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The
HANDI-Finder® Experimenter’s
Kit
by Bob Leskovec, K8DTS
6th Edition,
The HANDI-Finder® is a HANdheld DIrection Finder which
can be used to localize both AM and FM sources using only a single connection
to the antenna input of a VHF-FM receiver tuned to the frequency of interest.
The basic HANDI-Finder® when equipped
with open-loop wire antennas and a short handle, stores flat and ready-for-use
in a briefcase. It works well with just
an HT (handy-talky).
The HANDI-Finder® has been designed
for low power consumption, simplicity, and economy. The goal was to provide an easy-to-build kit
for the beginner and a basis for further experimentation by those with more
experience. Overall, it is a quick,
inexpensive way to introduce users of FM communications equipment to the
principle of direction finding and give them something with which they can help
locate sources of malicious interference.
A HANDI-Finder® Experimenter’s Kit
has been put together to conveniently supply the essential parts necessary to
build the control circuit powered by an on-board 9V battery. The user provides the coax, cable connector,
and appropriate antennas. By using two
open loop antennas made out of coat-hanger wire, the unit can be put into
immediate use!
An equally valuable part of the kit is this
instruction manual which describes variations in antennas, general construction
and detailed discussions of the circuit and components, to encourage
experimentation and ideas for improvements.
The HANDI-Finder® first came together
in October of 1986, and continued to evolve.
A write-up later appeared in May, 1993 QST Magazine, entitled “Build the
HANDI-Finder”.
The electronic circuit is based on a design
credited to Tom Feierabend SO/CM 03N18 circa 1979 which appeared in a manual
published in May, 1980 by Van Field, DCP XVIII, entitled “VHF Radio Direction
Finding Manual for Coast Guard Auxiliary Use”.
A similar circuit, referred to as the
“Double-Ducky” direction finder (DDDF) designed by David Geiser, WA2ANU, is
described in July, 1981 QST and reprinted in the 1983 ARRL Antenna Handbook.
The USCG AUX-03N18 version uses an LM555 driving
two successive stages of 7404 TTL to provide complementary buffered
outputs. Since TTL requires a 5-volt power
supply, yet another IC, a 7805 or 78L05 is needed, to regulate the stated 6-30
volt input range.
One problem is that the LM555 does not easily
put out a symmetrical square waveform, which is useful in this
application. The antenna assembly
consists of two vertical ½-wave dipoles (37” long) mounted 8” apart on a
boom. This assembly is rather bulky and
quite a bit of mechanical fabrication is required.
The QST circuit uses only one IC, the LM567
Phase-Locked-Loop Tone Decoder. This is
a complex chip that contains an oscillator and other circuitry, including an
output circuit which does put out a symmetrical square wave. However it does not have simultaneous
complementary outputs. In the DDDF the
single output is connected to the diode switches through a non-polar capacitor,
and some adjustment is required to achieve the proper switching level. Data sheet specifications show that the
LM567C typically draws 12mA. The antenna
assembly consists of two “rubber duckies” mounted 10” apart on a 4.25” X 18.5”
ground-plane. While the ground plane
requires much less work to fabricate than the USCG-AUX circuit, two “rubber
duckies” must be obtained.
The HANDI-Finder® is a good example
of integrated simplicity, wherein one simplification contributes to another.
First, it uses a single CD4047B CMOS IC, which
contains both an oscillator and a divider flip-flop to automatically provide
complementary symmetrical square wave outputs without special adjustments. Only a single resistor and capacitor are
needed to set the frequency. While many
have not heard of this IC, it continues to be available from standard suppliers
like Digi-Key, Mouser, and Jameco.
Second, very little current is used to bias the
switching diodes so the total current draw is only 1.7mA at 9V. Good service can therefore be provided by a
common alkaline “transistor radio battery” and there is no need for wiring to
an external source such as a 12V vehicular supply. This, in turn, eliminates the need for noise
filtering. The operation of the circuit
is not dependent on battery voltage, so a regulator is not required. Supply voltage for the CD4047 can be anywhere
from 3-18 volts. Finally, since all the
parts, including the battery are mounted on a single circuit board, the board
is designed to also serve as the mounting base for the two open-loop antenna
elements easily made out of bent wire.
