The LP-100 Wattmeter

A high-performance, microprocessor-controlled digital
SWR/wattmeter with wide dynamic range
By Larry Phipps, N8LP

The LP-100 project is really an
ongoing design exercise. The
main meter chassis has remained fairly unchanged through several iterations of the project, but thedirectional coupler has undergone
some radical changes, with several
new designs still undergoing testing
and modification. I will describe thefirst iteration of the project fully in thisarticle. At the conclusion of the article,
I will discuss some of the changes tothe coupler, and will refer the reader

49100 Pine Hill Dr
Plymouth, MI 48170
larry@telepostinc.com

to my Web site (www.telepostinc.
com) for the latest details.
Here are the basic specifications/features of the meter…
• Coverage from 0.5 to 54 MHz for
rated specs with the specified coupler designs• Auto-ranging scales covering 0-2,
20, 200 and 2000 W• Modular design to allow for different couplers• Scale for dBm measurements,~ –15.0 dBm to +63.0 dBm in
0.1 dB steps• Power display resolution of 0.01,
0.1 or 1 W depending on scale.
• Power accuracy better than ±.5 dB
to <10 mW, typically 5% after calibration• Displays actual power delivered to
the load (Fwd minus Ref)

• SWR display resolution of 0.01
• Overall SWR accuracy typicallywithin ±0.1 of actual down to 100
mW• Return Loss (RL) display of 0 to
49.9 dB• State-of-the-art PLED display withscreen saver• Fast responding logarithmic bargraphs for power, SWR and RL• Peak-hold numerical power readout
with fast and slow time constants• SWR Alarm system with set points
for 1.5, 2.0, 2.5 and 3.0• Separate calibration screen• Serial port for external software
control

From Jan/Feb 2006 QEX © ARRL• Virtual Control Panel (VCP) soft-

ware for Windows• Remote control supported directly or
through TRX-Manager
Among the most interesting aspectsof the design of the LP-100 are the coupler, the logarithmic detectors, themicrocontroller and the Windows VCP
software. The area that received themost attention (and created the mostfrustration) was the directional coupler. Over 50 designs were built andtested. Each had its good points andbad, but I settled on two for the final
versions.
Coupler

Before discussing coupler design,
let’s define a few terms.
Directivity—the difference (in dB)
between forward (Fwd) and reflected(Ref) power when the output port is
terminated in the design impedance…
50 Ω in this case.
Insertion Loss—the loss (in dB) between the input and output ports
Coupling Factor—the loss (in dB)
between the input port and the forward sample port, or between the output port and the reflected sample port.
Input Return Loss (RL)—theamount of power reflected back into
the source (in dB) when all other ports
are terminated with accurate 50-Ω
terminations.
I used eight criteria to evaluate mycoupler designs.
1. 50 Ω Z on all ports
2. cover 160 to 6 m in one design.
3. >30 dB directivity over the entire frequency range
4. Level passband response over theentire range
5. >25 dB input RL presented to thetransmitter over the entire range
6. Insertion loss < 0.2 dB
7. 1500 W continuous power handling capability
8. Easy to duplicate
First, let’s review how a coupler
operates. Probably the grandfather ofbroadband frequency-independent directional couplers is the design developed by Warren Bruene, W5OLY, of
Collins Radio. The best coupler article
I’ve read is his “An Inside Picture of
Directional Wattmeters,” published in
April 1959 QST.1 It is must reading
for anyone interested in the subject
because it analyzes the Bruene Bridge
as well as other coupler designs.
Underlying Bruene’s design, as wellas that of many other couplers, is acurrent sampling transformer in series with the transmission line combined with a circuit to sample voltage
1Notes appear on page 13.

From Jan/Feb 2006 QEX © ARRL

across the load. The voltage sampler
may be a capacitive or resistive divider, or a transformer. The current
and voltage samples are combined so
as to isolate the forward and reflectedcomponents. This is possible because
the voltage and current of the forward
wave are in-phase, while the voltage
and current of the reflected wave areout-of-phase.
Another type of coupler, which at
first would seem to be radically different from these, is also related. It is theline-section or loop coupler as
pioneered by Bird. In this design, aterminated loop is brought into proximity to the center conductor of atransmission line, and provides a Fwd
or Ref sample, depending on which endof the loop is terminated. This is possible because the loop provides two
kinds of coupling at the same time.
Inductive coupling to the line produces
a current sample, and capacitive coupling provides a voltage sample.
My first criterion—50-Ω impedanceat all ports—eliminated many published coupler designs, including theBruene bridge, which must be terminated in high Z loads for the forward
and reflected ports. After reviewing theavailable 50-Ω designs, I settled on acouple to try. One was the Tandem
Match,2 covered in depth in a QST article and recent issues of the ARRL
Antenna Book. Another version uses abinocular ferrite core and is describedin a comprehensive article by Michael
G. Ellis.3 These designs, and otherssuch as the Stockton coupler, are descendants of a design using two toroidal transformers patented by
Sontheimer and Fredrick in 1969. See
Fig 1.
By using identical transformers,
wired as shown, all four ports will havethe same impedance. The current
transformer, T1, has a one-turnprimary in series with the maintransmission line, while the voltage
transformer, T2, has its secondary connected across the transmission line. If
the two transformers have the sameturns ratio, and the current sample isterminated in a 50 Ω load, then the

