Shareware Overload Trio 2/Shareware Overload Trio Volume 2 (Chestnut CD-ROM).ISO/dir24/40mbeam.zip/40MBEAM.ASC

Text File | 1994-01-13 | 15KB | 270 lines

40MBEAM.wp
by Frank Kamp
K5DKZ



A Feasible Full Sized 40 Meter Beam



     This project started as a result of renewed interest in 40
meters coupled with the desire for an antenna system that would
be more effective than the simple dipole.

     Over the last several years I have concentrated on VHF.
Experimentation with VHF antennas had me retiring the HF beam and
quad to the garage for storage.  This made it a lot easier to
crank my homemade, tiltover, drill stem, tower down to mount
various VHF antenna designs.  It also gave me a large inventory
of telescoping aluminum tubing and fiberglass spreaders.  Making
sure I had the parts set aside for at least one triband beam, I
used some of the other materials in this project.

     Before starting this project I was using an 80/40 meter,
inverted vee, trap dipole as an all band antenna, and a 40 meter
broadband dipole flat topped at 35 feet for 40.  Those antennas
are still up and in use, but most of my serious 40 meter
operating is now done on a pair of full sized, phased, quarter
wave verticals spaced 35 feet apart.

     Implementing the first vertical was a simple matter of shunt
feeding my grounded tower.  I used the Gamma match.  A ten foot
section of one inch diameter tubing was spaced ten inches from
the fixed section of the tower.  Braid from a length of RG8 was
used to ensure a good electrical connection between the fixed and
tiltable portions of the tower.  An old ARC-5 variable capacitor
(approximately 275pfd) was used to cancel the reactive portion of
the Gamma stub.

     Your probably wondering where the 40 meter beam comes in.  I
guess I could squeeze out of this by implying that two phased
verticals do qualify as a two element beam.  Don't laugh.  VK9NS
uses four quarter wave phased verticals on 40 for about 7 db gain
and does a pretty good job of working into the states.  I won't
disappoint you though.  My design for the additional full sized
quarter wave vertical could easily be used to build a real full
sized two element Yagi should you desire to do so.

     A slightly heavier construction is used for the vertical
application and will be covered first.  By 'heavier' I'm
referring to weight, not necessarily increased structural
strength.  A base heavy vertical lowers it's center of gravity
giving it stability.   Using this design I was able to securely
mount my 31 foot vertical by securing the base and providing a
sturdy mounting bracket only three feet up from the base.

     To provide the weight, I used a 44.5 inch section of 1-1/4
inch steel TV mast.  This is driven into the end of a 70 inch
long section of 1-3/8 inch aluminum tubing.  The steel mast
presses into the tubing a distance of 1.5 inches.  A ten foot
section of 1-1/4 inch, schedule 40 PVC pipe was cut to a length
of 105 inches.  There is nothing critical about the length of the
PVC pipe as long as it is shorter than the steel mast/tubing
assembly.  Moreover, there is nothing critical about any of these
dimensions.  They are given in inches because that is the way I
took them.  The only 'critical' thing is to make sure your
finished assembly is less than 32 feet long or you will have less
leeway in tuning it to resonance.  The pipe is used to
electrically insulate the lower section of the vertical.  I had
originally planed to mount the vertical on top of the tower by
clamping it's base into the rotator coupling.  That was before I
decided to make it full sized and use it in phase with the tower.
The extended section of PVC pipe allows additional beams to be
mounted on the base of the vertical.  The PVC also adds strength
to the press fit between the first two sections.

     The inside diameter of the PVC is too small to slip over the
1-3/8 inch aluminum tubing and is a loose fit for the 1-1/4 steel
mast.  By cutting a 1/8 inch wide slot through 65 inches of the
PVC pipe the inside diameter can be enlarged to fit snugly around
the aluminum tubing.  The first two sections are pressed into the
PVC pipe until the pipe is almost flush with the base.  A 9 inch
long shim is used in the base to take up the slack between the
steel tubing and PVC pipe.  The shim is made by cutting a length
of 1 inch diameter aluminum tubing in half lengthwise and using
one half of it for the shim.  The slot in the PVC extends down
the tubing assembly a few inches past the 1.5 inch pressed
overlap.  Drill a hole sized for a self tapping stainless steel
screw through the overlap and centered on the 1/8 inch slot in
the PVC pipe.  The screw will prevent the press fit from working
loose and also help ensure a good electrical connection between
the two sections of tubing.  Install another self tapping screw
at the base of the assembly.  This second screw should pass
through the shim and into the steel pipe.  Cut off the head of
this second screw and wrap a couple of layers of electrical tape
over it (just in case even though this is a high current/low
voltage portion of the antenna).  Electrical connection to the
base is made by drilling a through hole for 6-32 stainless steel
hardware.  Use a solder lug or simply make provision for clamping
the feedline between two washers mounted on the bolt.