Thus, there are no ground-plane or vertical dipole elements to make, nor
“rubber duckies” to buy, and no case to drill, until later if you really want
to!
All three circuits described above are based on
the same principle. An electronic switch
alternately connects two antennas to the coax cable downlead going to the
antenna input of an FM radio receiver tuned to the frequency of interest. First one antenna is connected, then the
other, etc., back and forth with equal intervals. This is done at an audio rate, well within
the audio bandpass of the receiver, and usually in the range of 400 to 1500 Hz. A good frequency is 1000Hz.
Of the two antennas, if one is slightly closer
to the source, it receives the wave front slightly earlier in time (phase) than
the other. There is a phase difference
in the signal received by one antenna compared to the other. Since the receiver is being switched between
the two antennas, the switching action imposes phase modulation on the incoming
signal. This is detected in the FM
receiver and is heard at the audio output as a tone equal to the switching
frequency. The amplitude of the audio signal corresponds to the deviation, which depends on the physical
separation of the two antenna elements, up to ½ wavelength. In other words, if the antennas are farther
apart the circuit will impose a higher percentage of modulation or a larger deviation,
producing a louder tone, but the modulating frequency will stay the same.
If the antenna is rotated so that the plane of
the two elements is perpendicular or broadside to the direction of the signal,
both elements receive the signal at the same time (phase) and there is no longer
a difference in phase. Hence, the audio
tone disappears. This is perceived as a
rather sharp null in the audio as the antenna array is rotated into position
perpendicular to the direction of the signal.
This type of direction finder has the disadvantage
that it exhibits 180-degree ambiguity.
However, it has several advantages:
1)
It
works on a nulling principle rather than a peaking principle. The null is sharp and much easier to detect
than the peak from a directional or beam antenna.
2)
When
you null the superimposed audio, you are not nulling the carrier. This is unlike a conventional loop antenna or
cardioid array, which nulls out the carrier.
The problem with carrier-null, is that as you get closer to the null,
the signal you are trying to hear in order to null out, is getting harder to
hear! Also, when you null the
superimposed audio, you can still hear the audio coming from the source.
3)
Since
audio is being nulled, the operator does not have to watch a field-strength
meter. He only needs to listen, which is
something he can do while driving, riding a bike or walking.
4)
Since
this method uses phase information, it works well with strong signals, so no
attenuator is required. (By comparison,
the signal from directional gain antennas must be progressively attenuated to
keep the receiver RF within the range of the S-meter.)
REFERENCE
INFORMATION:
The HANDI-Finder® is an evolving
project which underwent several changes (hopefully for the better) during the
early stages. However, the basic circuit remains essentially the same. This manual might be supplied to help you
work on an earlier unit, as far back as 1986.
If you have a model that does not correspond to the pictorial
description in this document, you should be able to identify the components
with only minimal tracing of the etched circuit pattern.
Your unit may also have component values that
differ from the schematic. If you did
not buy a kit, you may have obtained the circuit board and documentation from
separate sources that copy and distribute such things and try to make them
“public domain”.
The circuit board versions are identified by a
prefix letter “A”, a 6-digit date code (YYMMDD), and an optional suffix. (The suffix, if present, indicates very minor
changes.) As of this writing, the
numbers used are: A861003, A860102, A870122, A890422, A01110, and A031123.
In general, if the board has a date code earlier
than the documentation, the component values in the documentation take
precedence, provided the integrated circuit type is pin-compatible. If the board has a date code later than the
documentation, and/or a different IC pinout, you would do well to send a
self-addressed-stamped-envelope (SASE) to the distributor for an update of the
layout and parts list. Of course, be
sure to furnish the number on your board, so you will be sent the correct
information. Please indicate whether the
RALTEC® or other trademark is present and we will help you figure it
out. Try contacting the author at ral@ralserve.net. (If you have trouble, you can also find the
author’s current snail mail and e-mail addresses through the internet Ham-Call
database.) For information, try the
website handi-finder.com.