Fig 1—Tandem Match Schematic.

voltage across the termination will be
the same as the voltage from the voltage sampler. If the two voltages are
combined, then the phase relationship
mentioned above will cause the Fwd
components to add and the Ref components to subtract.
The correct transformer turns ratio, N, is determined by balancing a
number of factors, including keeping
T2’s reactance high to prevent loading of the line, and keeping T1’s winding length short to minimize stray
capacitance which reduces high frequency response. N also determines
the coupling loss, and hence the required Fwd port termination power
rating, since the power at this port is
proportional to 1/N2.
The first version of this design that
I tried used a ferrite binocular core,
which allowed very simple construction techniques. I discovered that inorder to meet my bandwidth objectivewith available cores, T1 and T2’s
secondary windings had to be kept under 10 turns. Since the only commercially available binocular core that
could possibly handle 1500 W is about
1-inch long, each turn takes nearly
three inches of wire. Since the upper
frequency response is largely related
to the length of the wire in the secondary windings, keeping the coupler
functioning at even 28 MHz limited
my design to fewer than 10 turns.
But if T1’s secondary is only 8 turns,
for example, with an input power of1500 W, the forward coupling port termination must dissipate nearly 25 W.
Dissipating 25 W in a small box, while
keeping the temperature rise to acceptable levels is not easy.
My second attempt was the Tandem
Match coupler. One problem I notedwith this design duplicated the experience related in an article by N2PK.4
He discovered a problem with the
input RL of the Tandem Match at
1.8 MHz. He indicated that this wasa factor of inadequate XL at 1.8 MHz,
thereby shunting the line at that frequency with a relatively low value
inductance.
By replacing the powdered ironcores of the Tandem Match design withhigher permeability ferrite cores, thisproblem is eliminated. I used Fair-Rite# 5961000501 cores (Amidon part #FT-114-61), with an initial permeability (µi) of 125, compared with 8 or 10
for the powdered iron cores used in the
two Handbook designs. Although theferrite cores provide excellent RL performance at 1.8 MHz, because ferrite
is more easily saturated at high flux
levels than is powdered iron material,
I had to increase the size of the coresFig 2—Coupler directivity with a 50 Ω load. Vertical scale is
5 dB/div; mid-scale is –30 dB.

considerably. I determined experimentally that the chosen cores were thesmallest available cores that would
safely handle 1500 W.
The larger cores necessitated theuse of longer windings, so I reduced
the number of turns from the 31-40
used in the Tandem Match article tomaintain good response at 54 MHz. As
a compromise between Fwd port termination dissipation, flux density and
54 MHz response, I settled on 26 turns.
Even with fewer turns than theoriginal design, I had to spend time toachieve an acceptable input RL at
54 MHz. Stray capacitance and inductance from mechanical layout was
the culprit here. By making the lengthof the primary windings as short as
possible, I was able to achieve close to
30 dB RL at 54 MHz.
To measure SWR accurately, directivity must exceed 25 dB. I found that
it is difficult to achieve reliable measurements when testing a 50 Ω load.
This has to do with the vector addition of the Fwd and Ref samples. It is
possible to get wildly optimistic readings of directivity at 50 Ω, because
while the Fwd power can have a 6 dB
maximum range of addition, the reflected power can theoretically be zerounder certain conditions. I found it wasmuch better to use a load with an accurately known error. I chose 75 Ω formy tests, and computed directivity
from the measured SWR. Table 1
shows the range of SWR accuracy
available with various values of coupler directivity, for a 75-Ω load.
With even 20 dB directivity, theSWR error is sizeable. An indicated
SWR of 1.5 could actually be anywhere
between 1.2 and 1.9. Unfortunately,
20 dB is probably typical for many
ham-grade SWR bridges across the HF

Fig 3—Coupler directivity with a 75 Ω load. Vertical scale is
0.2 dB/div; mid-scale is –14 dB.