     The 1/8 inch slot in the PVC pipe was cut on a table saw
using a carbide metal cutting blade.  The same tool was used to
split the 1 inch aluminum tubing in making the shims.  If you
don't have access to a table saw, a hand held, rotary skill saw
will do the job if you clamp the pipe to a table.  However, for
safety I would recommend using a hack saw in making the shims.

               The next extension is a 72 inch length of 1-3/8 inch
aluminum tubing.  Since both of these tubing sections were
salvaged from my quad, it was a simple matter to slide the second
piece onto the coupling and secure it with a self tapping screw.

     The next section is a 129 inch long fiberglass spreader.  In
my case the spreader end had already been fitted with a length of
aluminum stock that was a snug slip fit into a nine inch length
of 1 inch diameter tubing.  The length of 1 inch diameter tubing
is mounted into the inside of the 1-3/8 inch tubing using shims
and self tapping screws.  The objective here is to have all
junctions tight and wiggle free so they will not work loose when
the antenna is subjected to wind stress.

     We now have an assembly that is 26 feet in length and have a
few options to consider.  The first is to run a wire straight
down the hollow tube of the fiberglass spreader, bring it out
through a hole drilled in the spreader at the bottom and secure
it to under the head of one of the self tapping screws.  The
other end of the wire can be bent over the far end of the
spreader and secured with electrical tape.  Number 12 or 14 solid
or stranded copper wire will do.  We now have one leg of a self
supporting dipole that can be mounted vertically on top of a
tower and feed with balanced transmission line (300 ohm twin lead
or open wire line).  The other leg of the dipole can be made from
wire and strung at an angle way from the tower.  This second wire
should also be 26 feet long and insulated at the end.  Now we
have a balanced feed, multi band, vertical dipole that can be
used on 40 through 10 meters with an antenna tuner.  On 10 meters
it is 3/4 wavelength and should provide a radiation angle of 7 to
30 degrees as well as some gain over a dipole.  On 15 meters, it
is 5/8 wavelength, with a slightly lower angle of radiation and
the '3db gain' we have come to expect from a 5/8 wave vertical.
On 20 it is about 0.4 wavelength and should have radiation angles
lower than what can be experienced with a quarter wave ground
plane.  On 40 it is equivalent to a base loaded vertical.  Even
though this antenna qualifies as a vertical, note that it does
NOT require any radials.  It is a balanced vertical dipole.

     Another option is to cut a 14 foot length of wire and spiral
wrap it onto the outside of the fiberglass spreader.  This coil
will then need to be pruned until the antenna is resonant at 40
meters.  This could then be used either as a vertical or as a
vertically mounted dipole with a 32 foot counterpoise as
suggested above.  It's performance should be very good on 40, 20,
and 15 meters.

     The third option, the one I chose, was to add another five
feet of tubing to the end of the fiberglass spreader.  This
tubing is electrically connected with wire loosely wound over the
outside of the spreader to connect to the sections of aluminum
and steel tubing.  The additional tubing was made from three
telescoping sections of brass hobby stock.  The kind you find in
most hardware stores.  The brass tubing is very light weight,
readily available, and comparatively strong for its size.  The
bottom section of brass tubing is chosen to be a little larger in
diameter than the inside diameter of the fiberglass spreader.
The brass is slit lengthwise, compressed, and pushed into the
spreader for a length of about nine inches.  The wire is soldered
to the brass tubing.  The brass sections are also soldered
together.  We now have an assembly that is 31 feet in length,
very close to the 32 feet required for a quarter wavelength on 40
meters.  The additional electrical length is provided by the
winding of wire onto the spreader until the entire assembly is
resonant.  Note that we do not have to worry with sliding tubing
or base loading to shift the resonant frequency of the antenna.
We merely change the length and number of turns of wire around
the spreader.  This thing would also make a very good center-
loaded whip for use on 75 meters.  Notice I said 'whip'.  That is
exactly what we have here.  A very large, light-weight, whip that
is not 'floppy' when extended horizontally.  The total weight of
my vertical was less than 25 lbs.  More than half of that weight
was in the steel base section and the unnecessarily long length
of PVC pipe.  I wanted it base heavy for stability.  The
additional PVC pipe allows me to mount a small VHF/UHF beam at
it's base.  The vertical droop when the assembly was extended
horizontally was about five feet.