If you received this instruction manual as part
of a packaged kit along with the circuit board and loose components, unpack all
the parts and check the quantities and values against the Component Parts List
before you do anything else because some parts have the same value but different
mounting positions and lead lengths.
Before proceeding with actual construction, it is suggested that you
read this manual to see if there are any changes you would rather incorporate
right away. Then proceed with
“ASSEMBLING THE KIT” which is located at then end of this discussion.
Considering that a small transistor radio draws
about 10mA, it can be seen that the battery will last quite a while if the user
remembers to turn it off. Unfortunately
a pilot light would be self-defeating, since it would draw 5-10 times the
current of the circuit itself!
How many times have you forgotten to turn of
your HT? After a while you finally learn
to double check. With the HANDI-Finder®
it should even be a little easier, because you will probably be disconnecting
it from your radio or otherwise storing it at the end of a “DF’ing”
session. That action should help remind
you to check that the switch is off.
Better yet, remove the battery.
That’s why we have included a top grade battery holder in the bare board
version.
For models prior to A890422, here are a couple
of things to help you tell OFF from ON in the absence of a pilot light. First, the “ON is UP” convention has been
followed. This is fairly commonplace on
most equipment. Second, you can make a
“passive” indicator by using a dot of bright paint, for example, typewriter
correction fluid such as “whiteout”.
Push the slider to the ON position, then paint a small dot on the lower
part of the slider that is now exposed.
Let the paint dry thoroughly so it doesn’t rub off. When you slide the switch to OFF, the painted
part should be hidden; when ON, it should be visible. On Model A890422 and later the switch has
3-positions: OFF is in the center, UP is for DF’ing, and DOWN is for straight
receiving or Standby. (Notice the design
date is embedded in this code as YYMMDD.)
There are a couple of subminiature slide
switches available from “experimenter” sources such as Mouser and Digi-Key, but
they differ slightly in the spacing and style of the connector pins. Circuit boards A861003 and A860102 used the
CW Industries switch available from Digi-Key as SW103-ND.
Starting with circuit board A870122, the switch
pads were made larger and the spacing changed to use the Mouser 10SM007 or
10SP001. The larger pads allow larger
holes to be drilled to allow for the wider flat pins. If the switch you have does not go right in,
do not force it. Study the problem and
carefully enlarge the holes only where necessary. A small modeling file is handy for this
purpose. Don’t make the holes any larger
than necessary, or you will have trouble bridging the gap and getting a good
flow joint when soldering. When you do
solder it into position, use ample solder and heat it enough so the solder
surrounding each terminal flows evenly into a nice even form similar to an
“Indian tepee.” A890422 and later use a
DP3T switch with 6 pins, which will not fit the previous models, but that gave
the ability to add the “standby” position which is very useful.
The 9-volt battery holder is fastened at one end
by soldering the two terminal tabs that pass through the board. The other end of the frame can best be
fastened by using 1/8” diameter “pop-rivets”, but you may alternatively try
hot-melt glue, epoxy, small screws, etc.
However, make sure that whatever you use won’t protrude and prevent the
battery from seating in the clip.
The bottom area of the circuit board contains an
area where the handle is attached. The
unit can be mounted on either a short handle or directly to a mast, whatever
you desire. To get going quickly, take
any convenient piece of wood or metal, lay the end against the board, mark the
holes, drill them through the handle, and attach with screws. If you use a round handle, you will either
have to make a flat cutout along one side, or cut a slot in the end so that the
board can slide in. A round handle is
best. The most utilitarian handle is
described next. .
Find an inexpensive paint roller, but one with a
handle which is threaded for an extension pole.
(Home supply stores sell them typically for $1.39). Study how the handle is attached and
determine the best way to remove it or otherwise adapt it so the HANDI-Finder®
can be mounted on it.
