Here's my Yagi project all done. No performance numbers yet, but as they
come i will forward along.
feel free to post this content if you like...
kryptoknight
Update: Dec 20 2002
as of today i have not tested the cables/connectors for loss. i did manage
to get fairly good #'s on the Yagi, it yields about +10dbm without matching
input transformer...
UPDATE: Nov. 28 2002
all,
PLEASE WAIT BEFORE YOU BUILD IT!! My initial test gain #'s did not agree with the Yagi Modeler, so i am having the cables and Yagi tested professionally (my dad's shop has all the test gear), hopefully next week. i need to find what piece of my setup is lossy (12ft of RG58 has loss per foot, etc).....
regards,
kryptoknight
Objectives:
To build decent Yagi for the 802.11b band using readily available materials.
The Design:
I started with a Aironet Yagi (blueprinted at http://www.andrewhakman.dhs.org/yagi/)
and used the Yagi Modeler to make design adjustments (http://fermi.la.asu.edu/w9cf/yagipub/index.html).
Adjustments were made to get favorable #'s such a "Z" and gain. I managed
to obtain close to +16dbi with a very nice "Z" value (no reactance, with nice
real part favoring 75ohm RG6 coax to build a matching transformer). The attached
Yagi Modeler pattern file gives the mathematical
design.
The Materials:
Using the Aironet blueprint as a guide, I headed over to Home Depot to see
what they had. I found ~1/8" (3.09mm) brass and steel rod. Brass was perfect
for driver since I can solder to brass. The "frame" of the Yagi is 1/2" lucite
rod (online at http://www.tapplastics.com/).
Note: if you use Lucite (acrylic) rod, I would use 0.625”x0.625” square rod
for better rigidity.
Tools needed:
· drill press
· drill bits
· vernier caliper
· heavy wire cutter
· small grinder
· masking tape
· fine point marker
· 2 sheetrock screws
· wood glue
· hobby syringe
· soldering iron
· dremel
Construction Method:
After receiving lucite, I drilled a 1/8" hole radially at each end. The
rod is then attached to a strip of wood with sheetrock screws. This allows
me to hold the lucite while drilling and setting elements.
Take some masking tape and wrap around lucite and wood in a few places to
hold the rod firmly.
Next is to take a 24" piece of masking tape and lightly attach it down onto
a table nice and straight. Then take a 24" straight edge and make a line down
center of tape. Now take the tape and attach it to the lucite making sure
the line is straight all the way down the rod.
Now mark a "zero" reference point on the tape (rod). You should start "zero"
about 2-3" from end of rod. Using vernier caliper, mark out the location of
every element, measuring from "zero" point every time (note, if you need new
reference point as you go down because your caliper does not open that far,
use last marked point as new "zero", DO NOT MEASURE FROM ELEMENT TO ELEMENT).
Next is to drill out the rod. Use appropriate size drill bit. The best type
to use is a flat-bottom twisted flute wood bit because they have awesome sharp
brad-point tips which make it nice and easy for starting right on the mark.
You’ll need some sort of pilot tipped bit. I lost my awesome wood bits so
I opted to use my dewalt set that has sharp pilot tips. My 1/8" bit was ever
so slightly bigger than the rod. On drill press drill out each hole (my press
states 3600 rpm for 1/8" in plastic, but 900-1200 also works ok). Don’t rush.
After drilling out the holes, I rotate the lucite 90degrees and re-tape/screw
it to the wood. Using a 3/64" bit, I drilled "setting" holes. These holes
create a small hole directly into each element hole (these "setting" holes
do not go all the way through, just a radius depth). These holes are used
to inject a setting glue that will hold the elements in place.
Ok, lucite is all drilled out. Next is to cut the elements. I used a heavy-duty
wire cutter to nip the elements to rough length. I then used my 6" benchtop
grinder and my vernier to get to finished length. I then used hand file to
ease the ends from grinder burr. Each element is indexed (I use a piece of
wood to keep the elements in order). Then I simply used some spray paint for
color.