     There is no reason why this antenna cannot be made shorter
and still work.  I chose not to do that because I wanted as much
bandwidth as possible, as high a radiation resistance as possible
and as a close match to the full sized vertical I was trying to
phase.  As it turned out, the new vertical rose to a height
almost equal with the tower.  The vertical is roof mounted to an
eave located 35 feet from the tower.  I'm still tempted to mount
it on top of the tower, but then I couldn't phase it the way I
want to for 40 meters.  Anyone for a full sized quarter wave 75
meter vertical?  Wonder how that arrangement would work on 160
meters.  With a ten foot insulated, fiberglass section at the
middle of this thing, we could wind enough wire on it to make it
resonant on 160 meters without the tower extension.  A mast
mounted switching arrangement could tie it to the tower for 75
meter operation.  It could then also be used as a vertically
polarized dipole coax feed for 40 meters or balanced feed for
multi-band operation.  Even then, with the insulation provided by
the PVC pipe, we could still mount our triband beam.  Or, maybe,
a 40 meter beam made from four more spreaders and aluminum
tubing.

     I couldn't possibly raise a 40 meter beam at my QTH.  My
trees are not the only ones that have grown over the past fifteen
years.  I can just barely clear a two meter beam without having
it mangled by the large oak in my neighbor's front yard.
However, if that were not the case, here is how I would build it.

     Four sections similar to the one described above would be
made, but I would substitute aluminum tubing for the steel in the
base.  I would also shorten the brass extensions by a few feet
adding additional turns of wire to compensate for the shorter
length.  The driven element halves would be connected using a
slotted length of PVC pipe.  Four muffler clamps (two per element
halve) would hold the element to a six foot section of sturdy
aluminum angle stock.  Each element halve would have been
inserted into the six foot length of slotted PVC pipe taking care
to leave a gap between them in the exact center.  Electrical
connection to the elements would be made by drilling through PVC
and tubing,  then installing self tapping screws.  I would
probably try using an inductive hair-pin match taking care to
make sure the driven element sections were electrically shortened
enough to provide the required capacitive reactance.  I would
mount the parasitic element the same way in order to ensure each
full element had very close to the same weight for balance.  The
boom would need to be at least 2 inch aluminum.  A good
application for the tired and true irrigation pipe.  I would most
likely opt for close spacing (.15 wavelength) and use a director.
Even at .15 wavelength we need a 20 foot boom capable of
supporting about 35 lbs at each end.  Remember, one wavelength at
40 is 136 feet.  Muffler clamps would also be used to hold the
elements to the boom.  I would opt for using four clamps per
element.

     Tuning an almost 60 foot element would be feat in itself.  I
can envision it suspended at the ends of the spreaders, but I
think it would be more reasonable to tune each of the four
sections separately as quarterwave verticals.  This would require
some very careful planing and careful work.  Checking resonance
with a dip meter verified by a frequency counter or
communications receiver would be a minimum requirement.  Remember
to allow for the 100 to 200 khz upward shift in frequency when
this monster is raised.

     Getting it up onto the tower could best be accomplished
using the PRCV mount for a stationary tower.  A tiltover would
allow you install the boom, install one element, raise it, rotate
it 180 degrees, and lower it so you can install the other element
all from ground level.  Well, at least nothing more dizzying than
a stepladder, anyway.  Note that if your tower is not at least 40
feet high, you may not be able to install this antenna at all
unless you DO have a tiltover tower.  Of course, we would like to
get it up to at least 60 feet, with 70 feet preferred.

     As you can see, a 40 meter, rotatable beam is a major
undertaking even with low cost, light weight materials.  My
method of constructing the 40 meter monopole is offered as a cost
effective way to achieve good performance with a minimum of
effort.  It could make the job of constructing a 40 meter beam
less formidable and at a lower cost.  I feel it is definitely the
way to go when multiple verticals are required.  My next antenna
project will most probably be a four element 40 meter beam.  A
vertical beam, of course!