Fig 4—LP-100 block diagram.

Table 1—SWR Measurement Range versus Coupler Directivity
for a 75 Ω Load

Directivity
10 dB
15 dB
20 dB
25 dB
30 dB
35 dB
40 dB

Minimum SWR
1.00
1.04
1.22
1.33
1.40
1.44
1.47

Actual SWR
1.50
1.50
1.50
1.50
1.50
1.50
1.50

spectrum, and they’re probably muchworse at 50 MHz.
The measured directivity of thiscoupler with 50 and 75 Ω terminations
is shown in Figs 2 and 3. These graphs
were output from my HP-87510A
Vector Network Analyzer (VNA).
Especially noteworthy is the 75 Ω
directivity, which is within 0.2 dB ofMaximum SWR
4.22
2.36
1.90
1.70
1.61
1.56
1.53

the ideal value of 13.98 dB, verifying
a directivity of >30 dB.
To obtain the best high-frequency
directivity, the toroids needed to berotated carefully. I am convinced that
this was a result of stray coupling: both
inductive and capacitive. For this reason, I took care to make the design
symmetrical to balance out the effects

From Jan/Feb 2006 QEX © ARRL

of this parasitic coupling.
The coupler is built in a separate
shielded box, and connected to the
LP-100 through a pair of short coaxial
cables. Employing a separate coupler
eliminates the problems associatedwith having a small box on the operating table connected to heavy, stiff
feedlines. Furthermore, it removes
high power levels from the vicinity ofthe microprocessor and other sensitive

Fig 5—LP-100 schematic diagram.

From Jan/Feb 2006 QEX © ARRL

circuitry, as well as reducing the possibility of RF noise from the microprocessor being radiated into the station
receiver.
Logarithmic Detectors

The genesis for the LP-100 detectors came from the auto-tuner sectionof a mini-beam project. I sought a fast,
efficient tuning algorithm permittingnear instantaneous tuning of the mini-

beam elements. To make this work, it
was especially important to resolve
accurately slight differences in SWR
at very low power levels, because very
little power is available when theantenna is far from resonance.
To compound the problem, a highdirectivity coupler—like the one I’musing—can place high demands on thedetector when measuring a high-quality load. This is so because the signal

at the reflected port is very small. For
100 W input, the reflected power detector must provide accurate level data
for an input signal of 1 mW or less. I
ruled out op-amp corrected diodedetectors, even though they have beenused effectively in circuits such as theTandem Match and Lewallen’s QRP
Wattmeter.5 To handle a wide range ofpower with these circuits requires anarray of precision resistive dividers to

set the ranges. I wanted a meter thatwould continuously display power overa very wide power range with consistent accuracy at any level in the range.
So how might we detect these verysmall signals while still handling thelarger signals generated by 1500 W
without overloading?
Enter the logarithmic detector.
Based on work I had done in anotherproject, I knew that a log-detector

would handle the dynamic range
needed for these projects. I chose theAD-8307 logarithmic detector from
Analog Devices.6 This chip was designed for use in the cellular telephonemarket and is used to convert RF
levels into a dc Received Signal
Strength Indicator (RSSI) output. The
chip can accurately translate an RF
input with about 80 dB dynamic range
into a dc output corresponding to the

