(~ -----------"""'-''''''''''''''''''"'''''~'''''

Gregor Storz, ZLlGSG, DLlGSG

Stripline Directional Coupler for400 MHz to 3.6 GHz

r was preparing to move to newaccommodation, and I began to thinkabout enendtng III)' measuring equip­ment. You can buy many things, hutgood directional couplers arc veryexpensive, assuming you can get themat all. Normal ",,/4 microstrip couplersarc fre quently used, hut their direc­tivity effect is nothing special; andthey are certainly not broad-band.

I.IlASIC FACTORS

In my search for a solution I found acomprehensive description of directionalcouplers in the book "Stripline CircuitDesign" by Harlan Howe [1]. WhyStr ipline (Triplalc)? It very quicklybecame apparent that Strip line hasmarked advantages, as against Micros­trip, in relation to the directivity. Adirect ional coupler which consists onlyof 2 }J4 long coupled lines has a bandwidth of one octave. If this coupler is

2

extended by add itional ,,/4 couplers withan expanded interaction gap (Fig.1). theusable frequency range is expanded. Atn = 3, we can already obtain a bandwidth of 4:I to 8:I - depending on thepermitted ripple , of course. If we want acoupler with a hand width exceeding9: I, we have to use a structure with n ~

4. The COil sequence is that the couplerbecomes very long and this also meansthe losses in the substrate increase.

Another option is to use an asymmetri­cal structure. This saves us almost halfthe A,!4 segments requ ired - withscarcely any deterioration in the charac­ter istics (Fig.2).

For a symmetrical coupler, the phasedisplacement to the de-coupled port isalways 90°.

For an asymmetrical coupler, the phasedisplacement IS frequency-dependent,but this is irrelevant for amateur radioapplicat ions.

One question troubled me here. Arethere differences between the 2/4 cou­pling and the 1/3? Simulations show that

VHF COMMUNICATIONS 1198

n=3,n=l,~ " , Fig.l:~ , , r-::~=j~~=::::: Symmet r ica l and

, Asy mme trical

Co up lers

the co upling for the 2/4 port is morestrong ly frequency-dependent than thatlor the 1/3 port. The reason for this isthe transmission loss through the seg­ments right up to the AJ4 segment,limit ing for the h igh freq uencies. withthe sma llest coupling gap.

2.IU:ALlSATlON

Once these bask principles were clear,the o riginal problem could a lso hesolved very quickly. The result was a

symmetrical coupler with lJ .." 3 onRogers R03003 mater ia l.

Since this solut ion is very expensive.because of the base material used, theidea of a vers ion which would givebetter value was pursued . So how wel lsuited. or how bad ly :-uiIOO, is FR4epoxy material 10 these high freqnen­. ,c res:

, discovered through simulation. usingSuper-Compact. that , with limitations. a"normal" assembly could be corded outright up to .1.5 CiH7.. As a result or thesimulation. 1 expanded the originalstructure (n = 1) by one )) 4 segment.

The usefu l frequency range goes from

•' M .,' 00

Coup l e .. 10ctlHz-3.6GHzSt ..ip!inl/' 2_2. 0Il'llll F'~"

J--J, ..",--,"

"'i~.2: Wiring Diagram or Asy meret rtea l Sir ip lillc Coupler

3

VHF COMMUNICATIONS 1198(~. - ----'''''-'0==''''-'''''''-'''"'''''

•a

CCMI'ACT son llM.l: _ SUP!:fl-CCttPACT PC V6 . ~

Pi t t or b - 4 01 11 &.... 4 2 121 Ell_ 4 ' 1) 1

•

o • .•• / [\'\\

/ \ \

\• \••

/ ~ l-.•• (/ .'- ,,\./ r"'-..e

"- -, \/ /.e

/ 1\ / 1\.c

/ <; \.00-

. "

-20 .

V KSll tll ill lItOI'Cl)C MSU t411 l f((l1'3)- 15 .

