

Fig 4. Test setup for calibration, as described in the text.

couplin g hole, and it should

be perfectly round with no

burrs or raised pieces stick-

irig down into the wave-

guide. The deburring job is

easy on the to p of the guide

and can be done with an-

other drill bit held by hand

and turned to li ft off the

burr. The inside of the

waveguide may be done

with a very small bladed

knife such as an X-actom

scalpel. This can also be

done fro m the top of the

waveguide by inserting the

point into the hole against

the burr.

Clean a piece of copper

pip e (shown in the assembly

drawing) as the cavity cyl-

inder.

t

should be free fro m

all burrs on each end and

polished clean with Dutch

Cleanser or a similar pot-

scouring compound, partic-

ularly on the inside of the

cylinder. O n one end of the

cylinder, using a machinist's

square and try, set the cen-

ter so that a pair of marks

can be scribed across the

open face of the cylinder

and on the end of its side

walls.

Using f lu x here, too,

solder the half-circle of

brass in place so that it lies

on the t wo scribe marlts just

made t o the bot tom edges

of the cylinder. Now the

bottom of the cavity will be

half closed. Use acid-base

so lder ing f lux fo r th is

operation.

The cavity may now be

mount ed to the waveguide

section by al igning the

scribe marks on the cavity

ou ts ide wa l ls w i th the

scribe line made on the

face o f t he waveguide. The

half circle of brass wi ll b ut t

against one side wall of the

wavegu ide p r ov id in g a

solder surface. Using

a

C-clamp and a small, flat

piece of wood, clamp the

cavity in place, placing the

wood under the clamp so

that it does not mar the top

of the cavity. Adjust the

cavity so that i t aligns with

the marks on its walls and

the waveguide. Make sure

that the half-circle butts

tigh tly against the side wall

of the waveguide. Now ap-

ply soldering paste to the

bott om of the cavity and to

the waveguide surfaces

that make contact wit h the

cavity parts. Heat and sol-

der, using just enough

solder so tha t it wil l run

when the assembly is hot

enough. Let the assembly

coo l thorough ly be fo re

removing the clamp.

When the soldering is

completed, the waveguide

section should have the

cavity mounted off-center

on the guide with the iris

hole off-center with the di-

ameter of the cavity (see

Fig. 3 details). The cavity

should be in line with the

scribe marks previously

described as the alignment

lines. If this is not the case,

but the alignment marks

a r e o n l y a v er y f e w

thousands of an inch off , do

not rework to im~rovehe

alignment; this small devia-

tion will not degrade the

operation of. the wavemeter

too badly. Inspect the in-

side of the cavity where it

mates with the waveguide

and be sure that solder has

c o m p l e t e d t h e r un ';

around the entire cavity

surface that is in contact

with the waveguide.

When all soldering is

comple te o n the waveguide

and cavity body, wash off

paste or acid will degrade

the future performance of

the instrument when i t

causes corrosion.

The most importa nt parts

of the assembly are at-

tempted next, These parts

are shown in the rnechani..

cal drawings as parts A

tlirough and should be

asse~r>bIedn this order. Be-

fo re assembly, i t yo .)desire

to silver-plate ail parts of

the cavity and waveguide,

some improvement of the

cavity wi ll result and the

~er fo rmanceof the wave-

meter wi ll be bette r. Silver-

plating can be dispensed

with, however, since the

unit wi ll perform very well

without it.)

Now the t uning disk and

lead screw can be assem-

bled into the bearing and

coll et assembly. To d o this,

place a lock nu t of the com-

pression type (as found on

most screwdr iver -adjust

locking pots) on the

col-

let-see A in Fig. 3. The

loc king nut is a device

which has inside tapered

threads, 3/8-24, and is abo ut

114-inch thick. It will com-

press the collet against the

lead screw threads causing

it t o move tightly in the col-

let bearing. Put the nut on

so that it is loose.

Now, from the opposite

side o f the bearing pla te (A),

insert the threaded end of

the brass l/Cinch rod and

screw it up through the col-

let until about an inch of it

is exposed. This item is

labeled in Fig.

3.

Now in-

stall piece C, the tuning

disk, on the end of the

brass-threaded rod. Gently

hea t this piece and solder

it

to the threaded rod, taking

care not to get solder in the

fine threads. When the

piece is cooled, wash of f al l

solder paste. Now, on the

back of the tuning plate,

add the powdered Iron as

described earlier.