From Jan/Feb 2006 QEX © ARRL

log of the input signal. When connected
to a high quality directional coupler, apair of these chips is capable of providing accurate forward and reflected
power detection over a dynamic rangeof almost 50dB at an SWR of 1.1. In
terms of the LP-100, this corresponds
to a range of ~20 mW to 2000 W for
full SWR accuracy, or less than 1 mW
if only forward power accuracy isneeded. The chip is capable of operation from dc to over 500 MHz, and provides a linear RSSI output of 0 to
2.5 V, with a conversion factor of
25 mV/dB and a typical accuracy of±0.25 dB.
For an input power of 5 W, thepower available at the Fwd power port,
expressed in dBm, is roughly+7 dBm (coupling factor is approximately 30 dB). The signal at the reflected power port, for an SWR of 1.1:1,
is ~26 dB lower, or –19 dBm. The
AD-8307’s rated linear input range is–65 to +10 dBm, so this is no problem.
But since we want to measure
power up to about 2000 W (+63 dBm)
we see that 30 dB coupling loss resultsin +33 dBm (2 W) of power at the input of the forward detector, grossly
exceeding the AD-8307’s maximum
permitted input signal level of+20 dBm. To keep the AD-8307’s input level within its recommended linear operating range, therefore, we willneed a pad between the coupler ports
and the log-detectors. My design uses
25-dB total attenuation, split 10 dB atthe coupler outputs and 15 dB at the
LP-100 inputs. The pads also help isolate the coupler and log-detectors fromvariances in the connecting lines.
In round numbers, the maximum
signal that the AD-8307 would normally see for approximately 1500 W
input is:+62 dBm (1585 W) – 30 dB (thecoupler loss) – 25 dB (the pad)= + 7 dBm
With this input power and an SWRof 1.1, the signal at the reflected power
port will be –19 dBm. Now, what happens with only 5 W input? Since 5 W
is ~25 dB below 1500 W, all the values
must be reduced by this amount. The
critical port is the reflected port, so
let’s look at that. The signal available
there would be –44 dBm (19 + 25). Thisis still no problem since the AD-8307
will accurately handle signals as low
as –65 dBm. The AD-8307 still has~20 dB of range left, indicating thatpower as low as ~50 mW (20 dB below
5 W) can provide accurate results at
an SWR of 1.1 or higher. I used an
SWR of 1.1 for this illustration since
it represents a challenge in terms of
testing the limits of the noise floor on

From Jan/Feb 2006 QEX © ARRL

the Ref port detector.
LP-100 Microcontroller

The “brain” of the LP-100 is the
microcontroller. This circuit is basedon the 16F876 PIC chip fromMicroChip.7 It is an 8-bit controller
with 8kB of EEPROM program
memory (14-bit width), 368 bytes of
RAM and 256 bytes of EEPROM working memory. The chip has an internal
A/D converter that can accept up to 5
analog inputs, and provides up to 22
I/O pins, which can act as TTL/CMOS
compatible inputs or outputs. The
LP-100 uses these ports for interfacing the log-detectors, implementing afrequency counter, PLED screen,switches, LEDs and an SWR alarm
relay. See Fig 4.
Operationally, the PIC sequentiallysamples the RSSI outputs of the forward and reflected power log-detectors
using two of its A/D inputs. The inputs
have 10-bit resolution and use a precision 2.56-V reference. The AD-8307
output is set so that the maximum
input power of 2000 W produces a dc
output of 2.56 V, corresponding to an
A/D output of 1023. Each sample of theA/D converter is therefore 2.56/1024,
or 2.5 mV. Since the AD-8307 has an
output slope of 25 mV/dB, the A/Dreadings equal 0.1 dB per step. (Theinter-sample interval is sufficiently
short that the transmitter output
power does not materially change between the forward and reverse
samples even when operating SSB.)
The firmware calculates the “true”
power in dBm and W then calculates
RL and SWR using the forward andreflected power samples. I define “true”
power as the net power delivered by
the source to the load, or forward
power minus reflected power. Thus, themeter always displays the actual
power delivered to the load, regardlessof SWR. The values are displayed on a
2×20 character PLED panel. I originally used an LCD display, but foundthe PLED was better looking and has
faster response. It is plug-in compatible with the LCD. The various displayed screens are shown in Fig 6.
The first range, labeled “A,” is notshown. It is an auto-ranging screenthat displays power at any level. The
next four screens display power inranges of 2, 20, 200 and 2000 W,
along with SWR. The fifth screen
shown in the figure is the overload
indication that will appear if one of
the fixed-power screens is selected
and the power exceeds the limit ofthat screen. The next screen shown
displays power in dBm along with
RL in dB, followed by the Alarm Set

screen, described later.
The final screen is the calibration
screen. The CAL screen shows the raw
Fwd and Ref power sample values simultaneously, making it easy to calibrate and match the two log-detectors.
It also allows for matching the power
readout to a reference meter at 1 MHz
intervals from 1-54 MHz. The current
frequency and A/D counts are displayed, along with the “trim” value,
which is adjusted with the UP and DN
buttons. This value is stored in nonvolatile memory, indexed to frequency.
The power scales use a logarithmic
bar-graph display, which is comprisedof custom characters to allow the display of 36 vertical bars. These bars
respond instantaneously to changes in
power. This was done in part to makeit easy to adjust things like an antenna
tuner or linear amplifier. Although the
response is quick enough to follow SSB
voice peaks, the display is smooth due
to a combination of the logarithmic
scale and over-sampling of the A/D
inputs.
The maximum power scale ischanged by pressing the SCALE button,
which increments the scale once foreach button press. After seven presses,
you return to the original scale. The
current scale selection is stored in the
PIC’s non-volatile memory, so that if
you lose power, the LP-100 will remember your last selection. The first
position is an auto-ranging screen,
which allows continuous display from0.01 W to 2000 W. If one of the fixed
power screens is used, and the power
level exceeds full scale on the bargraph, the over-limit message is displayed, indicating the operator should
increment to a higher scale.
Next to the bar-graph display is anumerical power value. Unlike thebar-graph, the numerical readout ispeak reading, so that it is easy to monitor power while operating SSB or CW.
The peak-hold algorithm freezes thereading unless a higher value is encountered during the hold period, inwhich case the higher value is displayed and the hold timer reset. The
FAST/SLOW button disables the holdfunction to allow the numerical readout to follow changes during adjustment of antenna tuners if desired.
The display resolution of the numerical readout varies with the maximum value of the scale, from 0.01-W
resolution on the 2-W scale to 1-W
resolution on the 200 and 2000-W
scales.
Below the power bar-graph is the
SWR bar-graph. Next to the SWR bargraph is the numerical SWR readout,with a display resolution of 0.01. The

