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73 Amateur Radio - Free and Open Source Software
73 Amateur Radio - Free and Open Source Software
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•<br />
,<br />
tang of a small file until the capacitors could<br />
be secured with a press-tight fit and finally<br />
secured with a dab of "super-glue." This<br />
arrangement has pro ved quite satisfactory<br />
but purists might prefer to solder the capacito<br />
rs to a small rectangle of tin plate and<br />
bolt the assembly to the side of the aluminium<br />
box. The wiring-up of the meters and<br />
the dual potentiometer should present no<br />
difficulty.<br />
Sett ing-up<br />
O nce the un it is assembled, you must balance<br />
the bridge. Connect the tra nsmitter 10<br />
one of the rear sockets via a short length of<br />
coaxial cable having the same impedance as<br />
the T X output. A further length of the same<br />
cable co nnected to rne other socket should be<br />
terminated by a non-inductive dummy load o f<br />
the same impedance. You can make this up<br />
from one o r more carbon resistors to obtain<br />
the resistance and wattage required.<br />
Now apply a rad io-frequency (RF) ca rrier<br />
at the highest used freq uency. One meter<br />
should indicate a hig h, and the other a low ,<br />
reading . Adjust the trimmer capacitor on the<br />
side of the bridge showi ng a low read ing<br />
(reflected power) to obtain the lowest posslble<br />
reading . Now, re ...erse the connections to<br />
the sockets and make the same adjustment<br />
with the trimmer for the second meter. Repeat<br />
this procedure once or twice until finally<br />
both meters , whe n indicating reflected power,<br />
read ze ro , and the br idge is then balanced.<br />
During this process, adjust the potentiometer<br />
so that .....hic he...er meter is indicating forward<br />
po.....er is set at full-scale deflection.<br />
C alib ration-SW R<br />
You can use either meter for forward or<br />
reflected power indication , dependi ng on<br />
.....hich socket is used for input or output . Fo r<br />
SW R readings, both meters arc used and that<br />
showing forward po .....cr need s only to indicate<br />
FSO . For reflected po.....er, op inions differ<br />
on the need for detailed calibration . The<br />
most important marking is at a point exactly<br />
halfway across the scale , which represents an<br />
SWR of 3: I . Any SWR in excess of that may<br />
be detrimental to the transmitter. An SW R of<br />
2: I or less is acceptable, especially with low<br />
po.....er.<br />
Therefore , you need only a center marking<br />
to indicate maximum permissible SWR. The<br />
aim is to get the reading down to as near to<br />
zero as possible .<br />
Calibrallon-c.Power<br />
It's easy to get forward power readings<br />
since the circuit pro ...ides a reasonably uniform<br />
indicatio n of RF po.....er, irrcspecti...c of<br />
frequency , o ...er its range ofoperation.<br />
Calibration modes, unfortunately , require<br />
an e xternal means of measuring RF power<br />
for comparison purposes. Those .....ho have<br />
access to RF measurement can set the forward<br />
meter to FSD .....hen the desired maximum<br />
RF power passes through it into a durnmy<br />
load. Note the setting of the pointer on<br />
the control knob by making a mark on the<br />
front panel . W ith the control at th is setting,<br />
feed successively lowe r RF powers through<br />
46 73AmateurRadiO . June,1989<br />
the un it and mark the meter<br />
scale accordingly.<br />
Operation<br />
Th e bridge should still be<br />
co nnected to the transmitter.<br />
Connect the output to an antenna<br />