When the three pieces

have been assembled, in-

sert the tuning plate into

Photo B Wavemeter parts and bo ttom view o f the cavity,

all soldering paste wit h very

the cavity and align the top

showing ho w i t is offset

from

the waveguide. hot water. Any traces of the p la te so th at the three

7 agazine March

1981

7/25/2019 Home Brew X Band Wavemeter

4/4

screw holes in the cavity

wiiil

top mate with the

th;-.e threaded holes fo r the

three 4-40 flat-head screws,

which can be found on the

s : d e of the top plate (A)

F ess the top plate down

Flrrnly into the c av ~t y nd

r~ st al l he screws to hold

l plate firm

Now screw in the tuning

p la te by ad jus t ing the

~h re ade d od: see that ~

moves in and out of the

cavi ty easily, and adjust the

collet lock nut so that the

rod moves firmly but not

:lard. Now put a dab of

Super Cluen on the lock

nut, taking care not to get it

in the cracks of the collet

and on the lead screw

threads. This will lock the

collet lock nut in place.

Now the last piece,

D,

can be assembled. Simply

glue the plastic disk to the

to p of the 1-112-inch diame-

ter tube. This tube can be

made from any available

plastic or metal material. It

fastens t o the extended end

of the threaded brass rod

wi th tw o 1/2-36 nuts. Place

one nut on the end of the

rod and engage enough

threads t o al low about 114

to protrude through the as-

sembled dial barrel

(D).

Then place another nut

ith the same threads on

he r od and tighten unti l i t is

rotate. Now you have a

micrometer barrel dial and

the cavity . Cover the out-

wall wi th

on marks can be added.

To calibrate the wave-

wi ll need a signal

e w hich can be tuned

d the band on

cluded. The source can be

your uncalibrated X-band

transmitter, a signal genera-

tor, or s imply a klystron and

power supply. A calibrated

wavemeter and an attenua-

tor ending in a crystal de-

tector mounted in a wave-

guide adapter make up the

waveguide components re-

quired for calibration. A

meter t o register the crystal

cur rent wi ll serve as the re-

action indicator. Fig. 4

shows how to connect these

devices for calibration.

Let's assume you are us-

ing a klystron and power

supply to do the job. Be

sure that the klystron is os-

cillating in a mode that will

not stop or squeg during

the operation. Once i t is os-

cillating , adjust the attenu-

ator t o provide a three-

quarter scale reading on the

current meter. Set the kly-

stron so that it is in the

range of 9.8 CHz t o12 CHz

by adjusting its frequency

control. Now adjust the cal-

ibrated wavemeter (which

we will call the reference

wavemeter from now on)

unt il you see a pronounced

dip in the output-current in-

dicating meter. By adjust-

ing the frequency control

on the klystron and the ref-

erence wavemeter, you

should be able to set

UD

the

starting calibration fre-

quency of 9.8 CHz. When

you have this frequency

and vou are sure of it. de-

tune the reference wave-

meter. Now tune the device

you have just constructed

unt il y?u get a similar reac-

tion on the output-current

meter. The dial barrel will

be quit e near the top of the

wavemeter cavity. Now

simply make a pencil or pen

mark on the calibration dial

paper, using the moving

barrel edge as a guide for

your pencil.

Computer

or

Electronics

USINESS

INVESTMENT

W LYH

s

looking to

buy

a small electronics business

based

in

the southeastern Uni ted States.

The business should be:

Well.managed

Profitable

Have sales between

$500,000 and $5,000,000

@Andhave growth potential

Be sure to contac t me wi th your lead:

Stew Siebens

1401 North Western Avenue

Lake Forest, Illinois 60045

Te le~hone312) 295-2010

side. If you decide to make

the calibration steps at a

finer set of divisions after

you have done a coarse cal-

ibration, do not remove the

coarse steps; they can be

your check points t o check

out how well the device re-

sets.

The drawings showing

the mechanical construc-

tion of the wavemeter are

supplemented by photo-

graphs so that you can veri-

fy how the assembly goes

together. It is not difficu lt,

and most amateurs who

have had experience with

hand tools should be able

to complete the unit satis-

factorily. It is hoped that

the meter will be helpful to

those amateurs who want

to know that they are within

your power-output indica-

tor, read off your frequen-

cy, and then detune the

wavemeter. You may leave

it in the line since it will

cause very lit tl e loss to your

output.

Materials for this device

were obtained from Lec-

tronic Research Labs, Inc.,

Atlantic and Ferry Ave.,

Camden, New Jersey.Al l of

the materials for waveguide

components are listed in

their catalog. There are

numerous other sources of

material for this band and

others, bu t this catalog

should be in the hands of a ll

microwave enthusiasts.

All letters with questions

regarding this and other ar-

ticles I have written wil l be

answered if the writer in-

Continue the calibrat ion the X-band confines. cludes an SASE. Al l tele-

process, sett ing the fre- To use the wavemeter in phone calls also wi ll be an-

quency changes in 50-MHz your rig, simply place it in

swered, but please remem-

steps, for a start. Put the t he wa ve gu id e ci r cu it , ber what your time zone is

coarse calibrations on one

which is the output toward so that I don't get your call

side of the calibrat ion line the antenna, then tune it in the early hours of the

and fine steps on the other

until you get a reaction in m0r ning .B

73

agaz ine

arch, 1981

7