Fig 6—LP-100 Display Screens. (N8LP).

SWR display is the same for all four“W” scales. An interesting idea I borrowed from the on-screen display of
one of my TVs, is the use of markers
to show the limits of the bar-graph.
This makes it easy to see what portion of the full range is being displayed. Additional larger markers areprovided on the SWR scale for SWRs
of 1.5, 2.0 and 3.0.
The dBm scale displays power in
0.1 dB steps from about –15 dBm (thenoise limit of the instrument in a
typical ham shack environment) to+63 dBm (2000 W). For this screen,
the SWR display is replaced with a RL
scale. The RL scale is limited to49.9 dB maximum, even if RL ishigher, to prevent overflow in some of
the internal calculations.
The LP-100 also includes an SWR
Alarm function. The “Alarm Set” button controls this function. Whenpressed, the button changes the display to an Alarm Set screen, as seen
earlier in Fig 6. Repeatedly pressing
the button cycles through the set-point
choices... Off, 1.5, 2.0, 2.5 and 3.0.
When you have reached the choice youwant, stop pressing the button and thescreen will return after about one second to the power screen displayed before entering the Alarm Set mode. The
set-point choice is stored in EEPROM.
If the SWR reading reaches your set
point in any mode, the front panel“Alarm” LED will light, the Sonalert
will beep and the relay will close.
The Alarm is deactivated by correcting the over-limit condition. There isa delay of about five seconds built into
this action, to allow for antenna tuning without constant interruption by
the relay. Setting the Alarm Set mode

Fig 7—LP-100 Virtual Control Panel.

to OFF at any time will also deactivate the alarm, and cancel any current
alarm. I included a jumper in the circuit to permanently disable theSonalert if desired.
The relay contacts are brought out
to the rear panel to allow disabling thePTT connection to a linear amplifier
or transmitter. The relay terminatesin a pair of RCA type PTT connections,which should make for easy interfacing to most amplifiers. This is a great
way to protect those expensive finals
from a catastrophic antenna failure!
I have included a serial port forsoftware control of the LP-100, which
terminates on the rear panel with astandard DB9 connector. I have also
included a programming port to allow
the PIC to be reprogrammed in-circuit.
The programming port connects
through an RJ-11 jack on the PCB toa Basic Micro8 programmer. Anyone
who doesn’t need either of these lasttwo capabilities can simply delete theassociated parts from the project.
Software Virtual Control PanelI have developed Virtual Control
Panel (VCP) software to provide computer control of the LP-100, as well
as offering extended functionality. The
Windows-based VCP mirrors the controls and display of the microcontroller box, but adds calibrated
scales below the bar graphs (see Fig
7). Further information can be foundin the SteppIR VCP Help. A copy ofthe help file is also available on my
Web site. The VCP program is available as a free download.
In addition, I am working on acharting program to automatically create an SWR-versus-frequency graphfor a selected frequency range. The
chart could be printed and saved for
future reference. In order to use thisfeature, the LP-100-Chart softwaremust be used in conjunction with TRXManager9 to control the transmitter.