Next, shop around for your choice of a wood or
metal paint roller extension pole, preferably the type which is made of 3
sections which screw together. You will
thus have a very flexible system. You
can use the unit with just the handle alone, or screw on up to three lengths of
additional “mast”. You may even wish to
obtain a second extension to have more lengths available. Be aware that some extension handles have
different threads in the sections than they have at the roller end. In any event, the pieces are easily stored
when disassembled.
There is a homemaker’s utility duster on the
market called a WEBSTER® which can be found for as little as $4 in
discount stores. Unfortunately, only a
small percentage of stores seem to stock it.
However, this is an amazing value for our purposes because it contains
not only a 6-inch detachable handle, but includes a removable, telescoping,
extension pole!
The dusting head consists of an 8-inch diameter
half-spherical array of bristles embedded in the end of the short handle. This end is easily sawed off to allow for the
HANDI-Finder®. The extension
pole is about 36” in the collapsed position and 60” when extended.
Mount the handle against the component side and put the screws through with the heads on the solder side, to keep a low profile for the coax cable which will come down the solder side
The HANDI-Finder® works best when the
first ¼ wavelength of coaxial cable downlead to the receiver is kept vertical
or parallel to the center line of the circuit board. (Measure the ¼-wave from the bottom antenna
terminal.) If it waves around, it can
throw off the bearing. Therefore, when
using the unit with a short handle, make sure to hold it straight and high so
that the cable hangs straight down. When
using the unit with a long handle it is preferable to attach the coax to the
mast for the first ¼ wavelength. Use
tape, nylon cable ties, etc.
RESISTORS
vs. INDUCTORS:
The HANDI-Finder® was designed mainly
out of the need to inexpensively provide Radio Amateurs with something they
could use to help locate interference on the 2-meter repeaters. But the unit operates over a much wider range
of frequencies because the chokes “traditionally” used in such RF circuits have
been replaced by resistors.
Inductors are usually used to feed direct
current into some point in a circuit where it is desired to allow an
alternating current signal to pass without attenuation. At radio frequencies these are called “RF
chokes”. Circuits which operate in the
150MHz range, for example, usually use small coils with a value of 1.0 to 1.2
microHenries. Using the expression Xc =
2(3.14)fL, where “f” is the frequency in MHz and L is the inductance in
microHenries, the corresponding reactance works out to about 1000 Ohms. At 1/3 the frequency, or 50 Mhz, this same
inductor would have a reactance of only 333 Ohms. At UHF this inductance would mathematically
exhibit a proportionately higher reactance, but other difficulties arise. Certain assumptions about the construction of
the coil are no longer valid and the math becomes more complicated. Factors like the “capacitance between turns”
and the length of the connecting leads can no longer be ignored. At some frequencies the coil looks like a
high impedance, but at others it may look like a short circuit!
Thus, the frequency band over which the circuit
can operate is limited by the chokes.
Different sets have to be installed to operate over different ranges of
frequencies. Normally it is necessary to
use chokes because they have a comparatively low resistance at d.c. so there
won’t be any significant voltage drop.
The key word here is “significant”.
In this circuit, the switching diodes are biased
by current. As long as enough current is
supplied to do the job, it doesn’t matter if a little voltage drop occurs
across the connecting element. The
chokes can simply be replaced by 1000 Ohm resistors. The currents are so small that the voltage
drop is negligible. Further, for all the
complications involved with determining how chokes will act at different
frequencies, it is no less risky, and a great deal easier, to assume that a
resistor will exhibit the same resistance over a much wider frequency range.
In this circuit R4, R5, and R6 would have
“traditionally” been specified as 1.0uH chokes for operation in the 150 MHz
region. As you can see, by using 1K
resistors, operation at 150 MHz should not be affected, and operation over a
wider range of frequencies should now be possible. The value of the resistors is not that
critical. Values in the range of 1K-1.3K
will do, but make them all the same value.
Using carbon-film resistors, we also get some useful inductance, but
since the resistance is high and distributed with resistance, these are
inherently free of self resonance, and broadband becaue they are low Q.
