AMATEUR RADIO
BY GARRY CRATT, VK2YBX
Build this relative field strength meter
It is often said that simple is best. This simple
relative field strength meter can be built in
less than an hour for just a few dollars.
One simple yet quite useful instrument that should be part of every amateur radio "shack" is a field strength
meter. Such a device can be used to
check the operation of a transmitter,
regardless of the frequency band. Simple relative signal strength indications
can also be used to evaluate the efficiency of antennas and the output
stage of transmitters.
Our unit, by necessity, is broadband.
Most published designs are quite insensitive, relying solely on the rectifi- .
cation of RF using a small signal diode. The resultant DC voltage is then
filtered and used to drive a DC meter.
Fig.1 shows this typical arrangement.
To overcome this shortcoming, we
have added an LM358 operational
amplifier, which is used to amplify
D2
DCNIDEO
OUTPUT
Fig.1: basic circuit for a field strength
meter. The filtered output appears
across C2 & can be coupled to a DMM.
the rectified RF voltage. Also, we decided that because most amateur operators have a digital multimeter, we
would use this as the indicator. Fig.2
shows the final circuit. Of course, virtually any op amp capable of single
supply operation could be used.
The RF field applied to the antenna
induces a voltage across the terminating resistor Rl. This voltage is rectified by diode Dl, which is a germanium or Schottky diode. Resistor R2
and capacitor Cl filter the resulting
DC voltage, while Cl also bypasses
The circuit is
assembled onto a
small PC board &
this is plugged into
a digital multimeter
via two banana
plugs to obtain the
relative signal
strength readout.
86
SILICON CHIP
pin 3 of the op amp, thus preventing
RF energy from being coupled into
the input.
The DC amplifier we chose is an
LM358, with an adjustable gain ofup
to 101. This is set by VRl, a 100kQ
potentiometer.
If additional sensitivity is needed,
then the bias arrangement shown in
Fig.3 can be used. Here, a small
amount of bias is used to almost turn
the diode" on", thus overcoming most
of the 0.25V drop characteristic of
germanium diodes.
The output of the op amp is fed via
current limiting resistor R4 directly
to the input of the DMM. By selecting
the 200mV scale, the multimeter can
be used as quite a good indicator. In
fact, it is possible to drive the meter to
read several volts and if the DMM is
autoranging, then so much the better.
Power for the circuit is derived from
a 9V battery. Diode DZ has been included to ensure that the incorrect
polarity can never be accidentally applied to the op amp.
Construction
Our prototype was built on a small
PC board (code SC06110921) but
there's no reason why you cannot assemble yours on a small piece of preetched circuit board (available from
most electronics stores). Fig.4 shows
the parts layout on the PC board.
The ideal mounting arrangement
would be in a metal box, large enough
to accommodate the 9V battery and
an on/off switch. The output plugs
should protrude through one end of
the case and be positioned so that
they plug straight into the "banana"
input sockets of the DMM. The only
critical component is Cl, which
should be located as close to the antenna terminal as possible. Apart from
that, the usual precautions when
ANTENNA
Fig.2: the final circuit
uses D1 to detect the
incoming RF signal. The
detected signal is then
filtered and amplified
using op amp IC1. D1
protects the op amp
against reverse
connection to the battery.
w
1
D1
c2
OA~5.
or s1m.
_047
c,
.oo,I
7
.,.
7
Protect your valuable issues
Silicon Chip Binders
OUTPUT TO
DMM
VR1
100k
RJ
1k
.,.
RELATIVE FIELD STRENGTH METER
··-+ . M G~ ~ LI
ANTENNA
fl
02$
.041W
+9V-----...
TO
1
ANTENNA-
-i 1
-.111.:_
OUP~TTO
DMM
------•
..._.,. E::!).001 .- {il'D-
..ffiD. .~
Fig.4: install the parts on the board as
shown here & be careful not to
confuse diodes D1 & D2.
R1
Fig.3: this modified circuit provides
greater sensitivity by applying
forward bias to the detector diode.
working at VHF and UHF should be
observed; ie, all component leads
should be kept as short as possible.
The entire circuit draws less than
2mA when fully driven, so battery
life will not be a problem. The antenna should ideally be telescopic, so
that it can be adjusted in length (corresponding to a quarter wavelength)
to suit the freque!lcy of interest. Alternatively, you can use an insulated
PARTS LIST
1 PC board, code SC06110921,
51 x 30mm
2 banana plugs
1 battery snap connector
1 9V battery
Semiconductors
1 LM358 op amp (IC1)
1 OA95 germanium diode (D1)
1 1N914 silicon diode (02)
1 100kQ trim pot (VR 1)
Capacitors
1 .001µF ceramic capacitor (C1)
1 .047µF ceramic capacitor (C2)
Resistors (0.25W, 1%)
1 12kQ (R1)
1 1kQ (R3)
1 4.7kQ (R2)
1 1.5kQ (R4)
These beautifully-made binders
will protect your copies of SILICON
CHIP. They feature heavy-board
covers & are made from a
distinctive 2-tone green vinyl. They
hold up to 14 issues & will look
great on your bookshelf.
* High quality
* Hold up to 14 issues
* 80mm internal width
* SILICON CHIP logo printed in
gold-coloured lettering on spine &
cover
Price: $A 11.95 plus $3 p&p each
(NZ $6 p&p). Send your order to:
Fig.5: this is the full-size pattern for
the PC board.
wire lead for the antenna (again cut to
the appropriate length).
The length of the antenna can be
easily calculated using the formula
L = 7140/F, where L is the calculated
length in centimetres and F is the
frequency in MHz. For example, if
F = 400MHz, then L = 7140/400 =
17.85cm.
At frequencies lower than UHF, the
quarter wavelength must be significantly increased to compensate for
the n egligible groundplane afforded
by the case and the meter body. A
larger metal case would partly overcpme this but this may not always be
convenient.
Our prototype showed good sensitivity and detected a 5-watt VHF handheld transceiver at a distance of 30
metres. We also checked the unit using both cordless and cellular telephones, with good results.
SC
Silicon Chip Publications
PO Box 139
Collaroy Beach 2097
Or fax (02) 979 6503; or ring (02)
979 5644 & quote your credit card
number.
Use this handy form l
----------Enclosed is my cheque/money order for
$_ __
or please debit my
O Bankcard
O Visa
O Mastercard
Card No:
Card Expiry Date _ _/_ _
Signature _ _ _ __ _ __ _
Name _
_ __ _ _ __ __
Address_ _ _ _ _ __ _ __
_ _ _ _ _ _ _ P/code _ __
.. _ _ _ _ _ _ _ _ _ _ _ ,1
SEPTEMBER1992
87