From Jan/Feb 2006 QEX © ARRL

The LP-100 is also supported
directly in TRX-Manager allowing
remote monitoring of LP-100 basic parameters using TRX-Manager’s Remotetelnet mode. Details of all of the LP-100
software are available on my Web site(www.telepostinc.com). The LP-100
can also be controlled and monitored
over any network connection, including
the Internet. For details of doing this,see my article in the Oct 2005 QST entitled, “Using Networked Equipment for
Remote Station Control.” 10Construction

Most of the parts for the LP-100 are
mounted on a single, double-sided PC
board, except for those associated with
the PLED display and coupler. Fig 8shows the completed board, with the
PLED board mounted in front of it on
the front panel. The PLED board isconnected to the main board with a 16pin header and ribbon cable.
A version of the main board withsilk-screened component placement,
and a solder mask, will be available
on my Web site for those wishing to
replicate this project. A pre-programmed PIC chip will also be available. Since I have already had numerous requests for a kit of parts for thisproject, I plan to offer that as well.
The main PC board mounts on fourstandoffs in a Pac Tec CM6-225 black
ABS plastic case. This case is available in plain ABS or with an RF shieldcoating. The rear-panel PC boardmounted BNCs are secured to the rear
panel. All connectors and switches are
PC board mounted for ease of assembly and reliability. All parts arethrough-hole except the resistors in
the input pads and dividers. This wasnecessary from a performance standpoint, but the parts are easier tohandle than you might imagine.
K8ZOA did the drilling and millingof the front and rear panels for myprototype. The panels are brushed aluminum, and the labeling is an overlay
I produced in Microsoft Word, which
is printed on adhesive vinyl transparency material.
The prototype coupler is mounted
in an aluminum clamshell box with
SO-239 connectors for the XMTR andANT ports, and BNC connectors for the
FWD and REF ports. The main connectors could be replaced with N connectors if desired. Figs 9 and 10 show the
prototype coupler in detail. I used external 10 dB BNC inline coaxial pads
from Mini-Circuits11 in my prototype.
It is important that these two ports
always have a load to prevent highvoltages from developing on the transformer windings when transmitting. I

From Jan/Feb 2006 QEX © ARRLFig 8—Main Board. (N8LP).

Fig 9—LP-100 Coupler photo. (N8LP)

plan to integrate the pads into
the case in the future.
The coupler box dimensions areimportant. Don’t use one that’s muchsmaller than mine, which might resultin stray coupling degrading high-frequency directivity. Also, be sure toinclude the shield partition between
sections to improve isolation of thetransformers. I used grommets in theshield to connect the transformer
secondaries between sections.
The primary of T1, which goes between the XMTR and ANT connectors,
is a short piece of RG-142 Teflon coax.
The shield is grounded on one end, andleft open at the other. This provides
electrostatic shielding, and reduces theeffects of capacitive coupling. (If you
ground the shields at both ends, it will
create a shorted turn and render the
coupler useless.) The primary of T2 isconnected between the FWD and REF
connectors.
Both secondaries are 26 turns of#26 (AWG) enameled copper wire,
spaced evenly over about 2/3 of thecores. The cores are supported byshort lengths of plastic rod, drilled
down the center. I used Teflon, but
any decent low-loss plastic shouldwork. I have seen small sink plungers with plastic handles of the rightdiameter, which could be cut andwould probably work okay.
A support package with complete
assembly details, schematics, parts
list, mechanical drawings, panel templates and graphics files for the front
and rear panels, is available on my
Web site if you want to duplicate thisproject.
The package should be downloadedby anyone interested in replicating the
project. The package contains a wealthof information that will make theproject easier to build.
Performance

Tables 2 and 3 show the measured
performance of the LP-100 prototype
at various frequencies and loads. In
general, power accuracy is dependent upon the reference against
which the LP-100 is calibrated.
(Calibration is necessary because of
the manufacturing variation of thelog amps.) After calibration, themeter will match the reference exactly at the calibration level andshould be within ±0.25 dB at any
power level. Table 2 shows a comparison of the LP-100 to Bird 4410A
and Alpha 4510 power meters. The
source is my Elecraft K2.) These are
exceptionally good meters, but evenat this level, it is apparent that there
are some differences. The LP-100

matches the Bird exactly because it
was calibrated against it. A spot
check of the meters at 100 W, without further calibration, showed a±3% variation between the three instruments.
SWR accuracy was calculated to bebetter than 5% based on device specs.
The SWR readings in Table 3 are compared with those from my HP-87510A
VNA. The table shows the displayed

SWR for nominal load values of 50, 75
and 25 Ω, using precision terminations.
Readings were taken at +20 dBm (100mW) for all load values, using my
HP-8640B signal generator.
SWR results below 1.1:1 would require 0.5 W to reach full accuracy. Thisis due to the reflected power signal
being below the noise floor of the logdetector as discussed in the coupler
description above.

Fig 10—LP-100 Coupler drawing.