However, if you wish to experiment with
inductors, you can either make them or buy them. According to the USCG AUX article, you can
make these with one layer of #28 close wound on a 1 Meg ½-watt carbon
composition resistor. Actually there is
nothing magical about the 1 Meg value, it is the physical size and shape that
is more important. The intention is that
the resistance be at least a factor of 100 times greater than the inductive
reactance. Therefore any value above
100k is acceptable.
Mouser Electronics has a line of good quality
subminiature RF chokes that are quite reasonably priced, roughly $.23
each. Consult their catalog for the
“43LQ” series. The 1uH value is part
number 43LQ106. Keep in mind they
probably will not fit on the HANDI-Finder circuit board supplied with the kit.
The HANDI-Finder® should work over a
wider range when resistors are used in place of inductors, but there will still
be problems with resonances (series LC) and anti-resonances (parallel LC) for
any given set of antennas. However,
ignoring those effects, let us discuss the effect of antenna spacing. Remember, you want the vertical members of
the two antennas as far apart as practical to get maximum modulation.
This will make it easier to tune for a
null.
For a given set of antennas, the upper frequency
limit should be that which corresponds to a ½ wavelength equal to the widest
spacing of the vertical members.
If the usual wavelength formula is multiplied by
12 inches/ft, a new “constant” factor is obtained: dividing 5616 by the
frequency in MHz will give the ½ wavelength in inches. For the open-loop antennas you will be
instructed to make elsewhere in this manual, the spacing between opposing
vertical sections is typically 17.5 inches.
Turning our formula around, and dividing 5616 by 17.5 gives 321 MHz,
which is also the frequency at which maximum deviation would be available. Some have reported operation to include the
70cm Ham Band.
Moving to 1/10 that frequency or 32 MHz, only
1/10 the modulation would be available, but the units should still
function. So it would appear that
operation over a 10:1 range might be possible.
At the lowest limit, it is important that the
coupling capacitors have a reactance of 50 Ohms or less so as not to attenuate
the signal by more than 3dB. If we
choose that limit to be 27MHz the value of capacitance having 50-Ohm reactance
is 117pF. Values of .001uF, or 1000pf
will be one tenth that or 5 Ohms, and work just fine.
Antenna connections to the board are made using
screw terminals. On early units, these
are formed by installing a clip-type “tinnerman nut” which is slipped onto the
circuit board edge at each designated point, and then threading in a 6-32 x
3/8” or ½” binder head machine screw.
Only four are needed for any given configuration. Later units use regular hex nuts, serrated
lock washers, and flat washers against the board to keep the lock washers from
tearing up the copper foil.
When fastening small diameter or stranded wires
to these, it is suggested that you first install crimp-on spade lugs or ring
lugs on the end of the wire. If you must
wrap the wire around the screw, do it under a washer and wrap it in the
direction that the screw tightens (clockwise).
BE CAREFUL NOT TO OVERTIGHTEN. If you experiment a lot, the tinnerman nuts
will strip and wear out. If you need to
have things very tight, you can use 6-32 screws with regular hex nuts. However, do not put serrated lock washers
against the surface of the board since they really dig in and cause
problems. It is better to use a slightly
longer screw with flat washers to protect the surfaces. If you are tempted to just solder directly to
the board, don’t do it! The heavy heat may cause the foil to delaminate. Further, if the antennas are bumped, the foil
may be pulled off the board. If you
want to have some sort of “rounded off” nut on the thread end, inquire about
“acorn nuts” in the screw specialty section of most hardware stores.
The HANDI-Finder® is quite versatile
in the way it can be used with different kinds of antennas. First of all, you can get it running
immediately without extensive fabrication because two simple wire antennas can
be attached directly at the circuit board.
For 150 MHz, take two EQUAL lengths of stiff wire about 19 to 20 inches
long and bend each one into a neat square “U” shape. The bottom of the “U” should be about 6”. Form the ends into a hook and fasten them to
the screw terminals on the circuit board.
