NOTES ON CONSTRUCTION AND USE OF THE
DQ RECEIVER AND BEACON
I suggest scanning the CREG articles on my website http://radiolocation.tripod.com
starting construction in order to understand how the units work and what
you can do with them. The schematics and parts lists in the CREG articles
are out of date, but the rest of the info is good. Refer to the schematics,
layouts, parts lists, and notes included here for the final word. Also
look at the picture gallery on the website to see the various loops etc.
THE "README" FILE on the WEBSITE and THE NOTES HERE BEFORE BUYING PARTS
OR STARTING CONSTRUCTION!
NOTE: 7/30/03. These notes
have been updated for the latest boards with the dates 2002 or 2003 printed
on them. All earlier boards were shipped with printed notes which
mention corrections that are no longer needed.
DQ RECEIVER NOTES
Detector Board notes
The actual board is 3.5" square. 3 of the 4 corners must be grounded to
the minibox thru metal standoffs
The simple version of the receiver is cheaper and easier to build (and
use) if all you need is accurate Ground Zero location without depth measurement.
One can still use the "field angle" method for rough depth measurement,
or an ACVM on "Test Point 2" to do a "Ratiometric" depth measurement when
the signal is strong and interference is low. The receiver can be upgrated
to the "complete" version by installing the rest of the parts on the board
and doing a tuneup. Note that there will be a ~1.3 dB variation in
the ACVM reading as beacon signal slowly drifts from one channel to the
other. This variation is not audible and is inherent in the receiver
The receiver is designed to be mounted in a metal box for proper grounding
and shielding. The boards are designed to fit in the specified 3x4x5 inch
"Minibox". The cover and all of the standoffs must be grounded to the box.
Remove paint if necessary. See the pictures on the Website showing
"12v" in the lower left of the PC board is a misprint. It should just say
The complete receiver uses two 9 Volt Alkaline battery. It can also
be used with a 12 Volt sealed lead-acid battery if U20 is deleted or bypassed.
The simplified receiver uses one 9 Volt battery or a 12 Volt lead-acid
battery without modification.
Warning!!! The metal crystal can must be raised above the board
to avoid shorting out the traces that run under it! Note that a different
crystal frequency should be used in countries that use 50 Hz power so that
the operating frequency fits between two 50 Hz harmonics. See "The DQ Receiver
- An Overview" on the website (near the end of the article).
Trimmer C19 has its metal adjusting screw connected to one lead (which
must be the ground lead).
The RF amp board derives Vcc (+12V) from the output of the voltage regulator
in the simplified receiver without the regulator in the complete receiver,
or directly from the on/off switch in the simplified receiver without the
The DVM derives its power from the regulated +12V on this board. Use a
3.0 or 3.3V zener in series to provide ~9V for the DVM.
To save money in the complete receiver, the built-in DVM can be replaced
by an external digital multimeter with a 200mV DC range. Connect it after
the divider/filter R51, R52, C40, which must be retained. Because a separate
battery is being used for meter power, U11A and its 4 100k resistors can
be deleted. In this case, connect R51 directly to the S6 "arm" and the
negative meter lead to the U3B pin 7 lead along with the "bottom" of C40
It is a good idea to use small coax to connect the volume control R34 to
the audio amp U9. The center wire connects the tap of the volume control
to R35 on the board while the shield connects the "bottom" of R34 to C28
on the board. Note that the shield is at V/2 and not grounded.
Stereo headphone jacks are specified in the parts list because this is
what most people will have. Any impedance will work, but 8 ohm phones may
cause problems. For the simplified receiver I do not recommend
headphones of less than 32 ohms, as they draw a lot of battery current
and will cause distortion and instability with weak batteries. One
solution is to use just the left or right element, which doubles the impedence.
Changing C28 to 100 uF or more helps somewhat. Wire the "tip" and
"ring" leads in parallel to use them as mono headphones. Lo-Fi mono headphones
(intended for voice use) of the type that block all outside noise are really
better in this application. Hi-fi phones have a somewhat annoying
"hiss" that is not audible in the lo-fi units. High impedance
phones have less risk of feedback than Low-Z units. My best phones are
mono crystal "stethoscope" style. I also have some Telex 600 ohm mono headsets
with full ear coverage, probably intended for "language lab" use. With
mono phones, connect only the "tip" lead to avoid a short circuit.
Radio Shack sells mono phones, and I bought some nice Ata model 1261 "home
hi-fi" stereo phones at Walmart for $10.
