The new 3-part Printed
Circuit Boards (marked
"2008") include the identical 3-stage receiver RF amplifier board that
has been included since 2003. The main (detector) board has only
two minor changes. The pads for the varactor diode (V1) have been
enlarged to accomodate a surface mount diode. The original
1N5470A diode (with leads) is no longer easily available. See the
"README" file for possible sources for the
leaded diode. The second
change is the addition of a voltage regulator to provide 5 Volt or 9
Volt power
to the LCD digital panel meter because only 5 & 9 Volt meters are
readily
available. I have run these meters on 12VDC, but why not do it
right? See
COMPLETE INFORMATION
FOR THE UPDATED RECEIVER, BEACON, AND LOOPS 7/30/03 prior to
constructing this receiver, and especially the
README
file for updates.
Parts layout of the three PC boards
(including the beacon)
Receiver Detector Board

Receiver RF Amp Board
The
New Class-E Beacon Design
The big change is the beacon
transmitter circuit
board, which is a completely different design. The old
parallel-tuned beacon design, which was technically a Class-E amplifier
with one inductor and one capacitor, has several problems:
- DC current (and heat dissipation) rises rapidly whenever the loop
antenna is not precisely tuned to resonance. This can happen
during initial tuning; failure of a tuning capacitor; or simply by
setting the loop antenna close to a large metal object.
- The 1/16 duty cycle causes large transients that must be absorbed
by the zener diode, causing power loss.
- The transients reach the battery, despite the large electrolytic
capacitor. The circuit will function properly only on
low-impedance batteries such as NiCad, NiMH, or sealed lead acid. This
circuit will not function properly on "Lab" type
power supplies, even supplies rated at several Amps.
The
new Class-E
design uses square wave drive, has no large transients, is
simpler, and will
work on any power source. Technically it is a Class-E amplifier
with two inductors and two capacitors. The impedances of the
series-tuned loop
antennas rise when they are de-tuned, which causes DC current to
drop. The loss in the MOSFET is so low that no heat sink is
necessary. The circuit efficiency is above 90%.
Different series-tuned loop impedances, power levels, and DC supply
voltages can be accommodated by changing
the number of turns on the secondary (link) winding on a
toroid. For more on this circuit
design see my article in Speleonics 25, June 2005, pages 10-12;
http://caves.org/section/commelect/spelonic.html
.
Assembly
of the
2008 RF amplifier and Detector
boards is identical to the older 2003 and 2007 boards (and will
not be repeated here) except to mention that the
new detector board has space for an on-board 5VDC or 9VDC regulator to
power
the digital
panel meter, which may not be rated to operate on 12 VDC. See the
Parts
Layout for the two additional
parts, U12 and C42, which are located at the extreme lower left corner
of the detector board. Due to software problems, the detector
schematic
has not been updated to show these parts. The panel meter power
leads are wired to
the new + and - 9V output terminals. The
receiver parts list has been updated
with
correct part numbers (mostly DigiKey and Mouser) and prices.
Warning about the digital panel meters: There
are two types (1) those that must use isolated power, i. e. they cannot
monitor their own DC power source such as the mpja.com PM438 9VDC unit;
(2) those that have a common ground for both the input and 5VDC DC
power source, so that they can monitor their own power source such as
the mpja.com PM128G-G. All of my receivers until now have used
the type (1) meters which should work directly with the new
boards. Changes must be made in order to use the 5 V type (2)
meters with common ground, which I have installed in some receivers
just completed. I first installed a10 V regulator in place of the
12 V unit. I could not find a 10 V regulator, so I used a 9 V
unit with two resistors: a 1k 1% between the 5V output and the negative
regulator lead; and an 82 Ohm 5% between the negative regulator lead
and ground. I then wired the type (2) meter's power leads as
follows: Positive lead to +10 V and negative lead to the low impedance
V/2 rail at pin 7 of U3B.
Warning!
You must disable the backlight if your DVM has one, as it draws
nearly 50 mA compared to 2-3 mA for the meter itself, which will
overload the V/2 rail! On my PM128G-G meters, the backlight was
disabled by removing the 24 Ohm resistor R2.
