-- o ICOM

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