The receiver must be nulled before first use. While using headphones,
set both RF amp controls to Lo gain to get rid of RF amp Noise. Pre-setting
both null controls to the center of their ranges beforehand will help.
Alternately adjust the two null controls (R5 and R9) for a perfect audio
null, turning up the volume control as the null improves, until only noise
is heard. If you can't get a null then you should check the receiver to
see that the correct resistor values were installed; the IC's are not backwards;
there are no shorts or unsoldered connections on the board; the front panel
controls are correctly wired; and the correct voltages are present. Each
IC uses +V DC, and each op-amp output (and most circuits) operate at +V/2.
The front panel null control will need adjustment from time to time (using
low RF gain with no loop as before). The internal null adjust should only
need adjustment after large changes in temperature (also for changes in
battery voltage with the simplified receiver).
In the complete receiver R41 can now be adjusted to align the DVM to exactly
zero at the perfect audio null. See notes on the receiver mainboard
After nulling, switch to Hi RF gain and 32 Hz mode, then turn up the RF
gain. Noise from the RF amp should be audible.
If a 1 turn pot is used for R9, it can be shunted with ~2k ohms to make
nulling less "touchy". It may be necessary to add a small resistance
in series with R10 or R11 to "center" the null on the pot.
The DVM should read "000" when the receiver is adjusted for a perfect audio
null. If it does not (but is close!), you will need to apply a small DC
offset to the DVM using R42, the 1-turn screwdriver trimmer on the detector
board. If the DVM reading is negative insert a jumper wire from the "0-adj"
pad to the adjacent "V+" pad on the detector board, then adjust R42 until
the DVM reads "000". If the reading was positive, jumper "0-adj" to "gnd".
All 3 pads are located between R11 and C39. Once the correct connection
is found, the jumper can be soldered.
The completed receiver must be tuned very close to the frequency of
your beacon in order to phaselock properly and give a steady DVM readout
of signal strength. The beacon should first be adjusted to freq by
either adjusting the crystal oscillator to 3.579545 MHz using the test
point, or installing a fixed capacitor of 20-30 pF. While receiving the
Beacon signal from ~30m away, slowly tune C19 while watching the green
phaselock LED, which will be blinking slowly on and off. Tune until the
blinking becomes slower and finally stops with the LED ON. Now switch the
DVM to "PLL Adjust" and continue trimming until the DVM reading is as close
to zero as possible, hopefully under 100. This will seldom need readjustment,
but can be done in the field during a Radiolocation if necessary.
RF Amp Board Notes
The simplified receiver must also be matched to the beacon frequency.
With an analog meter, monitor the DC voltage at pins 1 or 7 of U2 while
receiving the beacon signal. Slowly adjust C19 until the slow meter
swinging almost stops.
Note that R20 (2M) has become R20A (1M) plus R20B (1M) in series. This
is shown in the parts list and color layout, but not on the schematic or
on the actual circuit board.
For the simplified receiver, delete R20A, R20B, R21, and R55. Solder a
jumper between X100 and S3 to connect the loop input directly to U0.
Actual board size is 3.5" x 2.0"
All 4 corners of the board must be grounded to the minibox thru metal standoffs.
Solder 1/2" high shields along the long red pads bisecting U0 to help isolate
Use small coax such as RG-178 for the loop input and Sig Out lines.
Receive Loop Notes
Wrap the 3 wires of R22 with an electrostatic shield which can be grounded
to the output end of the RF amp board.
Ultimate receiver sensitivity is determined strictly by the design of the
loop antenna. Sensitivity is simply the ratio of the signal received on
the loop to the thermal noise of the wire. Resonating the loop does not
change the loop's sensitivity, but does increase the signal level and the
impedance, which raises the thermal noise level above the input noise of
The 18" dia receive loop described in the CREG article "Constructing the
3496 Hz D-Q Beacon Receiver" on my website is a very handy size for field
work. The 22" dia loop (red text) gives about 6dB increase in actual sensitivity.
I wound a second 22" dia loop and got 512 turns with 2 lbs of #28 wire.
I measures 432mH with roughly 4600pF for resonance. Whatever frame is used
for the loop should hold the loop rigidly in a plane. One possible frame
is a used (ie worn-out) plastic wheelchair wheel with the tire removed.
My receive loops are "frameless", wrapped in electrical tape, and sandwiched
between plywood sheets in a rigid box. I have also constructed loops using
a disks of rigid foam with a groove in their rims.
I now recommend using fixed 600 Volt mica capacitors for the receive loops.
The high voltage rating is to prevent damage (shorted caps) when the loop
is accidently placed close to an operating beacon (and it will happen!).
