Last semester, I designed and built the QROP Meter for my
graduate school independent study project. Thanks to those of you who
helped me with certain thorny issues, such as transformers and DC
amplification. Go to my web page at http://www.jasonhsu.com/ee.html
to learn how to build the instrument.

What makes my SWR/wattmeter different from the numerous versions
available from MFJ and the numerous homebrew designs out there?

1. Works from 200mW to 100W! You get the best of both worlds! MFJ
SWR/wattmeters don't work at QRP, and QRP SWR/wattmeters can't handle
100W. Tuning up at 1W instead of 100W reduces the QRM you cause by
20dB, or over 3 S units! You can also safely change the tuner's
inductance taps WHILE transmitting at 1W.
2. GOOD resolution at SWR values above 3 and even above 5! This can
be very handy at 160m and 80/75m.
3. 10-LED display for the SWR meter and another 10-LED display for
the wattmeter: No more squinting at cross-needle meters!
4. More accuracy at low power levels: Most SWR meters understate SWR
at low power levels due to the infamous diode drop loss. My design
compensates for this and allows accurate SWR measurements at QRP power
levels.

Let me know what you think, and feel free to contact me if you have
any questions or suggestions. If you decide to build the QROP Meter,
let me know how it goes.

Jason Hsu, AG4DG

Nice project. People interested in learning more about the diode
compensation circuit can find additional information in John
Grebenkemper's article "The Tandem Match -- An Accurate Directional
Wattmeter", QST, Jan, 1987, and a good deal more in my article "A Simple
and Accurate QRP Directional Wattmeter" in February 1990 QST.. It turns
out that you can reduce the diode drop to an arbitrarily low value by
terminating it with an arbitrarily high resistance. But that's not the
problem in detecting small signals. The problem is that the diode
reverse leakage current becomes a bigger and bigger fraction of the
forward current as the current gets smaller. That is, the diode becomes
more and more like a resistor as the signal gets smaller and smaller.
The problem, then, isn't a lack of forward conduction, but that the
charge conducted into the load during the positive half cycle is sucked
back out during the negative half cycle. The compensation circuit used
in these two articles and in Jason's circuit only approximately match
the RF conductivity characteristics of the diode, and the operating
points of the diodes have to be carefully chosen to insure tracking with
temperature changes as well as signal level.

Roy Lewallen, W7EL

Jason Hsu wrote:
Last semester, I designed and built the QROP Meter for my
graduate school independent study project. Thanks to those of you who
helped me with certain thorny issues, such as transformers and DC
amplification. Go to my web page at http://www.jasonhsu.com/ee.html
to learn how to build the instrument.

What makes my SWR/wattmeter different from the numerous versions
available from MFJ and the numerous homebrew designs out there?

1. Works from 200mW to 100W! You get the best of both worlds! MFJ
SWR/wattmeters don't work at QRP, and QRP SWR/wattmeters can't handle
100W. Tuning up at 1W instead of 100W reduces the QRM you cause by
20dB, or over 3 S units! You can also safely change the tuner's
inductance taps WHILE transmitting at 1W.
2. GOOD resolution at SWR values above 3 and even above 5! This can
be very handy at 160m and 80/75m.
3. 10-LED display for the SWR meter and another 10-LED display for
the wattmeter: No more squinting at cross-needle meters!
4. More accuracy at low power levels: Most SWR meters understate SWR
at low power levels due to the infamous diode drop loss. My design
compensates for this and allows accurate SWR measurements at QRP power
levels.

Let me know what you think, and feel free to contact me if you have
any questions or suggestions. If you decide to build the QROP Meter,
let me know how it goes.

Jason Hsu, AG4DG

Jason:

Ok for the QRP to barefoot operator, but what about someone that at least
occasionally uses an amplifier?

I've got a couple of swr/wattmeters (two meters) that provide everything I
need, though they are a bit difficult to read at QRP levels. Autek WM-1
is one of them and currently available, and the other is a SignalCrafter
30 (may not be full name and model, but also not available any more).
Both have ranges for 20w, 200w and 2000w full scale, and show average or
peak readings. They tend to match one another pretty well, especially
considering the WM-1 is less than a year old and the other is probably 20
years old....