A
word about batteries: I chose to power my new receivers
with five CR123A lithium cells, which are now available for about $1.00
each online along with sockets. I changed to 10 V regulation (as
mentioned in the panel meter note above) to provide more "headroom" as
well as directly power the 5 V meters.
To assemble the
2008 Class-E Beacon board, first install all of the parts that
are
not involved in tuning. Refer to the
parts list. Mount R1, R2, R3,
C5, C6, C7, C8, C9,
C10, C12, D1, D2, D4, F1, U1's socket, U2, and X1. Note that U2
is installed with its semicircular side facing to the right in the
parts layout diagram below. D3 must be
installed with the correct polarity (opposite to the polarity of
D2). Note that Q1 is specified as an insulated tab MOSFET. Also
note that it is a "logic level" gate type that turns on at only 2-3
volts. Ordinary MOSFETS may not fully turn on at 5VDC gate
voltage and may heat up! Q1 is mounted
with a 4-40 screw or the metric equivalent.
A MOSFET with a bare metal tab must use
insulating hardware to isolate it from the PC board!, but heat
sink
compound is useful.
There are 3 choices for C11,
which adjusts the
frequency of the crystal oscillator. The frequency is only
critical if this beacon will be used with other peoples DQ receivers
that have the digital field strength readout, or if you own more than
one
beacon. In either case, the frequency should be set precisely to
3.579545 MHz to match the other beacons using Test Point 1 (
TP1) and a good frequency
counter. I use a x10 scope probe to avoid loading down TP1.
The easiest tuning method is to install a 6-50pF trimmer
in either C11 position. I strongly recommend measuring the
trimmer's value, then installing fixed COG (NPO) ceramic capacitors in
its place. The trimmer's value will shift with time and
handling. If this beacon will only be used with your receiver,
and especially if the receiver does not have the digital readout, C11
can be a 27pF ceramic cap.
Class-E Beacon Parts Layout
Class-E Beacon PC Board
Wind the Primary of toroid T1
(provided) with 90
turns of #20 wire
(provided). This is the longer length of smaller wire. Start by feeding
half of the length of wire through the center of the toroid core. Now
wind one half of the wire onto the core, counting turns. Wind a compact
even layer. The easiest technique is to feed a loop through the
center hole, then insert a finger into the loop and pull the length of
wire through the hole. This is much faster than feeding the wire
end into the hole because the wire tends to tangle. The second half
will overlap into a second layer, which can be spread out evenly. It is
a good idea to leave one lead long (for now) just in case.
This will give close to 1242 uH, which is the correct
value when C6 is 1uF. Note that these values of L and C
deliberately do not
resonate close to 3496 Hz even though they form a "tank" circuit. C6 is
shorted out for half of each carrier cycle. These values are
adequate for power
levels from a few Watts up to 10 Watts or so. See the article in
Speleonics 25 mentioned above. This winding connects to the two
large "pri" holes on the board.
The secondary (link) winding,
which feeds the loop
antenna, is wound over the top of the primary winding with the shorter
length of larger #18 wire, spreading the winding more or less evenly
around the toroid. The number of turns required depends on the
particular loop antenna chosen; the power level desired; and the
battery voltage chosen. The number of turns could vary
(typically) from 15 to 25. The smaller number of turns is
associated with loops with lower series-resonant resistance, and/or
lower power levels. It is easy to tap this winding to allow two
or more power levels. The trick is to initially leave a long lead
(as much as 1 ft [0.3m]) on one end to allow the easy addition of turns
during
tune up by unsoldering a single wire. This winding connects to
the two "sec" holes on the board
Later, once the beacon is tuned and working
properly, T1 can be secured to the board with silicon rubber and one or
two tie wraps.
Tune
Up
The first step in tuneup is to apply
DC power to the
board with the key switch closed (ON), no loop antenna connected, and
an ammeter in series with the battery. The circuit should draw a
low current. My breadboard drew 25 mA (.025 Amps). A high
current indicates that either C6 or the primary of T1 are the wrong
value. A current close to zero indicates that the power MOSFET is
not getting its gate drive, which should be a 5 Volt 3496 Hz square
wave on the "G" pin. Check the operation of the 3.57 MHz
oscillator.