The loops do not detune by themselves. The fixed caps help keep the receiver
gain stable for depth measurements.
Adding a grounded electrostatic shield (with a gap of course) to the loop
winding is worthwhile if you are doing precision Radiolocations where wells
will be drilled, depth measurements, or are doing conductivity measurements
where signal strength at the bottom of deep nulls must be measured. The
shield eliminates small variations in signal strength when the loop or
receiver are touched. The shield lowers the Q of the loop slightly. All
of my loops have shields, but they worked pretty well without them.
You MUST mount a sensitive cylindrical bubble level (not a round one)
on the top edge of the receive loop, set to be centered when the loop is
PRECISELY vertical. This is an absolute MUST for accurate Ground Zero location!!!!!!!!!!!
These are available as inexpensive plastic "line levels" at Home Depot
or other hardware store
A useful addition to the loop is a 6 ft (2m) low stretch line attached
to the center of the loop on one side, with a "T" bar of thin wood or fiberglass
on the loose end to stand on. This allows one to make rapid Ratiometric
depth measurements by raising the horizontal loop over ones head a precisly
known distance above the ground. The reasonable depth limit for accurate
measurement with this short line is about 120 ft (40M), but I have made
reasonably accurate measurements at twice this depth. For the best results
at larger depths, I use a rigid pole to raise the loop as much as 25 ft
in the air. See the electrostatic shield notes above.
If only one beacon will be used, C2 can be replaced by a 33pF fixed cap.
Otherwise, insert C2 without soldering then connect a freq counter to C1
(using a x10 scope probe for isolation if available) and adjust C2 for
3.579545 MHz. Measure C2 then replace with fixed ceramic or mica caps.
Actual board size is 3.5" x 2.0".
The metal crystal case must be raised above the board to avoid shorting
out the traces underneath.
Caps C4-C8 are all in parallel to resonate the loop. A capacitance decade
box is very helpful in loop tuning. With the loop away from all metal objects,
momentarily key the beacon with an analog current meter in series with
the battery, and tune for minimum current. Initial current may be very
high and will cause overheating. Use a real 12V battery (Gel, AGM, or NiCad)
to power the beacon. This is critical! I use small AGM (sealed lead-acid)
batteries of 2-3 Amp Hours, although even 1 AH batteries will work. A "Lab"
supply, even one rated at several amps, may cause strange effects such
a drifting DC current. Once tuned, current should be roughly 0.5 amps for
any of my loop designs. Note that maximum output does not occur exactly
at minimum current, but maximum efficiency does. Most of the installed
capacitance should be low loss 250V polyproplyene, although the smaller
"C8" trim caps can be Mylar.
Caps C4-C8 can be mounted on the loop. This will allow use of a 2-wire
feedline, and will increase Q slightly.
To prevent overheating in case of accidental loop detuning in the field,
a small heatsink formed from a strip of sheet aluminum ~3" long can be
fastened under Q1 and bent in a "U" over Q1. Note that this heatsink is
"hot" with +12V and output signal.
Note that the LED will only light if all 3 connections to the loop are
intact and a real signal is being transmitted. It is real "BITE"!
Locating Ground Zero
See the CREG article "3496 MHz Beacon and Loop" on my website for general
Beacon info. Note that the end of the article has general loop design info
and gives the design of a 4 ft diameter loop in addition to the standard
2 ft dia loop. The laws of physics dictate that the Magnetic Moment (beacon
field strength) increases linearily with loop diameter if battery power
drain and the weight of the wire in the loop remain constant. I now use
a collapsable frame with the 4 ft loop to make it easier to level (with
a line level) and obtain a repeatable output. The beacon loops must be
precisely leveled in order for the axis of the magnetic field (ie Ground
Zero) to be precisely overhead.
I have a small rectangular loop that I use for caves with tight passages.
It consists of 26 turns of #10 plastic insulated solid "house" wire with
the tap at 4 turns from the +12V end. The center of the winding is 13"
by 16.5". The winding is sandwiched between pieces of 1/4 inch plywood,
with cutouts for a handle and a round bubble level for precise leveling.
This loop resonates with just slightly over 4 uF and draws about 0.4 Amps
Radiolocation requires practice. Even though this gear often gives a
useable signal close to 1 km away, always try to position yourself as close
as possible to the expected location by using topographic overlays, GPS,
compass course and distance from an entrance, etc. Try to always be higher
than the cave passage, ie wait uphill rather than downhill. If you are
within a horizontal distance of about 1.4 times the expected depth of the
beacon, the locating will be very easy.
