Monster Media 1994 #1/monster.zip/monster/HAM/TRAP01.ZIP/TRAP01.TXT

Text File | 1994-01-02 | 10KB | 201 lines

traps01.wp
by Frank Kamp
K5DKZ
Dec 1993



                    Multi Band Antenna Traps


     These devices are called Traps, but they are actually more
like frequency sensitive switches.  They are parallel resonant,
high Q, tuned circuits which provide a very high impedance at their
frequency of resonance.

     An example of their use can best explain their operation.
Take, for example an 80/40 meter trap dipole.  The center section
of this antenna is a normal 40 meter dipole of conventional length.
One end of each trap is connected to the ends of this dipole.  The
other end of the trap is connected to additional lengths of wire
(typically 21 feet) to allow the complete antenna to resonate on 80
meters.

     The resonant frequency of the traps should be the frequency
you wish to use in the 40 meter band.  The additional 21 foot
lengths of wire should be adjusted so that the antenna resonates at
your choice of frequency for the 80 meter band.  Note that each leg
of the dipole is only a little over 50 feet long making it about 20
percent shorter than a full sized 80 meter dipole.

     On 40 meters, the traps have a very high impedance,
effectively disconnecting the two 21 foot lengths of wire needed
for 80.  On 80 meters, the traps act as loading coils permitting
the antenna to be shorter than a conventional 80 meter dipole.

     The 80/40 meter trap dipole example will work on all bands
(80, 40, 20, 15, and 10), but will work most efficiently on 80 and
40.  If this sounds to good to be true (shorter antenna with
multiple band performance), there are some compromises to be
considered.  On 80 meters even a full sized dipole cannot provide
an SWR of less than 2:1 across the entire band.  On 40 meters, we
have the same problem to a lesser extent.  A trap dipole exhibits
an even narrower bandwidth than a full sized antenna.  Still, if
your use of these bands can be served with a 200khz to 250khz
bandwidth, a trap dipole can be a good solution.

     Another compromise of a trap dipole is the requirement for the
traps.  Conventional traps are constructed of high voltage
transmitting type capacitors and heavy B&W miniductor stock.  They
are not particularly difficult to make, but the parts are expensive
and they are subject to drift in frequency when exposed to adverse
weather conditions.  However, there is a method of building traps
that is very inexpensive, can withstand full legal power limits,
and is relatively stable under even the most adverse weather
conditions.

     The 1988 ARRL Handbook alludes to this method of trap
construction but does not give any specific data.  I have built a
set of traps using this method and would like to share the
information with anyone interested in homebrewing a trap dipole.

     My traps were built to be used in an 80/40 meter dipole as in
the example given.  The same method of construction can be used for
other frequencies with the turns reduced to cover the higher
frequencies.

     The method of construction alluded to in the handbook uses a
coil of coax to form the tuned circuit of the trap.  The shield of
the coax forms the coil.  The center conductor of the coax on one
end of this coil is connected to the shield at the opposite end.
This allows the capacitance between the center conductor and the
shield to act as the capacitance that resonates the assembly.

     Aside from the low cost, this method reduces resistive losses
in the coils to an absolute minimum.  The shield portion of the
RG62AU coax I used is electrically equivalent to using quarter inch
copper tubing.  The capacitor formed by this assembly is capable of
withstanding several thousand volts of RF allowing the use of high
power on 40 meters.  Although I used RG62AU, RG58 or RG59 would
serve as well.  RG8 may also be usable, but it's stiffness might
require a larger diameter coil form and may result in a heavier
assembly.

     My traps were wound on two 6.5 inch long pieces of schedule 40
PVC pipe 1.25 inches O.D.  I found that 20.5 turns would resonate
at 7.285 mhz, my chosen 40 meter frequency.

     Each end of the PVC pipe was prepared by drilling two opposing
holes in it about 0.25 inches in from each end.  Solid number 12
copper wire was inserted through these holes and bent around the
PVC to form a loop with the wire inside the pipe.  These
terminations were used to attach the antenna wire as well as
provide a tie point for the coil of coax.

     Each trap will require 80.5 inches (6.7 feet) of coax.  Start
with seven feet and trim it up to the frequency you want in the 40
meter band.  Each length of coax is prepared by stripping about 2
inches of outer insulation from each end.  The shield is unbraided
and twisted at each end.  The center conductor at one end is
stripped of insulation for a length of about 1 inch.