Capacitors C1-C4 are all wired in
parallel to
series-tune the
loop antenna. Often, only one or two capacitors are necessary.
Polypropylene capacitors should be used for C1-C4 because they are much
lower loss than the smaller (and cheaper) polyester (Mylar) capacitors.
If additional small capacitors are needed for
precise tuning, they can be be mounted between C1-C4, or under the
board. These small capacitors could be polyester because they are
such a small part of the total value. A 250 VDC rating is
adequate for most wire
loops, and for all of the examples given here. Higher Q
loops or higher power levels may require a higher voltage
rating, up to 600VDC. A capacitor decade box is very useful
here. The tuning procedure is to adjust the C1-C4 value to maximize
battery current drain and/or AC loop voltage, both of which will occur
at the same value of capacitance, or very close to it. Record the
final value of DC current. There is
no danger of overheating the circuit unless the power level is very
high (15-20 Watts). The next section gives several examples of actual
loop antennas that were tuned up using a breadboard version of this
beacon circuit.
NOTE: Prior to each use
of a beacon, it is wise to connect it to the battery and measure the DC
current drain to make certain that the tuning is still OK. If the
DC current is more than 10-20% lower than the value recorded when the
beacon was initially tuned, the tuning should be checked.
Examples
of Antennas for the New Beacon Design
This section describes several actual beacon
antennas followed by a
table giving tuning capacitor values and the number of turns on the
link winding for different power levels and operating voltages.
All values are only approximate for loops that you wind. In
particular, the actual PC board should give higher output power and DC
current than the breadboard, which had some thin wiring and several
clip-lead connections. All of the loops are wound with wire
available at Home Depot, Lowes, or electrical wholesalers. It is
insulated, solid or stranded (as indicated), THHN rated, on 500 ft
spools. Unfortunately the cost of copper has doubled
recently. Currently, a 500 ft roll of #14 solid THHN is $30.00 US
while stranded is $35.00. For those outside of the US, #14 bare
copper wire is
about 1.65 mm dia. and #12 is about 2.05 mm dia.. Feedlines for
all of the antennas can be 2-conductor appliance power cord. I
use a few feet of rubber-covered cord with wires of at least #16 (1.3
mm dia.). Because I plan to mount the beacon electronics in the
center of the smallest loop, I fed the loop with a piece of loudspeaker
wire, which is acceptable in such a short length.
The first antenna is a small wire
loop.
The winding is 50 turns of #14 stranded THHN wire wound on a rigid form
12" (30.5 cm) in diameter and 1.25" (3.2 cm) wide. Inductance is
1.419 mH for my loop. The loop form is 14" (35.5 cm) dia.. The
"filler" ring is some scrap stiff closed-cell foam. A
bubble level is installed in the form. Remember that this is just
an example. The dimensions are only critical if you wish to make an
exact duplicate. There is plenty of room to wind more turns or to use
larger
wire. This loop is suitable for locations up to
perhaps 250 ft (75 m) deep.

12 Inch Loop
The next antenna is 37 turns of
#12 solid THHN wire
which was initally wound on a plywood wheel which had a circle of nails
22" (56 cm) in dia.. The resulting winding was tightly
tie-wrapped every few inches before removing it from the wheel.
The inductance is 1.802 mH. The loop is rigid enough to be used
by itself without a frame, making it the easiest to construct, cheap,
and lightweight for its very strong signal. It can be covered
with duct tape or other material to make it even more rugged. It
can be made even more rigid by wrapping narrow fiberglass sailboat
batten material around its perimeter. The only drawback is
the need to carry a line level on a string to level it. Line levels are
$1.29 at Home Depot. This antenna will easily do locations
300-400 ft (90-120 m) deep.

