A SIGNAL METER FOR ALL:
Lowe HF150 is a really superb little radio and I have enjoyed many
hours of fun with the set. The one major drawback that I found
was the lack of any signal strength indication which was a drawback
when attempting to make aerial
comparisons or trying to compare the strength of
This short article describes how
a very useful signal meter can be added to the Lowe HF150 (or any other
communications receiver that lacks a signal meter for that
matter). The circuit design of the S meter described here is the
one employed by Lowe themselves in their add-on audio-filter and S
meter unit and is easy to construct and reliable in operation.
The input to the meter consists
simply connection to the AGC line within the receiver - which does
require a very minor modification described later.
HOW IT WORKS:
op-amp ( IC1 ) is being used as an 'inverting amplifier' in this
circuit, as the AGC voltage applied to the inverting input of the
op-amp is reduced (because the
received signal strength rises) the result will be an increase in the
voltage supplied to the meter so thereby indicating an increased signal
Below are the circuit details
the meter circuit (top section of circuit) and the power regulator (lower part of circuit) which provides
the +5 volt rail and a -5 volt rail required for the meter circuit, and
derived from an external 12 volt power supply. I have in fact
used the output of the 12V power supply used by the HF150 radio.
The complete circuit
diagram. It is very easy to assemble this onto a piece of
incorporating the signal
meter circuit and the power
regulator circuit that derives the required
positive 5 volts and
negative 5 volt output that the signal meter circuit requires.
The op-amp for the
signal meter circuit is a TL061 or equivalent (see below), while the
voltage regulator for the power supply is a 78L09 which is rated at 0.1
amps, although I have also tried the higher
rated (2 amp) 78S09CV , so this could also be used. A suitable
available from Maplin Electronics.
12 volt DC
power adapter or
transformer. In my own case I used the Lowe 12 volt power adapter
to supply both the radio and this signal meter. It seems somewhat
confusing at first, but the signal meter part of the circuit (built
around IC1 the TL061 op-amp) needs a power supply of both +5 volts and
-5 volts. This is achieved by the power supply portion of the
circuit, the main component of which are the 78L09 voltage regulator
and the 5.1 v Zener Diode.
Two connections from the signal meter circuit - Pin 7 of IC1 and the
680k resistor go to the +5 volts point on the output from the power
supply section. Another two connections from the signal meter
circuit - Pin 4 of IC1 and the 460k resistor are connected to the
-5volts output from the power supply circuit. The -ve side of the
meter movement itself is connected to the 0 volts point on the output
of the power supply. This all looks a bit odd since the -5v
negative output is actually connected to the chassis and the -ve input
from the 12 volt power supply so seems to be at zero potential.
In this case, however, -5 volts is simply the relative potential
against the centre 0 volts connection point at the output of the power
supply. The + 5 volts output is merely 5 volts above the
potential of the 0 volts output. (and actually +10 volts above
the potential of the chassis i.e. the -5 volts point).
Don't worry about all this, it's just that the signal meter circuitry
needs to see a +ve 5 volts and a -ve 5 volts supply in relation to the
0 volts at the -ve side of the meter movement.
[ Note from Bob Warriner:
Bob wondered if it was absolutely necessary to fit the two 1000uf caps:
"Hi Mike, having recently acquired an HF-150, I was keen to build this
"S" Meter circuit. I planned to use the original Lowe power supply
which, as you know, is already well regulated and perfectly smoothed (as
tested on my oscilloscope). Omitting these components allowed me a bit
more space in the enclosure that I to used. I built and fitted the
circuitry into a meter case powered via the original HF-150 PSU. Without
the smoothing caps, all is well as can be seen in the
attached photographs (below). I have also carried out the backlight mod
on the HF-150 using a cheap Chinese display unit, dismantled the
backlight portion and fitted it behind the HF-150 display, connected up
the incorporated LED to the 12 volts on the on/off switch via a 2.2k
resistor. Best wishes, Bob."]