Will your meter be able to perform as well? I've found that most LED
based metering circuits are horribly inaccurate compared to a needle based
meter....

On the other hand, I'd love to see a new meter available, particularly one
that would work from qrp levels all the way up to legal limits.... while
providing both SWR and Power metering. I really really dislike dual
needle interpreted displays.

Thanks
--Rick AH7H



Jason Hsu wrote:

Last semester, I designed and built the QROP Meter for my
graduate school independent study project. Thanks to those of you who
helped me with certain thorny issues, such as transformers and DC
amplification. Go to my web page at http://www.jasonhsu.com/ee.html
to learn how to build the instrument.

What makes my SWR/wattmeter different from the numerous versions
available from MFJ and the numerous homebrew designs out there?

1. Works from 200mW to 100W! You get the best of both worlds! MFJ
SWR/wattmeters don't work at QRP, and QRP SWR/wattmeters can't handle
100W. Tuning up at 1W instead of 100W reduces the QRM you cause by
20dB, or over 3 S units! You can also safely change the tuner's
inductance taps WHILE transmitting at 1W.
2. GOOD resolution at SWR values above 3 and even above 5! This can
be very handy at 160m and 80/75m.
3. 10-LED display for the SWR meter and another 10-LED display for
the wattmeter: No more squinting at cross-needle meters!
4. More accuracy at low power levels: Most SWR meters understate SWR
at low power levels due to the infamous diode drop loss. My design
compensates for this and allows accurate SWR measurements at QRP power
levels.

Let me know what you think, and feel free to contact me if you have
any questions or suggestions. If you decide to build the QROP Meter,
let me know how it goes.

Jason Hsu, AG4DG

Jason,
The circuit looks very useful. The layout of the LEDs is not
very typical, which makes it a bit unnatural to use. That is
easy to change. Can you provide some inside photographs
to show how you did the circuit layout?

I am not sure why people even use SWR as a measurement for
reflected power. Perhaps it provides a nice mathematic
simplification, but it is a very quirky and unnatural scale
for human operators: 1) it does not go to zero, 2) it is
hard to read/write/and even say, 3) without a forward
reading, a SWR of 1:1 could simply indicate no forward
power, and the useful range ends at 3. What is needed
is a scale that based on a 2-dimentional vector. For
example, magnitude and angle of a vector that is formed
from an X-axis of forward power and a y-axis of reflected
power. The difficulty is mostly converting this to
a graphical representation. An array of 100 LEDs is
not very practical (10 x 10), but perhaps a tiny LCD
is. A small LCD and microprocessor could be cheaper
and easier to read than cross needles.

Andy
WA3LTJ





Jason Hsu ) wrote:
: Last semester, I designed and built the QROP Meter for my
: graduate school independent study project. Thanks to those of you who
: helped me with certain thorny issues, such as transformers and DC
: amplification. Go to my web page at http://www.jasonhsu.com/ee.html
: to learn how to build the instrument.
:
: What makes my SWR/wattmeter different from the numerous versions
: available from MFJ and the numerous homebrew designs out there?
:
: 1. Works from 200mW to 100W! You get the best of both worlds! MFJ
: SWR/wattmeters don't work at QRP, and QRP SWR/wattmeters can't handle
: 100W. Tuning up at 1W instead of 100W reduces the QRM you cause by
: 20dB, or over 3 S units! You can also safely change the tuner's
: inductance taps WHILE transmitting at 1W.
: 2. GOOD resolution at SWR values above 3 and even above 5! This can
: be very handy at 160m and 80/75m.
: 3. 10-LED display for the SWR meter and another 10-LED display for
: the wattmeter: No more squinting at cross-needle meters!
: 4. More accuracy at low power levels: Most SWR meters understate SWR
: at low power levels due to the infamous diode drop loss. My design
: compensates for this and allows accurate SWR measurements at QRP power
: levels.
:
: Let me know what you think, and feel free to contact me if you have
: any questions or suggestions. If you decide to build the QROP Meter,
: let me know how it goes.
:
: Jason Hsu, AG4DG
:

Rick Frazier wrote in message ...