     Start your winding by drilling a hole just large enough to
pass the coax through the PVC pipe.  This hole should be located
about 0.5 inches in from the end of the pipe.  A close fit of the
coax through this hole will help secure the winding until the holes
are filled with epoxy.

     Insert the end of the coax that has the center conductor
stripped through the hole and wrap the shield of the coax around
the number 12 wire at this end.  Solder the shield to the wire.
Use a 50 to 100 watt iron and do it quickly so that the heat will
not travel up the braid to melt the insulation to the center
conductor.  Let the soldered connection cool completely before
starting the winding.

     Now wind about 22 turns of coax onto the pipe.  I estimate
that 22 turns will resonate at the low end of the 40 meter band.
If you are interested in the higher portion of the band, stop at 21
turns.  Remember, you can always cut off coax to raise the
frequency, but if you get too high in frequency, your best bet is
to start over with a new length of coax.

     Mark the pipe at the end of the winding and drill another hole
in the pipe at this location to pass the coax.  The close fit of
the coax into this hole will keep the windings in place.

     Prepare a length of wire.  Hookup wire #20, #18, #12, is
adequate.  Cut the wire to 6.5 inches in length.  Strip off 1.5
inches of insulation from each end of the wire.  Solder one end of
the wire to the center conductor of the coax at the end where you
started your winding.  Pass the wire down through the center of the
pipe and twist it's bare end to the coax braid where you finished
the winding.

     Pull the hookup wire through the center of the pipe so that
the soldered bare end of the coax center conductor is pulled down
away from the soldered coaxial braid at the end of the coil where
you started the winding.

     Now you will need a grid dip meter to check the resonant
frequency of the trap.  Don't rely on the grid dip meter's
calibration.  Use a frequency counter or your communications
receiver to verify the frequency.  (My homebrew grid dip meter
doesn't even have a calibrated dial.  Only it's coils are marked as
to frequency range covered.)  I found that by inserting the coil of
the grid dip meter about 1/8th inch into the end of the PVC pipe an
easily recognizable dip could be obtained.  For your final
frequency check you may want to reduce the coupling between the dip
meter's coil and the trap.  In my case I found a 50khz shift in
frequency as I reduced the coupling.  The dip obtained at the
reduced coupling is the more accurate one.  Also, make certain that
your dip meter is on the right frequency range and that you have
the receiver tuned to the fundamental frequency and not a harmonic.
My grid dip meter had enough output to register an S9 +20db at the
receiver with the receiver's antenna disconnected.  Use the
receiver's S-meter to zero in on the dip meter's output when
determining frequency or zero beat as you would on an AM signal.

     Your first frequency measurement should fall somewhere in the
low end of the 40 meter band.  If it doesn't, and if you did get a
dip on the meter, you may be too low in frequency.  If so, cut off
about two inches from your winding and try again.
               In my case, I found that a one inch reduction in coax length
resulted in an approximate 50khz frequency shift.

     As you cut more and more coax from the winding, you will need
to drill additional holes in the PVC for proper termination of the
winding.  The PVC is easily drilled.  This cut-and-try method
requires a little patience, but it is very repeatable.  My first
trap took me two hours to build.  The second was done in 15
minutes.

     The coax winding is tight and close spaced onto the PVC form.
After your final frequency check, trim the finished end of the coax
winding and solder the braid and the short length of hookup wire to
the #12 copper wire termination that you installed in the pipe at
that end.

     That completes the trap.  Now all you have to do is build
another one and solder the antenna wires to the copper wire
terminations at each end of each trap.

     Note that the number of turns required will only hold true for
RG62AU coax.  Other types of coax may well be used, but the turns
required may vary.

     Initially, I was a little concerned as to whether or not the
end terminations I used would hold the strain of supporting the
traps.  The terminations have held through the weather conditions
we have experienced in the last two months.  However, I would not
recommend using anything less than schedule 40 PVC pipe.

     If you are interested in building a trap dipole, but not
interested in making the traps, I can offer you a matched set of
traps for the frequency of your choice for $25.  Leave me a message
on this board if you have any comments or require additional help.

             Ham Distribution Net BBS [phone]