THE CONNECTION BETWEEN
THE METER AND THE RADIO: A detailed description of how the meter
unit is connected to the radio is offered below, but suffice to
say: Two 2.5mm jack sockets were used; one was mounted in the
rear panel of the radio casing and the other in the back panel of the
signal meter housing. The tip connection of the 2.5 mm jack
socket within the radio is taken to the AGC (Automatic Gain Control)
circuit of the HF150 at Pin 16 of Q32 via a 470k Ohm resistor. This is
shown in detail below. The tip connection of the 2.5 mm jack
socket within the Signal Meter is taken to the 47 k resistor, and
thereby on to Pin 2 of IC1, as shown in the circuit diagram above.
The radio and meter are then 'hooked up' via the two 2.5mm sockets
using a suitable length of thin screened cable with a 2.5 mm plug on
Note that the 0 Volts
connection between the negative side of the meter and the
0 Volts output from the power regulator circuit should not be connected
to the metal case or ground (since this is at a relative -5 volts
potential which would make the meter malfunction). A 2.5mm jack socket was fitted
to the radio and the signal meter case to make an easily removable
connection between the signal meter unit and the radio.
METER PARTS LIST
pin socket to
mount Op Amp IC
eg 2.5mm jack
A Page Containing
Some Links To Suppliers Of Electronic Components >
THE OP-AMP: The
Op-Amp used for the signal meter circuit is a TL061. An LF351
works just as well, however and has the same pin layout. If you
have a TL062 or similar the differing pin-outs are shown below so that
you can design your circuit board successfully.
AMP Pin Layouts
The diagram above shows the pin
layout of the
TL061 and some alternative op-amps that can also be used in the signal
circuit described on this page. The
LF351 op-amp also
works in this design and is pin compatible with the TL061. Another
pin compatible op-amp that is usually easy to obtain, or that may be
knocking about in the 'junk box' is the very common LM741. I have not
tried an LM741 in this circuit, but I can see no reason why it would
The TL06* series are low
power versions of the TL8* series and are pin compatible
with the TL7* series.
Use D.I.L Sockets:
Rather than solder any op-amp directly to the circuit board, I would
strongly recommend that you use 8 Pin D.I.L. sockets so that the I.C.
can be easily inserted and removed.
The completed circuit
housed in an aluminium box.
back panel are the 12v power input sockets (2 are fitted to allow
'daisy-chaining' of the power to another unit) NB - the 0 Volts connection
between the negative side of the meter movement and the 0 Volts output
power regulator circuit should not be connected to the metal case or
on the left is the 2.5mm Jack that allows connection of the signal
to the radio. You can just see the single grey wire that connects
the 2.5mm socket to the circuit board. The top half of the board
accommodates the signal meter components (ignore
the second preset), while the bottom half of the board
accommodates the regulator components.
original scale provided with the Maplin
signal meter that was marked from 1 to 5, with a very simple one that I
printed on ordinary paper using an ink jet printer. I produced a simple
scale marked in 'S' units: S1 S3 S5 S7 S9
+30 +50dB. I copied these
gradations from a photograph of the original Lowe signal meter to
ensure reasonable accuracy and conformity to the original Lowe
design shown below.
Above, The Lowe HF-225
|IMPROVED SIGNAL METER SCALE
I found an interesting
page by Frank OK2FJ who produced an excellent meter scale for the Yaesu
FT-857 and FT-897 amateur radio transceivers. The meter for the Yaesu
rigs shows much more information than just the signal level, so I
decided to simplify it so that it could be used in an S Meter project
such as this. I saved the image that Frank produced made some changes
using an image editor. The finished S Meter scale is shown below. It is
a large image being 1795 pixels wide so that the print quality will be
When printing, you will
need to scale the image very accurately to fit the particular meter
that you use. This can be done by trial and error or by mathematics!