Looking at the component side of the board, you will see there are three
terminals along the left side and three along the right side. Fasten one end of one antenna to the very top
terminal on the right side. Fasten the
other end of that same antenna to very bottom terminal on the same side. Then repeat this procedure for the left side.
Note that the bottom terminals are merely
mounting points. They are electrically
isolated. On some board designs there
are circuit pads to allow installation of a grounding jumper. DO NOT INSTALL THIS JUMPER!
It is desired that the antennas be open loops. If you ground the bottom of the loop, you
will create a closed loop that will cause a carrier null in the direction of
the signal. This is not desirable. On later circuit boards provisions for
grounding was eliminated to avoid confusion.
The wire you use should be thick enough to
provide desired rigidity, but thin enough to allow fastening under the screw
terminals. If it is too thick, you might
be better to first solder on some spade lugs or similar terminations. Of course, it helps if the wire is a good
conductor, but steel coat-hanger wire or welding rod will work satisfactorily. A better choice would be brass rod or brazing
rod, between 1/16” to 3/32” thick. If
you wish, you can make the loops even larger.
Note the ACTIVE ELEMENT of each antenna in this
application is the vertical part of the open loop supported in space by the
horizontal part. A greater separation of
the vertical elements will produce a larger deviation and more audio. However, the longer a vertical element is,
the more signal it will receive, provided there isn’t some gross impedance
mismatch. This shows up as more carrier,
better quieting, or a stronger S-meter reading.
Of course, when that vertical section is maintained in space by an
unshielded horizontal section, determining the resonance or tuning gets very
complicated. Also consider that the
horizontal part does receive some signal, and this degrades the intended signal. Therefor, larger loops may work worse. Feel free to experiment; that is the whole
object of this project!
Alternatively, the circuit board is designed to
accommodate connections by coax cable to other kinds of antenna arrays. Just below the top antenna mounting screw on
each side, is a ground screw. This is
not used with the open loop antennas, but is used for the coax shield. Thus, if you do wish to make a “Double-Ducky”
direction finder as described in the ARRL Antenna Handbook, you can connect the
two equal-length coax cables to the HANDI-Finder® circuit
board. Similarly, you can fabricate the
dual half-wave vertical dipole array described in the Coast Guard Auxiliary
Manual and run it with the HANDI-Finder® board.
One suggestion for a more extravagant system is
to position two multi-element Yagi antennas with vertical orientation at
opposite ends of a horizontal boom.
Support the boom at its center on a vertical mast so that it can be
rotated. Use equal lengths of coax from
each beam and connect them to the HANDI-Finder® board which should
be mounted in a protective enclosure at the center of the boom. The beams will give greater forward gain and
reduce the 180-degree ambiguity. It is just a little hard to use with the
mobile!
Since the antennas have no path to ground, there
is no need for DC blocking capacitors.
On A890422, the positions of C6 and C7 are jumpered out with board
foils. If an application is encountered
which requires DC blocking, slit each foil at each end carefully with a sharp
hobby knife, then heat the center section with a soldering iron and peel it off
the board with the knife. Holes are
provided to install the capacitors.
Refer to parts list. Later boards
don’t have this connection, and come with capacitors. Check the board. If you don’t have the capacitors, then be
sure to insert some jumpers.
As you can now appreciate from these
discussions, there are many ways you may end up using the circuit board and
enclose it accordingly.
In its simplest “quick & dirty” constructed
form, the unit can be put to immediate use.
The long narrow profile was purposely selected to minimize wind
resistance for the benefit of a vehicle operator who may be trying to hold on
to the unit mounted on a narrow mast protruding through the drivers
window.
You will discover that an enclosure will only
increase the drag and may not be worth the effort! With only minimal care, these units have
rattled around in many trunks and back seats without anything getting shorted
out. The battery in one of the demo
lasted over three years, even though it had been left “ON” for several weekends
during that time!
However, you might want to consider that
numerous sharp edges from component leads could snag cloth upholstery, or
scratch leather and vinyl.