22 Inch Loop
The largest wire antenna is the
same 4 ft 4" (1.32
m) dia. loop
shown here
for use with the old beacon circuit,
except that in this design the tap (third wire) is not used. It
consists of
18 turns of #14 stranded THHN wire taped every few inches into a
circular bundle, then inserted into split plastic wire loom which is
used to cover wiring harnesses in vehicles. Mine is 3/4" (2 cm)
size, but it is not critical. The loom is then tie-wrapped every
few inches. The inductance is 1.30 mH. It can be
"figure-eighted" twice to form a bundle small enough for a cave
pack. The frame, made from 1/2" (1.3 cm) CPVC pipe and 45 degree
elbows, threaded over a loop of shock cord like a tent pole, is
required to stretch the loop out for tuning purposes, and to keep it in
one plane. The loop is attached with Velcro strips. I no
longer use the "stick" legs for leveling as they
are too much trouble. It is easier to use rocks or sand. This loop also
requires a line level.
I have done locations over 600 ft (180 m) deep with this loop!
Ferrite Rod Loop Antenna

Self-Leveling Ferrite Rod Antenna for Diver Tracking, with Floatation
NOTE: This photo shows a
no-no! The SS bands detuned the antenna by acting as shorted
turns.
I removed the floats as soon as I figured out what I did!
The final antenna has a ferrite
core. It is used as a self-leveling
loop for caver and diver tracking. This is
NOT recommended for your first beacon antenna! Build and use a
wire loop first! The ferrite core is a model VRF-12C made by
Stormwise.com. The
"rod" is actually assembled from a stack of ferrite toroid cores potted
in a PVC pipe,
with end caps to seal it. It weighs 2 lbs 3 oz (1 kg) without
wire. This assembly is much more rugged (and cheaper) than a solid bare
ferrite rod. The permeability is 850. The assembly is 14" (36 cm)
long. The winding area is 10 7/8" (28 cm) long by 1.32" (3.4 cm)
diameter. I wound 186 turns of #14 stranded THHN wire in
two smooth, tight, layers. The completed loop weighs 3 lbs 7 oz
(1.6 kg). The inductance is 4.375 mH for mine. This antenna is
self-leveling if held by it's feedline, which exits from the top at the
exact center. This makes it possible to track the underground (or
underwater) operator from the surface as he progresses through the
cave! It is possible to do this at depths of 200 ft (60 m) or
more with a skilled operator at the receiver, if the
rod is driven with its maximum power of 6 Watts (0.5 Amps from 12 VDC).
Note:
There is a problem with using series-tuned
rods for transmitting. The permeability of a ferrite
rod used for transmitting will
gradually increase as the RMS current through the loop winding
increases. This happens far below the level that will saturate the
ferrite. This will cause enough increase in inductance to de-tune
the loop. The result is that the final loop tuning must be done
while transmitting at the power level that the beacon will actually
use. The final capacitance value will always be less that the
value calculated using the small-signal inductance value such as that
obtained with an LCR meter. Another effect of high power is that
losses increase, i. e. the Q drops from the small-signal value.
What limits the maximum power level is the ability of the circuit to
"re-start" after it has been shut off. If the loop has been
re-tuned too far at high power, it will present such a high impedance
at low power (i.e. when the circuit is again turned on) that not enough
loop current will flow to "snap" the lindutance back to its high-power
value! In effect, the circuit becomes bi-stable. It can be
tuned for a nice high output power (high loop current), but when shut
off then turned on again, it operates at a much lower power level and
stays that way. I found the maximum power level by trial and
error once I understood the problem.
I have also found that this
ferrite inductance instability can occur at very low power levels such
as an 0.3 Watt unit tested recently. I believe that this may occur
because the very high Q at the low power level makes tuning very
critical. The trick to making the unit work was to vary the tuning
slightly while the DC voltage was varied from 6-13 volts until I found
a tune setting where the power level gradually dropped off with voltage
as one would expect, instead of an abrupt drop to a very low level.
Using
parallel resonance with ferrite rods: If a ferrite rod loop
antenna is wired for parallel resonance, as shown below, its impedance
will
drop off-resonance.