Click on the image below to save it.
can download and save the above image to print out at a size that
suits your particular meter movement
Printing The Scale: The
scale can be printed on paper or thin card and possibly laminated,
which is what I did. White paper or card might be the obvious choice,
but cream, light green, yellow or light blue card would also make a
good background colour.
When printed, the image will need to be scaled quite accurately to suit
the size of the particular meter movement being used, otherwise the
needle will not line up properly with the scale and the indication will
be inaccurate. This can be done by trial and error until the correct
size is found - a bit of a fiddly and a rather wasteful method.
Alternatively a bit of simple math's can be used.
My image editing program allows scaling of the print-out using a
sliding scale that shows the total width of the image when it's printed
and the dpi (dots per inch) output to the printer. Knowing the total
image width isn't especially helpful since what is needed in this case
is the dimension that is the distance between the left and right end
markers of the S scale - the top curve. My simple image editor does not
allow an accurate measurement of a portion of the image, so I did a
test print, estimating that the resultant image would need to be 50 mm
wide, the output in this case was 920 dpi. I then measured the width of
the top curve on the test print, from end marker to end marker - it was
40mm. The scale of the original microammeter is 34mm wide, so the print
had to be scaled down in size.
The magnitude of the size reduction can be found by dividing that
measurement, 40mm, by the required measurement - in this case 34mm.
40mm ÷ 34mm = 1.176 (the scaling factor)
The original test print produced an scale that, at 40mm, was too wide.
It needed to be 34mm wide. The original image width of the test print
was 50mm and therefore this needed to be divided by the scaling factor
50mm ÷ 1.176 = 42.5mm
The calculation suggests that 42.5 mm is the width required for the
whole image. The image was printed again at that width and the reulting
print measured. It was found that the width across the top curve from
end marker to end marker was, indeed, the required 34mm.
The other way of doing the scaling is to note the dpi output of the
original test print, in this case 920 dpi, and multiply (not divide)
that by the scaling factor. The original dpi figure is multiplied,
rather than divided, becuase the dots per inch will increase as the
original image size is shrunk. In this case the new, and correctly
sized print, is 1082 dpi. Whichever method is used, the second print
should produce a scale of the correct size.
Reference: http://www.radio-foto.net/radio/ftmeter2.png This is the original meter
scale image that was produced by Frank OK2FJ, I altered this to produce
the meter image that is shown above.
sense of 'S' units, the table below shows the relationship between 'S'
units and terminated voltage - in microvolts:
Note how relatively small changes in
voltage at the lower end of the scale ( in the S1 to S8 range) produce
quite noticable swings in the readings, while really quite large
changes in signal voltage at the higher end of the scale ( S9 to S+50 )
produce quite small variations in read-out. This non-linear
effect is quite intentional: Increases in signal strength from S1
to S9 will produce dramatic improvements in the received signal to
noise (S/N) radio while above around the S9+10dB signal level the
receiver is approacing the best acheivable S/N ratio and further large
increases in signal strength will make less if any improvement to the
audible S/N ratio, so the S Meter does not really need to reflect these
changes in such minute detail.
The signal meter is invaluable when making aerial adjustments and
comaprisons as well as being useful for comparing the strength of
A Note About S-Meter
Although there is no set standard for S-Meter calibration, it seems
widely accepted that the S9 reading is calibrated for a signal input of
50µV. (Although some manufacturers may calibrate S9 at
100µV). Additionally the gradation between each S unit is
generally accepted as 6dB. i.e. S2 is 6dB greater than S1 and S9 is
12dB greater than S7 etc. As can be seen in the table above 6dB
represents a doubling of the voltage received by the radio.