Where you really need to, the circuit board can
be mounted to surfaces on standoffs using 6-32 screws through existing holes.
Model A890422 and later are designed so that it
can be more easily modified to fit into a case.
This is somewhat irreversible since it requires that the bottom part of
the board be cut off, unless you can find a really long, flat plastic
case. Some have come and gone off the
market, so it is there is no particular model case that can be recommended. It is strongly suggested that the circuit be
completely built up on the existing board and thoroughly checked out. After you are familiar with the unit’s
operation, you may NOT wish to put it in a case.
It is best to start off with a “test” situation
where you know the location of the source, and experiment with “getting a feel”
for the null. The null itself is fairly
sharp, but it does not always manifest itself as a total null in the audio
tone. Sometimes, you will observe instead,
a jump in tone one octave up or down.
(Refer to the ARRL Antenna Handbook article for more discussion.)
At other times, you may hear a “buzz” or a fast
“twiddle”. This is usually due to
multipath, so moving just a few feet may help clear up the null. Also, if there is a strong transmitter in the
area, such as a 1kw paging system, 3 or 4 MHz away, you may experience more
“de-sense” than normal when the HANDI-Finder® is switched on in the
DF-ing mode. The sharp edges of the
diode switching waveform cause it to be rich in harmonics, all of which
modulate that strong RF source and cause it to broaden its normal spectrum at
the input of your receiver. (These are
the same problems that bother the more complex “DOPSCANs” that switch 4 or 8
antennas.)
Using the unit with a synthesized scanner or one
of the new extended-coverage HT’s will not only allow you to work in the Ham
bands, but other frequencies as well.
The prototypes were tried over the range of 49 MHz to 450 MHz and worked
well even though the 450 MHz test was well above the suspected ½-wave limit
frequency of 321 MHz calculated previously.
As you get considerably above or below the 150 MHz design frequency you
may find that your unit has a “null” point which is no longer perpendicular to
the plane of the circuit board. However
this also sometimes occurs due to multi-path in high reflection areas,
especially indoors. (Don’t try to do
anything meaningful indoors!) In any
event, don’t jump to conclusions. Take
several measurements at different locations a few feet apart, and consider your
findings carefully before deciding whether the problem is due to the location
or a characteristic of the way you have set up your particular unit.
When driving through neighborhoods it is
interesting to scan the bands and see what sort of frequencies are in use. For example, one of our experimenters found
several homes with 49 MHz “baby monitor” intercoms. You may also hear cordless phones near that
frequency.
Another application is to drive around and look for
Cable-TV leakage on 145.250 MHz. You
will quite easily pinpoint hotspots on poles and at the lawn boxes used for
underground installations.
Perhaps you can promote some Ham Radio good will
by helping local law enforcement officials find mobiles with “stuck
mikes.” Such a “free” service can help
educate personnel about Amateur Radio and keep you from getting “pulled over”
when you are out hunting.
If you are an avid radio-controlled model
airplane enthusiast, and have had a plane get away from you and get lost in the
woods, this could save you time. Put a
low-power transmitter and “crash switch” in your plane. Then if it gets away or crashes in the woods,
you can track it down!
Some areas have rather extensive park reserves
or other situations where naturalists “tag” wild animals with radio
transmitters to track their migration.
They are quite interested in reports on these
animals, and will give out the frequencies to individuals with a genuine
interest in helping. Invite your local
naturalist to speak to your club and demonstrate how you can help. But keep in
mind they DO NOT put radio collars on deer and turkey just to make it easy for
game hunters to track and kill them!
A round handle is best, because it allows a calibration
mark at any angle. You would do well to
try your unit outdoors in an open field by walking in a circle around a central
source. A person keying an HT held up at
arm’s reach is easiest, but caution him to hold still. The null should always occur perpendicular to
the plane of the antennas, but your radio or other factors may be introducing
phase shift. If the error is totally
consistent as you walk around the circle, then you will want to mark the true direction
on the handle.