Table 2—Power measurements of the LP-100
LP-100 measurements (watts) compared with those of Alpha 4510 and Bird
4410A wattmetersf (MHz)
1.80
3.50
7.00
14.00
21.00
28.00

LP-100
7.10
9.50
9.60
8.40
7.80
6.70

Alpha 4510
7.20
9.45
9.45
8.45
7.70
7.00

Bird 4410
7.10
9.50
9.60
8.40
7.80
6.70

Table 3—SWR Accuracy of the LP-100
LP-100 measurements (SWR) compared with those of an HP-87510A VNA

Frequency
(MHz)
1.80
3.50
7.00
14.00
28.00
56.00

75 Ω
LP-100
1.55
1.55
1.56
1.56
1.57
1.54

75 Ω
VNA
1.55
1.55
1.53
1.48
1.47
1.48

25 Ω
LP-100
1.95
1.97
1.99
1.99
2.03
2.14

25 Ω
VNA
2.02
1.98
1.94
1.90
1.95
2.20

50 Ω
50 Ω
LP-100 0.5 W +
1.06
1.03
1.06
1.03
1.06
1.02
1.07
1.02
1.08
1.05
1.10
—

50 Ω
VNA
1.02
1.02
1.02
1.02
1.04
1.09

From Jan/Feb 2006 QEX © ARRLTable 3 compares the LP-100 with
my HP 87510A VNA. The table shows
the displayed SWR for nominal load
values of 50 Ω, 75 Ω and 25-Ω, using
precision terminations. Readings weretaken at +20 dBm (100 mW) for all
load values, using my HP-8640B signal generator.
Remember, accurately measuring
extremely low SWR levels at very low
power requires an RF environment
with low interference levels. The normal noise floor as indicated on the dBm
scale should be between –20 and–10 dBm with no signal applied. Thisis dependent on a number of factors
such as proximity of other gear, or a PC.
If it is much higher than that, you may
have a strong local broadcast stationthat is affecting the noise floor. Contact
me for a suitable filter design in such acase.
Continuing Development

In an effort to eliminate the parasitic coupling, and provide a repeatable design without the need fortweaking of the transformers, I decided to try a line-section coupler. Instead of the Bird approach of inserting a small loop into a transmissionline, I chose to make the line a microstripline sampler using a PC board.
The loop was a length of Teflon wire(from the center conductor of RG-142
coax), placed directly on top of a
stripline trace. This loop design was
settled on after dozens of different
configurations were tried by both me
and Jack, K8ZOA.
There is one major drawback to linesection couplers, however, and that isthat the response varies with frequency
at the rate of 6 dB/octave. This meantthat I would have to find a method offlattening the response. To level the response, I added a small coil in series
with the feed to the log amps. After testing dozens of coils, I settled on a SMD
part from CoilCraft. This was necessary
to keep the self-resonant frequency ofthe coil above 54 MHz. Small remaining variations can be easily compensated by calibration.
In an effort to minimize the effectsof the connecting lines to the mainchassis, I decided to move the log amps
to the coupler chassis. I also added a
12-bit A/D converter to the coupler
case to increase resolution, as well asa temperature sensor, precision reference chip and RF pickup for the frequency counter.
Fig 11 shows the stripline coupler.
The stripline is on the underside forisolation. The cable is an 8-pin DIN
cable, which carries power, ground, theSPI bus data to/from the A/D converter

From Jan/Feb 2006 QEX © ARRL

Table 5—Parts List

Part ID—Description
Main Chassis
Caps—Keycaps
Case—CM6-225
C1—1 μF
C3, 4, 19—10 μF
C5-7, 9-13 —0.01 μF
C4, 8, 14-18—0.1 μF
D1, 5—1N4148
D2—LED
D3—LED
D4—1N4001
JP1-2—Header 2 Pin
Jumper
J1—16-pin ribbon connector
J3—dc JackJ2, 7—BNC
J4—DB9 D-SUB
J5—RJ-11
J6—Dual RCA Jack
L1-3—1 mH choke
L4, 6, 8—100 µH
PLED1—PLED Display
P1—16-pin ribbon cable assembly
Q1—2N4401
Q2, 3, 4—2N3904
R1—1 MΩ,- 1/8 W
R2, 4, 15—10 k 1/8 W
R3, 17—390 1/2 W
R6, 9—50 kΩ pot
R7—330 Ω
R8—20 kΩ pot
R11, 12—47 k Ω 1/8 W
R13, 14, 16—1kW 1/8 W
R24, 25—52.3 Ω ±1% 1/4 W
R10, 20—121 Ω ±1% 1/4 W
R22, 26, 27—49.9 Ω ±1% 1/4 W
RL1—Omron G5V-2-H1-5
S1—CEM-1212C piezo transducer
SW1-3—Tactile Switches
U1—16F876
U2—LM7805T
U3-4—AD8307
U5—LT1634
U6—MAX232CPE
U7—74HC4053
Y1—20 MHz resonator
Coupler
J8, 9—Chassis BNCs
J10, 11—Chassis SO-239s
FT1, 2—Ferrite Cores
Case—Bud CU-3003A minbox