Creating the driver element is easy. I used two 30mm pieces of brass (final
length will be done later). Ends are ground flat and cleaned (use emery paper).
Then I used some copper from the center of RG6 coax to make the connector.
The copper is soldered to the end of the brass at 90degrees.
Next is to modify the driver hole in the lucite so the brass driver elements
can fit in. I simply used my dremel with cutting disc to cut a keyhole slot.
This creates a keyhole so that the element can slide right in.
Next is to set each element. I marked each element with a "stop" mark. This
mark tells me when the element is centered in the lucite. My calculator comes
in handy. For 0.5”dia. rod [element length divided by 2, then minus 6.25mm],
this mark lines up with the outer edge of the hole. I then used my hobby syringe
to inject some wood glue into the "setting" hole (rotate element 2 or 3 times
so glue gets all around element in hole). Don’t set driver. Let sit overnight.
At this point, you should have a lucite rod held down onto a piece of wood
with all elements (except driver) glued in place.
You can now glue the driver in place. I used JB Weld to set the dipole in
place, leaving 5-6mm space between the copper elements (estimate this, and
it should be the distance between elements where the copper is soldered to
brass).
After epoxy sets, I used my vernier to mark the desired location on the
driver elements to be cut (total length of dipole is 54mm). I used my dremel
with cutoff disc to rough-cut the brass, and then I used my bench grinder
to get to final length of each piece of the dipole.
Next was to make the input transformer. The whole idea from the get-go was
to design a ‘Z’ that would allow me to use either RG58(50ohm) or RG6(75ohm)
coax to build a matching transformer. It turns out that RG6 works best (it
was easier to adjust the design for RG6). Two 1” pieces of RG6 soldered together
in parallel (they act just like resistors do) yield approx 37.5 ohms of ‘Z’,
which is a very very close matching ‘Z’ transformer (meaning I am matching
the 50ohm transceiver to the antenna ‘Z’). The design of the transformer is
outside the scope of this doc, the general equations say to use a quarter
wavelength of cable for transformer, but in brief, a short length of coax
cable looks electrically longer than it’s physical length. So the transformer
is slightly shorter than quarter wavelength (I did the math and 1” works just
fine).
Next was to somehow make a transition from the dipole to cable. I opted
not to just solder a cable onto the antenna. I use it at work so I tote it
via old flute case. Having cable permanently attached would really suck.
So I soldered the transformer to dipole and is tie-wrapped in place. Then
I had to bore out the end of the Lucite to accept a panel-mount BNC(F) connector
(TNC, SMA, etc would also work, but 50ohm BNC cable is off-the-shelf at radioshack).
The bore was just wide/deep enough to accept the connector. I simply soldered
some copper lead to the BNC connector and then used epoxy to cement the BNC
into the bore.
Once epoxy was dry I simply soldered BNC leads to transformer (making sure
to solder ground-to-ground and center-to-center).
That’s it for construction
Cabling:
I now needed some cable to get from antenna to card. Good ‘ol local Radioshack
had a ton of stuff. RG58 (50ohm) BNC patch cable, BNC panel mounts, but no
N-to-BNC adapter or N crimps for RG58. I got N-to-BNC adapter at another electronics
shop.
Voila, simple easy-to-use Yagi.
Note: you can reduce costs by using wood dowel instead of Lucite and by
soldering cable directly to transformer. If you use Lucite, I recommend using
the square rods. At this point, the Yagi is missing a way to be held while
in use. To deal with this issue I will obtain some 0.25” plastic threaded
rod which will be used as a mount. A small length of the plastic threaded
rod will be used, and one end will be epoxied into a short length of 5/8”square
lucite rod. The other end of the square lucite rod will receive a metal threaded
insert so it can be attached to my camera tripod. I will then simply drill
a 0.26” hole through my antenna lucite somewhere near the middle. The plastic
threaded rod will fit through the hole and will be held to the antenna with
a plastic wing-nut.