Ok for the QRP to barefoot operator, but what about someone that at least
occasionally uses an amplifier?

The only thing I can think of is using more turns in the transformers
and thus lowering the coupling ratio. This would increase the QRO
capability but at the expense of QRP capability. I designed the QROP
Meter to just barely handle 200W so it could easily handle 100W. It's
not designed to be used with an amplifier. I don't have an amplifier,
so 1500W capability was not a priority for me.

The closest thing to what you are looking for is the Tandem Match
Directional Wattmeter project that Roy alluded to. It's available in
the _ARRL Antenna Book_. In fact, I got the idea of separating the RF
and DC grounds from the Tandem Match Directional Wattmeter. You'll
also notice my QROP Meter and a few of the homebrew SWR/wattmeter
designs out there have similarities to the Tandem project, such as the
directional coupler design and the noninverting logarithmic op amp.

Believe it or not, I wanted even more QRP capability (like 1mW instead
of 200mW) than my project actually delivers. However, I found several
constraints:
1. Op amps have offset voltages, and these constrain the accuracy of
low-power measurements. Chopper amps have extremely low offset
voltages, but giving them a 10V input while the power supply is turned
off destroys them.
2. The active rectifier has limited accuracy. It's fine in dealing
with 100mV, but it would be useless in rectifying 10mV.
3. The LM3914 chip has limited accuracy. The internal comparator
amplifiers have a few mV of offset voltages.

I've got a couple of swr/wattmeters (two meters) that provide everything I
need, though they are a bit difficult to read at QRP levels. Autek WM-1
is one of them and currently available, and the other is a SignalCrafter
30 (may not be full name and model, but also not available any more).
Both have ranges for 20w, 200w and 2000w full scale, and show average or
peak readings. They tend to match one another pretty well, especially
considering the WM-1 is less than a year old and the other is probably 20
years old....

Will your meter be able to perform as well? I've found that most LED
based metering circuits are horribly inaccurate compared to a needle based
meter....

My QROP Meter needs to have diode pairs that are properly matched, and
the proper resistance value for the noninverting logarithmic op amp
will vary from one unit to another. If you don't bother to properly
match the diodes or if you don't select the right resistance value,
the instrument will be just as crude as the conventional products
manufactured by MFJ and other companies. Of course, the need to
customize every single unit would kill the manufacturability, and this
would explain why the products on the market are so crude.

On the other hand, I'd love to see a new meter available, particularly one
that would work from qrp levels all the way up to legal limits.... while
providing both SWR and Power metering. I really really dislike dual
needle interpreted displays.

That's even MORE ambitious than my QROP Meter. I'm sure this
QRP-to-1500W instrument could be designed and built, but it would be
MUCH more complicated and expensive than my device. I'm not sure the
market would bear the cost of such a device, which may cost $1000 for
all I know. I highly doubt that even my QROP Meter has a viable
market. Also remember that the QRP-to-1500W device would have to
compete with antenna analyzers, which are only around $300 and show
FAR more detail than an SWR meter. True, you can't transmit 100W
(much less 1500W) into an antenna analyzer, but antenna analyzers
allow you to tune up without emitting a single mW from your
transmitter.

Jason Hsu, AG4DG

On 23 Sep 2003 08:31:17 -0700, (Jason Hsu)
wrote:

Believe it or not, I wanted even more QRP capability (like 1mW instead
of 200mW) than my project actually delivers. However, I found several
constraints:
1. Op amps have offset voltages, and these constrain the accuracy of
low-power measurements. Chopper amps have extremely low offset
voltages, but giving them a 10V input while the power supply is turned
off destroys them.
2. The active rectifier has limited accuracy. It's fine in dealing
with 100mV, but it would be useless in rectifying 10mV.
3. The LM3914 chip has limited accuracy. The internal comparator
amplifiers have a few mV of offset voltages.


Hi Jason,

1. Op Amps have offset voltage compensation circuits (either
internal, or you can access 50 years of literature on how to do it
externally). Your choice for a self-destructing chopper amp is your
own problem, not an inherent failure of the class of device. Again,
there are 50 years of literature on how to build your own if the
devices your limit yourself to are either a. too expensive, or b. too
fragile.