(It may also be worth pointing out that for a receiver to receive
double the voltage from a transmitting station, the transmitting
station would have to increase its effective radiated power level by
In real life the signal meter on many radios may not be accurately
calibrated to 50µV at S9 and each individual S unit may not,
indeed, accuratelt represent 6dB intervals. Some meters may represent
the interval as 5dB or 4dB. However an S Meter's visual representation
of relative signal strengths
will still be very useful in judging differences between signal
strengths of different stations and particularly useful with antenna
However, without an accurate signal generator, it will be down to
individual judgement as to how this "home-brew" signal meter is
DETAIL : CONNECTING
THE SIGNAL METER TO THE LOWE HF150 - Or Other Radio
A minor modification to the HF-150 is required to enable an external
signal meter to be connected. The AGC (automatic gain
control) line on the receiver's printed circuit board needs to be
identified and an explanation was proveded by Lowe for the HF-150, but
a similar connection could feasably be made to the AGC line of any
other receiver. This may be
marked as a pin on an IC or next to a transistor or other
component. You may be able to find the circuit diagram for your
radio in the handbook or be able to abtain one from the manufacturer or
from a search of the internet.
A very thin wire can be soldered to the board at this AGC point and
taken, via the 470k resistor, to a socket on the back or side panel of
I used a 2.5mm jack on the
back of the HF-150 to facilitate easy connection and avoid the method
described by Lowe below which involves cutting the PCB track to the
earth terminal and exiting the signal meter feed by that
terminal - thus the radio would no longer have an earth terminal.
Instead I carefully drilled a neat hole in the back plate and fitted
new dedicated socket for the signal meter. The radio and
the meter are then easily connected together by a short length of
screened cable with a 2.5mm plug on each end.
This is the modification as described by Lowe:
for 'S' meter operation
Modification of the HF 150 for use with the 'S' meter if your set has a
serial number of 142617 or less it will require the addition of a 470k
To modify your HF150 you will need to remove any power including
batteries if fitted, lay the set upside down on a piece of cloth or
similar to prevent scratching the case.
Now remove the bottom panel; With the front panel facing you
locate the wire aerial connector, the black terminal being earth, if
you now look at diagram No.1 you will see a thin track connecting
this pad to the earth plane, you will need to cut this tiny track*, now you can solder a 470k
resistor .25W from the black terminal to pin 16 of IC Q32 at the point
where C97 is soldered onto the circuit board. Please refer to
For sets after No. 142617 the 470k resistor will be fitted and all that
will be needed is to cut the thin track to earth as to allow the agc
signal out from the black terminal.
* see my notes
in the text
The panel below shows the modification required to be undertaken to the
Lowe HF150 itself. The original modification shows that the 470k
resistor is taken from Pin 16 of Q32 to the Earth terminal. I
instead decided on the much neater option of fitting a dedicated 2.5mm
jack socket on the back panel for the signal meter output:
* The Lowe
of how the signal meter is connected to the AGC line of the radio via a
470k resistor, and output through the earth terminal. I instead
opted to fit a dedicated 2.5mm socket to the back panel to take the
out the the signal meter - the 470k resistor is therefore connected to
pin 16 of I.C Q32 and taken
directly to the 2.5mm jack socket. This
makes a much neater way of connecting up the external signal meter to the back of the Lowe HF150 I feel -
as can be seen in the photo of
the rear panel below:
Rear Panel -
showing additional 2.5mm socket
photograph above shows the rear panel of the HF-150. The
additional 2.5mm signal meter output socket is mounted in a neat hole
drilled just below the letter 'F' of the HF-150 logo top centre, above
the battery housings.
There is just one single thin wire to connect on to the circuit board
within the radio at the AGC line. This wire is taken to the 2.5mm
jack socket via the 470k ohm resistor. The body of the 2.5mm
socket is grounded to the aluminium panel
of the receiver. It is then a simple matter to use a short piece
of screened cable, terminated with a nice solid 2.5mm plug at each end,
to connect the radio to the external signal meter - the centre
conductor of the cable taking the AGC voltage to the meter while the
braid screen simply connects the ground of both the radio and the
signal meter casings together.
Using this type of shielded connecting cable arrangement is probably
really necessary if you are running the signal meter and the radio from
the same power supply transformer, as I do, and a single piece of
hook-up wire between the radio's AGC and the signal meter circuit could
do the job - but I used this method for neatness and reliability.