If such is the case, expect this calibration to
change at different RF frequencies, or if you change the CD4047 oscillator
frequency. If the error is not consistent, and changes as you walk around the
source, you are experiencing (multi-path) reflections from the surroundings. Try reducing the power of the source. For example, if a 100 mW HT is too strong,
remove the antenna and substitute a 51 Ohm, 2 watt carbon resistor. If you still get erratic readings you will
have to go to another location.
Once you have established the calibration mark,
fasten the cable along the side of the handle or mast so it runs over the
mark. That way you can feel it in the
dark. Now you are ready to do some serious
DF-ing or Fox Hunting.
Notes:
1.
The
abbreviation “uF” is used throughout this text to denote “micro-Farads”.
2.
Bob
Leskovec, K8DTS, has been licensed since 1957 and promises not to change his
call letters so you can always locate his mailing address via the Ham database!
3.
A
less detailed description of this project appeared as “Build the HANDI-Finder!”
QST Magazine, May, 1993.
4.
HANDI-Finder®
and RALTEC® are Registered Trademarks.
The HANDI-Finder®
Experimenter’s Kit
ASSEMBLING THE KIT: (Please read the whole manual before
assembling)
1) Check the circuit board and make sure all the
holes are drilled and it is otherwise finished and ready to accept the
parts.
2) Having read the discussions about the ON/OFF
switch, locate that item and make sure that the type you have does indeed fit
the hole pattern on the circuit board.
Do not install it yet.
3) Locate the integrated circuit socket, carefully
straighten the pins, and insert it onto the component side of the board with
the proper orientation. You may have to
study the IC socket to determine how its design denotes the position of pin
1. Some sockets have a beveled corner at
pin 1. Others have an indentation at the
end between pins 1 & 14, some are not clear!
Note, for packing purposes, the IC may have been
installed in the socket. It is NOT
necessary to remove it, unless you can see it has been inserted in the socket
incorrectly. The IC will incur much less handling and there will be less chance
of damaging it if you just leave it in place when you solder the socket onto
the board. Make sure the socket is
properly oriented and seated flat against the board when you do the final
soldering.
4) In the following order, mount the diodes,
resistors, and capacitors, starting with the smallest parts first. If you have only one capacitor with long
leads, SAVE that, and use the precut ones first. There is one location that needs the
capacitor to be formed with longer leads.
5) Examine the ON/OFF switch and make sure that the
terminals are clean and not tarnished, before soldering. If needed, carefully
scrape them with a small hobby blade.
Then mount the switch and solder it in position. The 6-pin switch also requires that the two
diagonal frame tabs be soldered as well.
NOTE: if you later use a solvent to remove flux from the board, be
especially careful not to get any into the switch. Some literally melt. Some have four frame mounting pins. Carefully
clip off only the two diagonal pins that prevent insertion.
6) Refer back to the discussion about the battery
holder and mount that item.
7)
Prepare the end of a length of RG-58/U or the miniature RG-174/U and attach it
to the board. Note that the hole for the
shield should be 1/8” so that the shield can fit through. The excess braid is cut off so that only
about 3/16” lies flat against the circuit foil where it is soldered down. Install a nylon cable tie through the holes
provided in the circuit board. Pull it
tight so that it anchors the coax securely against the board to prevent flexing
at the cable end. This is known as a
“strain relief.” Clip off the
excess. Depending on what type of handle
you use, you will need to figure out how to route the coax so that it crosses
to the center and comes down the handle equidistant from the two antennas for
best symmetry. Later circuit board
layouts have the coax routed down the centerline in the solder side of the
board on the side opposite where the handles should be attached.
8) Slide on “tinnerman” nuts (if such nuts have
been supplied) at the four points used to attach the open loop antennas. These are the two top-most and two
bottom-most locations.
9) Fashion two open-loop antennas according to the
instructions described previously and attach them to the board using the 6-32 x
½” screw hardware. That’s it! Refer to the section “TRYING IT OUT”.
The manual includes discussions of several other
options and variations in the way the unit can be wired.
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