If the rod has been tuned while transmitting, for minimum DC
current/maximum AC loop voltage, then when the battery is connected
again there will be an initial AC current surge that will "kick start"
the rod by changing its inductance enough to hit resonance. This
avoids the "bistable" problem of series resonant operation. The
drawback, of course, is that anything that de-tunes the antenna will
cause higher DC current and possible circuit damage, just as with the
old beacon circuit! In other words, don't set a parallel-resonant
antenna on a steel table while it is running!
Loop Antenna
(see above)
|
Tuning
cap, uF
|
Vdc
Volts
|
Idc *
Amps
|
Pin *
Watts
|
Link
Turns
|
Loop V *
Volts RMS
|
Loop I *
Amps RMS
|
Magnetic Mom *
A-T-m sq
|
Weight
Lbs/kg
|
12" loop 50 turns
|
1.456 uF
|
12.7 V
|
0.92 A
|
11.7 W
|
17
|
135 V
|
4.33 A
|
18.5 A-T-m sq
|
4/1.8
|
12" loop 50 turns |
1.456
|
12.7
|
0.72
|
9.1
|
15
|
120
|
3.85
|
16.5
|
|
12" loop 50 turns |
1.456
|
~6V AA
|
0.32
|
1.9
|
15
|
54
|
1.73
|
7.4
|
|
22" loop 37 turns
|
1.141
|
12.7
|
0.85
|
10.8
|
19
|
142
|
3.59
|
35.6
|
5.8/2.6
|
22" loop 37 turns |
1.138
|
12.7
|
0.55
|
7.0
|
15
|
120
|
3.03
|
30.0
|
|
22" loop 37 turns |
1.138
|
~6V AA
|
0.24
|
1.4
|
15
|
52
|
1.31
|
12.9
|
|
4' 4" loop 18 turns
|
1.608
|
12.7
|
0.92
|
11.7
|
19
|
109
|
3.81
|
94
|
5.8/2.6
|
14" ferrite rod 187 turns |
0.4278
|
~6V AA
|
0.09
|
0.54
|
15
|
57
|
0.59
|
2.64
|
3.5/1.6
|
14" ferrite rod 187 turns |
.4190
|
12.7
|
0.20
|
2.5
|
15
|
117
|
1.22
|
5.42
|
|
14" ferrite rod 187 turns |
.4190
|
12.7
|
0.26
|
3.3
|
17
|
133
|
1.38
|
6.16
|
|
14" ferrite rod 187 turns |
.4190
|
12.7
|
0.33
|
4.2
|
19
|
152
|
1.58
|
7.04
|
|
* NOTE: These values were measured with
a breadboarded circuit and will likely be higher with the actual PC
board.
Antenna
Tuning Table and comparison of a few actual beacon loops with the new
Class-E circuit
For those who care, a method
to calculate the number of link turns needed for any antenna is given
here. Note that if you adjusted your T1 winding to other than 90
turns to obtain the exact inductance, use that # turns instead of
90. The following calculations are for the 5th antenna from the
top of the table above.
If you measure the inductance and the resonant Q of the
parallel-tuned
loop (ie shorting out the link), you can calculate the series-tuned
resistance as just the inductive reactance/Q. For my 22" 37-turn
loop
I get L=0.51 + j40 Ohms, Q=78 by direct measurement. The link is
loaded with ~0.51 Ohms (plus feedline resistance and a bit of cap
loss). Mark Mallory's "Z" equation in
Speleonics
25 gives
the load
impedance that the circuit should see directly on the MOSFET drain for
a given supply voltage and power level.
Z=[1.2638(Vsquared)]/P
Z=load impedance at MOSFET drain, Ohms: V=battery voltage, Volts:
P=battery power drain (Volts x Amps)
If we want 12.7VDC and 0.55A, as in one of my examples, then the
desired load Z=29.2 Ohms (across the entire 90 turn winding).
Impedance changes as the square of the turns ratio. To convert
29.2 to
0.51 ohms should take a link of sqrt(0.51/29.2) x 90=12 turns.
The
reality is that 0.51 Ohms is really a higher value, thus the 15 turns
specified
in my table