-- o ICOM

•
Number 13 on you, Feedbllck urd
QRP SWR Bridge
This simple project lets you monitoryour antenna system's SWR.
by Tony Smith G4FAI
L
OW
power (Q RP) operating enjoys an
enduring popularity in amateur rad io.
Much ofthe equipment is relatively simple to
home-brew. and there isn't the need for many
ofthe precautions required whe n operati ng at
higher power levels.
Opinion on what constitutes low power
varies between different countr ies. and between
different operators in those cou ntries.
The G-QRP C lub's definitio n of less than 5
Wans input. and the (American) QRP Amaleur
Radio Club International' s maximum of
5 Watts output. are the power levels referred
to in this article.
Transmitters need to "see" a specified
load , usually 50 10 75 ohms . at their output.
Whether the a ntenna in use is already
matched to the requ ired impedance , or
whether the match is obtai ned through an
ante nna tuning unit (ATU). a standing wave
ratio (SWR) bridge lets you monitor and adjust
the effect of the load on the transmitter
(Figure I ). (Many modem transmitters automatically
reduce their output power if the
antenna system presents a mismatch.) The
bridge is thus a usefu l device to help obtain
optimum perfonnance at all times.
When a transmitter transfers power
t o a fe ed e r l ine o f t he correct
impedance. and the feeder terminates
at an antenna also presenting the correct
impedance , the antenna accepts
and rad iates all ofthe power coming to
it. When the antenna has the wrong
impedance, e.g. because the feeder is
not correctly terminated, a portion of
r
the power is reflected back down the
feeder in the form of standing waves.
The ratio between the forward power
and the reflected power is the standing
wave rat io, and the funct ion of an
SWR bridge is to indicate that ratio at
the point where the bridge is located in
the feede r line.
The Circuit
The design shown in Figure 2 is a
simple unit for QRP operation on all
authorized frequencies up to 30 MHz ,
based on a toroidal transformer TI .
The secondary winding of T I sam ples
a small amount of RF power (both
forward and reflected) which is d ivided
by the bridge circuit and rectified by
diodes 01 and D2. Forward and reflected
readings are obtained simultaneously
on the two meters M I and M2,
and the bridge is matched and balanced
at the required load impedance by CI
44 73 Amateur Radio • June, 1989
and C2 . See Figure 5 for an alternative. less
expensive , single meter version. The bridge
also measures forward power.
Although it should not be regarded as a
laboratory instrument, the bridge is sufficiently
accurate for all practical hamming
purposes.
The project is housed in an easily constructed
wood/hardboard case, partly to keep
the cost down and partly to give the builder
the satisfaction o f creating a completel y
" home-brewed" unit. If you want to use a
metal case, feed-through capacitors C4 and
C5 are then unnecessary.
Construcnon
There are any number of ways to fabricate
a case for the bridge. I built my case out of
wood , using nails and glue to hold it together.
I suggest d rilling the holes for the nails,
slightly undersize, to keep the wood from
splitting. The front panel is secured by panel
pins and glue, and the top and rea r panels are
secured by woodscrews to facilitate access
and setti ng-up.
In the prototype. the nails and panel pins
" -
,.,
-
A
t>rl(lg ~
0'
,., -
t> ,,(lg ~
n l ~nna
-
lunIng uni t
Figure 2. Circuit diagram of the QRP SWR bridge,
"
Ant ~nna
were punched below the surface level o f the
case and all gaps, holes, and irregularities
made good with filler and rubbed down. The
case. plus rear panel. was painted inside and
out with matt black paint, and the top cover
with black gloss. The fron t and sides were
covered with Fablon" after the meter holes
had been cut out.
The holes for the meters were cut by marking
the position of the meters on the front
panel and drilling a se ries of small holes
round the inside ofthe circle. The meter holes
were then finished off with a half-round file.
Exact details and measurements for meter
and potentiometer mounting will depend on
the particular meters obtained for the project.
You can use virtually any meter having a
. n l ~ nn a
Figure 1. SWR bridge placement- keep the feeder length
between the transmitter and bridge as short as possible.
.... ------------ -------- - - -- - - -,
•
" 14S"G •
~<