VendorPart #

Digi-Key
MouserDigi-Key
JamecoJamecoJamecoDigi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
JamecoJamecoCRYSTALFONTZ
Digi-Key
Digi-Key
JamecoDigi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Digi-Key
Mouser
Mouser
MouserDigi-Key
Digi-Key
Digi-Key
Digi-Key
JamecoAnalog Devices
JamecoJamecoDigi-Key

401-1221-ND
616-63075-510-000
P4675-ND
29891CX
15229CL
25523CL
1N4148FS-ND
67-1236-ND
67-1235-ND
1N4001DICT-ND
A1911-ND
A26242-ND
A26271-ND
CP-202B-ND
A24495-ND
A2100-ND
A9073-ND
CP-1433-ND
208240CA
208194CC
CFAP-2002A-Y-ECS
M3AAA-1606J-ND
2N4401-ND
38359CC
P1.0MBACT-ND
P10KBACT-ND
P330W-1BK-ND
3386P-503-ND
P330BACT-ND
3386P-203-ND
P47KBACT-ND
P1.0KBACT-ND
271-52.3
271-121271-49.9
Z108-ND
102-1123-ND
EG1822-ND
PIC16F876-20/SP-ND
51262CX
AD8307AN
LT1634-CCZ-2.5
24811CX
272111CX
X909-ND

JamecoMouserAmidonDigi-Key

159484CL
530-CP-AD206
FT-114A-61
377-1090-ND

and the RF sample for the counter. I
am thinking of changing to a VGA
video cable, which offers shielded coaxial wires.
This design improves on thealready very good SWR accuracy, whileeliminating the problems associatedwith stray coupling in the toroidal design. By moving all RF circuitry to theremote coupler, it should be possible
to allow for switching of multiple couplers to allow for VHF and UHF couplers, for instance. I have played a bitwith such a design, in fact.

6 www.analog.com/UploadedFiles/Data_Sheets/4833689284721AD-8307_b.pdf.
7 http://ww1.microchip.com/downloads/

en/DeviceDoc/30292c.pdf.
8http://www.basicmicro.com.
9www.trx-manager.com.
10Oct 2005 QST, page 40.
11 www.mini-circuits.com.

Larry Phipps, N8LP, has been licensed
since 1965, first as WN8PSD, later as
WA9PSD and finally as N8LP, when heupgraded to Amateur Extra. Larry also
earned an FCC First Class Radiotelephone license in 1967. He is retired from
TelePost Inc, a television post-production
facility that he and his wife Janetfounded and owned for 16 years. Larry
worked for WJBK-TV, Detroit, while attending the University of Michigan College of Engineering. He left school in1971 to work full-time at WJBK as abroadcast engineer. He then went to
NET Television Inc (a subsidiary of
WNET-TV, New York), where he was instrumental in launching one of the firstcomputer controlled videotape-editing
systems in the country. Larry now devotes time to his house, his wife and ham
radio (not necessarily in that order!), allsorely ignored while he and his wife operated the company.
Fig 11—LP-100 stripline coupler. (N8LP)

Because of the need for accurate
calibration of the LP-100 before use,
I am working on a couple of inexpensive calibrator designs to support the
project. For complete details on the
stripline coupler design, and for thelatest news on the project, check out
my website under the LP-100 link.
I am also interested in any feedback
about feature enhancements that
could be implemented in the LP-100.
In closing, I would like to thankmy wife, Janet, for her patience andsupport with all my projects, and toJack, K8ZOA, for his help and expertise in making this project a success,including design help, machining
and testing.
Notes
1http://www.arrl.org/tis/info/pdf/5904024.pdf
2 http://www.arrl.org/members-only/tis/
info/pdf/99hb2236.pdf.
3http://members.tripod.com/michaelgellis/
direct.html.
4http://users.adelphia.net/~n2pk/
RLPmtr/RLPv1c.pdf, also appeared in
QRP Quarterly , Oct 2002.
5 http://www.arrl.org/members-only/tis/
info/pdf/9002019.pdf.

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From Jan/Feb 2006 QEX © ARRL