...,

r n q IliHt l

Fig.J : Simulation Resu tts for Co uplinJta nd Ilirt.'CIi\'iI)' for variou" values of t:r

400 MHz to 3.5 Gll z at 20 :tJ d Bcoupling

The bigges t disadvantage of FR... is theattenuation. which here reaches a '"l .5dn a: 3.5 GIlL and is thus close tomaximum. A certain amoun t of unce r­ta inty comes from the &r. which oscil­lates. depending Oil the manufacturer.between ,; - ....0 and 4.6. In mysimu lations, I use a value of I;, = ....2 forFR4. T he effects are d isplayed ma inlyby the fad that the operatin g range ofthe coupler is displaced (Fig.J).

2.0 mm. was selected as the th ickne ss ofeac h substrate. The Trip late was rein­forced b)' a 2 mm. thick aluminium plateto give suffic ient mechani cal stability.

2 ), 100fJ: SMD 080 5 res isto rs were

integrated into the Triplate between two2 .0 mm. thick epoxy printed c ircuitboards. In order 10 obtain enough roomfor the resistors. some epoxy had to he"ca rved out" of the second boa rd. Theimportant th ing here was that the eart hcontact for the resistors was so ldered 10

all 3 layers with 2 pieces of wire W8mm. long). Yl'U should also make surethat there is a good con tact between thescrews and the two earth surfaces(FigA).

In a reflection measurement. the twoeh ip resistors, which added up 10 0.25W. limited the preliminary power to 25Watts.

l-"ig's .5 and 6 show the structure of thecoupler.

4

VHF COMMUNICAT IONS 1198

3.UNEX PECTE D OIlSTACLES

You can calculate every thing on thecoupler beautifully, usc a high-qua litysubstrate and have a good seal. and stil lnot obtain good direc tivity. The reasonfor th is is usually the socke ts used.These components. or the way they arematched. have 0 1' has a direct influenceon the directivity. The best so lution is tousc special sockets for striplines with"pennants" as centre contacts (e.g. fromRosenberger).

As I had "0 such Neockcts availableduring asse mbly, I tried my luck with"normal" Neockets. which possess athinner dielectr ic (d "'" 5.8 mm.) on rhcflange side.

After completing the coupler I had toadmit that the return loss with thissocket was still onI)' RL = - 22dB at 2.4GHz.

The reason for this is that this sockethas a sect ion about 2 mm. longer withonly 450 . Another type of socket was

/GROUNOPLANE~2.0mm FRi

_-STRIPLINE-.......2.0mm FRi~GROUNOPLANE

'-2.0mm I'llOHM SMD 0805

F i~"':

St r uct ure of Tri platc with AluGrcund plane, mounted f'lt -..ockct an ti..oldc red -en SMD Rest..tor

even worse. It had an area 5 1111ll . longwith an impedance of only 40 U .

1'0 get things clear in my mind. I sawedthe sockets do wn and rested them on thespot.

4.MEASUlU,MENT IU;SlJLTS

:-.'0 mauc r how attract ive the simulationresults may have been. ihe measurementresults (Fig's.7. 8) for the prototypeslooked comple tely differen t. However,this was mainly in relation 10 the returnloss. which at times was only 14dB(Fig .9).

The reason for this lay in the soc kets. orin the way they were mounted.

In th is case. however, I la id greateremphasis on a simple, mechanicallysolid construction which, for example.did not immediately collapse when twoAircom cables were connected.

5

VH F COMMUNICATION S 1/98(~--------------"''--== = ''-== -''''''

Fi~.5:

Structure ofCo upler : P rintedC irc uit Boardbefore Assembly

5.MAKING YO UR OWN

of printed circuit boards, 1 am givingonly the dimensions here.

For those who want to start making theirown version straight away, here are thedimensions for a coupler, with n '" 5,mounted on 2 x 1.5 mm. FR4 epoxy(Fig. I 0).

Since there will certainly be no problemwith access to programs for the design

- All lines lie on a 5 mil (0 .005 °) basicgrid.