The best time of day for radiolocations is during the morning, from about
8AM to 1-2PM. The worst time is during the night when "skip" brings the
noise of far distant thunderstorms. This assumes no local storm activity.
For this same reason, winter is much quieter than summer. Even so, it is
possible to do moderate depths on a summer evening. I find it hard to even
get cavers moving before the "crack-of-noon"!
Coordinate times with the underground party so you know the earliest time
they will "turn on". In smaller caves or with experienced cavers, you can
have them operate on a schedule. At first, plan to have the beacon turned
on for 30 minutes or more. With careful planning, I have used a little
as 4 minutes with divers in springs. You go to the first location, turn
on the receiver a little ahead of time, and null the receiver (after disconnecting
the loop, set the switch to low gain and the dial below 5.0), then reconnect
the loop using quite high RF gain, 1 Hz bandwidth, and the loop on the
ground. Carry a small screwdriver in case you have to adjust the internal
null adjustment or the frequency (if it won't phaselock). You can turn
on the alarm and remove the headphones. The alarm works quite well, although
false phaselocks can occur with strong interference from Thunderstorms
(atmospheric noise), power lines, or electric fence controllers. Simply
reduce the RF gain until there is only an occasional false alarm. Don't
wait directly under a noisy power line or close to a metal fence line!
The receiver should be kept out of direct sunlight, as the heating will
cause the null to drift, which may set off the alarm and may make it impossible
to null the receiver until it cools. With the simplified receiver, after
nulling, just keep listening in high gain mode.
When the beacon is heard, hold the loop vertical and slowly rotate it for
a null, adjusting volume as desired. RF gain should remain high enough
to give deep nulls. Ground Zero lies along the null line. Gamblers can
simply walk in one direction along the null line while continually swinging
the loop thru the null to update direction. If the signal gets weaker,
then simply go the other way. In an open field, one can walk perpendicular
to the original null line a short distance and then null again. If you
walked far enough, the two null lines will not be parallel and will intersect
near Ground Zero. This is the 2-LOP method. If all else fails, you may
be very far from the Beacon. In this case make a quick signal strength
measurement with the loop on the ground then walk 50-100m along the null
line and measure again to see of the signal really is getting stronger
or weaker. Note that this method will fail if you are within roughly twice
the estimated depth of the beacon where the field lines are not vertical.
It is easy to "overshoot" Ground Zero. As one gets close, within 1.4 times
the estimated depth of the beacon, the best way to check progess is to
occasionally rotate the vertical loop perpendicular to the null line that
you are walking along, then tilt the top of the loop towards yourself.
At first this null may occur with the loop nearly horizontal, but as Ground
Zero is approached this null will occur with the loop closer and closer
to vertical. Very close to ground zero the loop will null while rotated
in any direction, but the signal will become incredibly strong when the
loop is tilted even slightly from vertical. At this point, for highest
accuracy, it is a good idea to re-null the receiver.
"Ratiometric" and "absolute"(signal strength) depth measurements are discussed
in "Operation" in the CREG article "Constructing
the 3496 Hz DQ Beacon Receiver" on my website, and are discussed in
more detail in "Depth by Radiolocation: an
Extreme Case". Using 2 independent depth measurement methods helps
to give confidence (or lack of it) in the results. The only missing information
is the receiver amplitude calibration for depth by absolute signal strength.
The procedure given here is based on a calibration table (which follows)
generated for the first receiver I built using the new circuit boards.
It allows one to adjust the receiver RF gain for a reasonable DVM reading,
without overload, with the receive loop horizontal at ground zero, and
calculate depth by absolute signal strength later.
Now the vertical loop is placed on the ground and tilted back and forth
to find the null. At any nearby location except exactly at Ground Zero,
the magnetic field will tilt towards you as it exits the ground. The cylindrical
bubble level shows loop tilt. If the loop nulls when it is tilted slightly
towards you, then move the bottom of the loop slightly away from you and
re-null, repeating until the loop is precisely vertical when nulled. The
plane of the loop is now precisely on a line of position that passes thur
Ground Zero. With practice, and quiet conditions, it is possible to detect
a 6 inch (10cm) change in the position of a beacon at 300 ft (90m) depth!
Mark this line at the loop, then rotate the vertical loop 90 degrees and
repeat the process to obtain a second line, which should cross directly
over the first line. Mark the intersection of the lines as ground zero.
To cancel out most of the errors in the receive loop and its bubble level,
repeat each measurement in the last step with the vertical loop rotated
180 degrees from its original position. This will usually give slightly
different positions. The result will be a small square box of marks with
ground zero at the center.