2. Active rectifiers having limited accuracy is strictly a problem of
GBWP. Choose a crappy one, and you fulfill your nightmare. For HF,
you should be using one with at least 1GHz. Your problem then becomes
one of selecting an amp that is stable at unity gain (or at least
stable at the gain you choose/need). Complaints about not being able
to handle 10mV signals only suggest you need an amplifying buffer
before the detector. With a little leg work (researching that same 50
years worth of application design), a second diode, and feed back, you
could linearize the power/SWR meter too.

3. LM3914 should be designed with the usual offset compensation if
that is a problem. However, even with a few mV out of 10V, the
dynamic range is considerably greater than your instrument's range.
Your problem is one of scaling your signal (it shows in the other
complaints). Dynamic range, 20 log (10V/10mV), borders on 60dB where
accuracy would tend to go to hell at the low end. You show only 30dB
of dynamic range, 10 log (200W/200mW). You are not using the full
potential of the device.

Compensation circuits can run from simple to complex. If you have
zero compensation, you are abandoning your fate. Reading National
Semiconductor product literature would be a simple approach to a great
deal of variety of design:
http://cache.national.com/ds/LM/LM3914.pdf

73's
Richard Clark, KB7QHC

Richard Clark wrote in message . ..

1. Op Amps have offset voltage compensation circuits (either
internal, or you can access 50 years of literature on how to do it
externally). Your choice for a self-destructing chopper amp is your
own problem, not an inherent failure of the class of device. Again,
there are 50 years of literature on how to build your own if the
devices your limit yourself to are either a. too expensive, or b. too
fragile.

I am aware that some op amps have offset voltage compensation
circuits, but they require adjusting a potentiometer. I don't like
potentiometers and prefer to design a circuit to avoid the need for
one. There probably are better solutions than the active rectifier
design I went with, but they would have been more complicated.

2. Active rectifiers having limited accuracy is strictly a problem of
GBWP. Choose a crappy one, and you fulfill your nightmare. For HF,
you should be using one with at least 1GHz. Your problem then becomes
one of selecting an amp that is stable at unity gain (or at least
stable at the gain you choose/need). Complaints about not being able
to handle 10mV signals only suggest you need an amplifying buffer
before the detector. With a little leg work (researching that same 50
years worth of application design), a second diode, and feed back, you
could linearize the power/SWR meter too.

I had originally thought of using RF amplifiers in my design, but RF
amplifiers are not available in DIP format. So I switched to
rectifying the RF with Ge diodes and then using a noninverting
logarithmic op amp to compensate for the diode drop loss. The
experience also taught me to be less reliant on PSPICE in the future.
Just try to model an LM324 with the negative power supply grounded.
An LM324 doesn't need that negative power supply in real life, but it
does in PSPICE.

3. LM3914 should be designed with the usual offset compensation if
that is a problem. However, even with a few mV out of 10V, the
dynamic range is considerably greater than your instrument's range.
Your problem is one of scaling your signal (it shows in the other
complaints). Dynamic range, 20 log (10V/10mV), borders on 60dB where
accuracy would tend to go to hell at the low end. You show only 30dB
of dynamic range, 10 log (200W/200mW). You are not using the full
potential of the device.

In the original version, I had 2 LM3914 amplifiers cascaded for 20
LEDs for the SWR display. When I wasn't transmitting, several of the
LEDs would often come on, presumably because of the near-zero voltages
on the low, signal, and high voltage ports. The instrument also
didn't have the accuracy needed to justify 20 LEDs, and a 7-inch by
3-inch face does not have enough room for 40 LEDs.

You can always try to improve on my QROP Meter, just as my QROP Meter
idea was inspired by my dissatisfaction with conventional
SWR/wattmeters and the numerous homebrew designs I had looked at. And
yes, I obtained many of my ideas from other people's designs. I found
an LED-based SWR meter for VHF in the ARRL's archived articles. The
noninverting logarithmic op amp to compensate for the diode drop loss
and the directional coupler for sampling forward and reflected
voltages came from several homebrew projects on the net plus the ARRL
Antenna Book's Tandem Wattmeter project.

Jason Hsu, AG4DG