rf - Free and Open Source Software

A
Direct -Reading
J. M. Prosser, K3WRW
P. O. Box 68
New Marke t , Maryland 21774
SWR
Indicator
T h e d irect reading swr ind ic a t or,
The device described here was conceived
as a result of my somewhat frustrating experience
in tuning an antenna matching circuit.
The proced ure usually recommended
goes something like the following:
I. Tune u p the transmitter into a dummy
load o f the proper impedance.
2. Remove the dummy load and connect
the antenna matching unit to the transmitter
through the SW R bridge.
3. With SWR bridge set to forward
(FWD) position and transmitter energized.
set the meter pointer to full scale.
4. Switch the SWR bridge to reflected
(REF) position. and adjust antenna matching
unit fo r minimum reading on the me ter.
This sounds simple enough. b ut all too
often the REF reading is decreasing because
the transmitter outout is decreasing as a result
of mismatch. "10 guard against such J
condition ....aftcr an adjustment is made. the
SWR bridge must be set to FWD . the meter
adjusted to full scale. and the SWR bridge
aga in switched to REF. Ho pefull y. the SWR
has been decreased. but this is not always the
case. What is involved . then. is a repetit ive
cycle of setting the meter to fu ll scale in t he
FWD position. switching the bridge to REF.
making an adjus tment to decrease the rea d­
ing. and then checking the result. This can
get pretty frustrating after a while. Then:
should be an eas ier way. and there is.
What is needed is a way to compare the
forward and reflect ed readings continuously
wh ile making the adj ust ments to the matching
unit. Of course this could be done by
using two meters to show forward and rcfleeted
power simultaneously. and adjust ing
everything to ma ximi ze the read ing on t he
fo rward meter while minimizing the reading
o n the reflect ed meter. I feel that the d evice
described here is eve n simpler t o use and it
requires only one meter. Basicall y. it is a
voltage compara tor, as shown in Fig. I . The
meter should have a high impedance for best
results. This circuit has some limitations.
'0 <
t--
'0'
•
--< J.f----1
F ig, 1 . A simple VO ltage comparator, Th is
c ircuit h as so me l imitatio n s since '3 f al se ~wr
r eading may result d epending o n cov-er.
For example. there is no way to determine
wha t the SWR is; that is. high po wer with
high SWR may give the same reading