Using metal cased 2.5mm jack plugs looks more professional too!
ADJUSTING THE METER READOUT: Once the meter is connected
to the radio the sensitivity is adjusted by
the 100k Ohm preset potentiometer - in essence this involves
disconnecting the aerial from the receiver and ensure that no radio
signal is being received (even hash or interference) and then to set
preset so that the needle rests at minimum deflection i.e. as far left
as possible or zero .
** I actually ended up constructing
signal meter for my listening post within the case of an existing audio
filter unit that I had previously constructed and which had some space
I hope you have a go
at building this little project. A signal meter can be a very
useful tuning aid and I have found it invaluable for making various
aerial adjustments and comparisons, as well as for general listening.
writes regarding the Lowe SP-150 loudspeaker, audio notch filter and
signal meter unit: "Last
summer I considered myself lucky to find a matching
speaker/notch-filter/S-Meter at a HAM-fest, the SP-150. The missing
instruction-manual I downloaded from the internet easy enough. When I
connected the set to my HF-150 with an appropriate cable I found the
S-Meter not to be working at all and the sound to be distorted at
times. Some damage in the SP-150’s circuitry I thought and so began
searching the internet for a source of repair.
That’s how I came upon
the information that for receivers below a certain serial-number (just
like mine) a “modification” has to be done by connecting the
ground-terminal to an IC by means of a resistor (after having isolated
this ground-clamp from the circuitry-ground first of all). That was
easily done as well. While it struck me as absurd to rid the radio of
all means to ground it, the obvious question has been on my mind ever
since: How does the AGC-voltage that’s now available at the receiver’s
ground-terminal (or at the tip of a 2.5 mm connector as per your
approach, which I wish I’d read earlier), how does this AGC-voltage get
to the SP-150’s S-Meter???
As per the manual a
cable-connection must be made between the radio and the SP-150’s input.
There is only one such connector at the set’s backside, which (sure
enough) connects to Pin 2 of the IC described in the circuitry on your
website (only in the SP-150 it’s IC 5 or 6). So where (and how) does
one feed in the AGC-voltage? I cannot possibly be the only user facing
this problem which I why I hope you’ve had reason to cope with this
situation in the past and help me out.
What I will do in any
case is re-connect the radio’s ground-lug to circuitry-ground, drill a
small hole like you did and solder the 470K-resistor to a separate 2,5
mm-connector. Some advice and support would be highly appreciated! Many
thanks in advance and all the best from Hamburg".
Solution - How to connect
the audio and AGC signal meter drive to the Lowe SP-150: Well,
fortunately the solution is relatively straightforward. Looking at the
Lowe SP-150 circuit digram shows that both the audio from the Speaker
Out socket on the rear ofthe HF-150 and the AGC voltage derived from
the modification to the HF-150 described above is connected via a three
conductor jack socket ('stereo jack') on the rear of the SP-150. The
audio input socket serves this dual function which, sadly, is not made
clear in the manual.
The three conductor ' stereo' jack on the SP-150 is wired as follows:
The body of the plug is ground, the tip is audio and the centre ring is
for the AGC voltage for the S-meter.
Above: cut-out section of the Lowe SP-150 circuit diagram showing that
the three conductor audio input jack socket ('stereo jack') also
carries AGC for signal meter.
Body of the plug is ground; the tip is audio; the centre ring
carries the AGC voltage for the S-meter.
Loudspeaker, Filter & Signal Meter User Manual
Hans Kröger replies to our
clarification: "Good morning
I just wanted to let you know that the cable-connection suggested by
did indeed make my HF-150 and SP-150 work together. Just a quick and
dirty job I had time for, pretty much “on the fly”, but it worked as it
should. Thanks a million for your assistance and all the best from
Sincerely, Hans Kröger".
Bob Warriner's "S" Meter
Bob Warriner's "S" Meter
Lowe HF-150 back-light modification by Bob Warriner
& Short Wave