After locating Ground Zero, place the loop horizontally at ground zero
and level it with a round bubble level.
In the 1 Hz mode, adjust the RF gain switch and pot for a high resolution
reading on the DVM, say 900-1500. Now adjust the gain pot precisely
to the nearest "1/2", ie 7.0, 7.5, 8.0, 8.5. For highest accuracy, it is
best to avoid the extreme ends of the pot near 0.0 or 10.0 if possible.
These are the points where gain is calibrated in the table that follows.
Make certain that the RF overload LED on the receiver is not lit!
If is is lit, back off on the RF gain.
Calculating Ratiometric Depth
Record 3 pieces of info: The position of the gain switch (High or Low);
the position of the gain pot (1-10 in increments of 1/2); and the DVM reading
(>900 if possible). For Ratiometric depth,
without changing the receiver settings, raise the horizontal loop an exactly
known distance (say 6 feet) and record the new (lower) DVM reading and
the distance the loop was raised.
The two DVM readings and the 6 foot spacing are used to calculate the "free
space" Ratiometric depth using the equation below. V1 is the DVM reading
with the loop on the ground and V2 is the reading with the loop raised
the distance H above the ground. The cube root requires a scientific calculator
such as the one in Windows. The calculated Ratiometric depth will normally
be equal to, or less than the actual depth, with the error growing as the
depth gets greater.
Next look up the gain pot setting in the following table and record the
corresponding "inverse ratio". These numbers are accurate for my receiver
and probably OK for yours, but are not guaranteed. Contact me if you are
interested in custom calibrating you own receiver. I used a precision 3-decade
50 ohm step attenuator.
Pot Setting Inverse Ratio
Pot Setting Inverse Ratio
Use the DVM reading, switch setting, and inverse ratio to calculate Vz,
the normalized receiver input voltage, which will be used to calculate
"free space" absolute depth. Vz=(DVM reading)(100 for Lo, 1 for Hi Gain
setting)(Inverse Ratio). Example: if DVM=900; Lo Gain mode; Pot Setting=7.5;
Calibration for Depth by Signal Strength (Absolute Depth)
The beacon/receiver system must be calibrated, with both units on the surface,
before or after the trip. The best location to do it is a level area with
low conductivity ground, such as the granite in my backyard since you are
trying to do a "free space" calibration.
Set the Beacon loop exactly vertically with its axis aimed exactly at the
receive loop exactly 100ft (or exactly 30m for metric) awayat distance
"Dcal". Likewise, the receive loop is also set up exactly vertically with
its axis aimed at the Beacon. This is called "coaxial" alignment.
Turn on the receiver, disconnect the loop, set to 1 Hz BW, Lo RF Gain,
alarm off, meter Fast and connected to the receiver output. Now null the
receiver for exactly "000" on the DVM (the audio doesn't matter). Reconnect
the loop, then turn on the beacon with a fresh battery and let it warm
Calculating Absolute Depth
Adjust the RF Gain pot to a setting listed in the table above that gives
a DVM reading of at least 3 digits, preferably above 500. Record the RF
gain setting, Hi-Lo Gain setting, and DVM reading for use in the 100 foot
(or 30m) absolute depth calibration. Calculate Vcal exactly as Vz was (or
will be) calculated using the actual Ground Zero readings.
Use the following equation to calculate Absolute Depth, which will normally
be equal to or greater than the actual depth, with the error growing as
the depth gets greater.
A SIMPLE WAY TO START DOING DEPTH MEASUREMENTS
At shallow depths (say under 30m), the two calculated depths should be
similar. At greater depths, the Absolute depth should be somewhat greater
than the Ratiometric Depth. If it is, then one can add the two numbers
and divide by 2 to get an average which should be quite close to the true
depth if you have uniform rock from the surface to well below the cave
passage. See the CREG article "Determining Depth by Radiolocation, an Extreme
case". My deepest reasonable result was 550 feet at Jewel Cave, South Dakota,
which meets the uniform rock requirement.
For a start, you can do the Ratiometric measurement
without any calibration at all. With the receive loop horizontal
on ground zero, just set the receiver dial to give a reading of 1000 or
more (if you can), then raise the horizontal loop a known distance for
the second reading, without changing any receiver settings. You then
use the equation to calculate ratiometric depth. For shallow depths
(say <100 ft [30m]), this is all that you need to do.
For the Absolute measurement,
simply record the precise receiver gain settings and DVM reading that you
obtained with the loop resting on ground zero.