- The printed circuit boards are always56 mil wide, and the length of acoup ler segment is 690 mil.

- The length of the entire cou pler IS

app. 88 mm. and thus fits crosswiseunto a Euro-card.

Fig.6:View of Prototypeswit h Neockets

6

s

.~- " '9 ....0 . e cI'I

~ 10 . 0 c£V-a" .~ ~

"I,"" ER

o G z. 3 1I p<un

-;; I".., /V\

11'1 - -_..~

y.

S"M' S"~

0 .I~ Giz E> . 1IIIII0O'IZl'I1 G b

Fig.7:Meas ureme ntDiagram for CouplingPort t ttl 3:

JOt) MilL - 3 G lb . ­20 % IdB;

Mar"L.er I: 400 Mli z..

Ma rker 2: 2.3 1 G II7.

Marker 3: 3.()GH z,

Marker 4: 4.0 r ;H"l

. ' .__~"~d6t';S~

'''H"1 5 n z

4'

Fig.S:Measurement Diagramfor Dtrecttvtty Po rt ::z

J v ~rV ' 103,

400 MHz - 2.5 GHz O!::

ISdB,

400 MHz - 3.5 Gllz 2:

.<d B

7

Fig.9 :Mat[hill~ of Ports I, 2and 3;

400 MHz - 2.5 GHzRL ~ 16dB,

400 Mllz - 3.5 GllzRio ~ 13dD

VHF COMMUNICATIONS 1198

U!l .12l dO"~

llZ1.11I dY-2:l. . ~ d3 - l E . l1I01 d3

"I""" ER

. C.. 3 1po.n

-~~ A:

'1 ::~l"iX,

) Itt\ lY.ww.JIft

(~ ----Y!:!E.£?~~~!!l!!!

ST""0. 1~ Gtz:

- The intervals of the co upler lines(centre of track in each case) are 5 1= 17S mil. 52 - 140 mil. 53 ." 120mil. S4 = 100 mil and 55 = 85 mil >coupling gap: 119 mil. R4 mil, 64mil, 44 m il, 29 mi l.

The layout was printed onto film on aI: I scale using a laser printer andengraved using normal engraving equip­ment.

As regards sockets, an SMA construc­tion with " pennants" was used on theprot otype , and an SMA seal was used atport 4 .

The assembly coupling is 2 1 ± IdB (350MH7- to 3.5 GHz). The di rect ivity isbetter than 20dB up to 2 GJIz and betterthan ISdB up to 3.5 Gl lz.

a

Fi2·10:Coupler made from2 I: 1.5 mm. Epoxywith SMA Sockets

VHF CO MMUNICATIONS 1198

6.SUl\I~IARY

The d irectional coupler described abovecou ld certain ly he dew loped further inrelation to the matching and the d irectiv­ity. Better sockets - e.g. SMA' s - and anS\1 A seal resistance improve the resultsmark edl y.

[ would like to than k Kf . ichcl.DI.6SES, from the TSS company forsup po rt with Supe r-C o mpact. andDLi TR for measuring the prototype.

7.LITER ATURE REFER E"'CES

( I) Ha rlan Howe, Jr.: Strip line CircuitDesign (1974), Artech House

I2l Elements Lih rary Super-Compact,Com pact Software . USA

The Parabolic 24cm PreamplifierA fully weather-proofed very high-quality preamplifiercovering 1200 MHz to 1360 MHz. Mounted in a sealed

diecast enclosure with weatherproof N·type sockets forinput and output. DC powered via the output socket for

remote mast-head mounting.

GAIN >40dB across the bandNOISE FIGURE <1.6dB

£135 + £7.50 post and packing

KM Publications, 5 Ware Orchard, Barby, Nr .Rugby,CV238UF, U.K.

Tel: 01788 890 365; Int: +441788 890 365Fax: 01788891883 ; Int: +441788 891 883

ALL MAJOR CRElHT CA RDS ACCF.PT[n - abon~ pric~ +5°/.

9