system.<br />
Apply power and check the<br />
meters. Adjust the tuner for<br />
minimum SWR ( I : I if possible).<br />
Take care that the forward<br />
meter reads FSO, but that<br />
it isn 't pegging against the endstop,<br />
as that ca n damage the<br />
meter.<br />
If you are using an antenna<br />
system without a tuner, and the<br />
antenna is cut forthe frequency<br />
for the input signal, you may<br />
want to check the bandwidth of<br />
you r anten na. Do this by<br />
checking the SWR at diffe rent frequencies<br />
across a ha nd. This will often sho w you ho w<br />
to alter the antenna d imension s for a d ifferent<br />
resonant frequency .<br />
Lea...ing the bridge permanently in line lets<br />
you monitor the effect of the antenna system<br />
on the transmitter output. Bear in<br />
mind, however, thai a low SWR<br />
indication does not necessarily mean<br />
an antenna is performing wel l. A<br />
dummy load, for example, presents<br />
a near I : I SWR through a matching<br />
feeder, and yet is virtually non -radiating.<br />
Too, it measures SWR only<br />
at the point in the feedline where the<br />
bridge is located-not at the antenna<br />
itself.<br />
An SWR bridge is a ...aluable pa rt of<br />
every radio operator's station. It is a<br />
useful tool when constructing anten <br />
nas and exercises an essential control<br />
function whe n they are in use. Because<br />
of its simplicity it is an ideal<br />
project for home construction, especially<br />
for beginners, and .....ith good<br />
quality components it will last for<br />
years. QRP operation itself offers<br />
eno rmous scope for home construction<br />
and experimentation, and a unit<br />
such as this should be an integral part<br />
of e...ery QRP statio n.<br />
!w"wu41<br />
n,<br />
50~"" I<br />
n,<br />
J. 1<br />
O,odn<br />
~nde 'l..t<br />
r-' w<br />
'"<br />
I I ..<br />
Figure 4. Simple diode matching circuit.<br />
Match the two resistors with an<br />
ohmmeter. As the voltage i$ increased<br />
b)' rotating the potentiometer,<br />
the meter shouldnot deflect rrwre<br />
than I ~ from its no-curre11l setting.<br />
It may be necessary to test several<br />
diodes 10 obtain a matched pair.<br />
TRACK BREAK S AT :<br />
A4, 14 , 2 4<br />
B4 , 24<br />
C4 . 24<br />
H"<br />
K' 0 . 14 . 18<br />
Figure 3. Componens tocanons on the veroboard. Drill the<br />
mounting holes \ 'n ) ' carefully to avoiddamaging the board.<br />
Reslslora<br />
Carbon film V.W5%<br />
330 2<br />
Polenllometerl<br />
Oual-ganglKi<br />
22'" ,<br />
CapacltOf1l<br />
CeramiC<br />
,-,<br />
n<br />
...~,<br />
~'l L><br />
n,<br />
"<br />
2<br />
2<br />
COMPONENTS<br />
-<br />
r<br />
n<br />
'" '"<br />
, ~ ~,<br />
"<br />
"""<br />
R3<br />
06.1<br />
04.5<br />
C3<br />
C1,2<br />
I<br />
I-<br />
n op m ~<br />
'm'<br />
I<br />
Figure 5. Single meter version oflhe S WR bridge.<br />
u<br />
21 23 2!t 27<br />
Warning<br />
Th e unit as described is suitable only for<br />
low power operatio n. The circuit is capable<br />
ofoperation up to about 100 Watts.<br />
Taken from Practical wireless , October<br />
1983. DI<br />
'....<br />
FeeD-through<br />
'.'<br />
2<br />
Polystyrene<br />
330pF 1<br />
Miniature trimmer<br />
2-22pf 2<br />
SemlconduetOf1l<br />
[);ode<br />
QA91<br />
Mi_Uaneoul<br />
R1,2 (Matched, see text)<br />
01 .2 1Matched, see texl)<br />
Mater 1()()pA FS0(2); RF Choke 1.SmH (1); Toroidal<br />
COfe T68-2 (1); Veroboard 0.1 inch matrix 24 holes x 10<br />
tfllcks; Metal box 73 .51 x25mm: Phono sockets (2);<br />
Pointer knob; Enamelled copper wire 28 s.w_g. (1.3m);<br />
Insulated wire 24 sow_g. (l00mm).<br />
rw Awm l