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GP PATCH ANTENNA
(Goof-Proof
Patch)
Building a high performance
Patch Antenna may seem like a daunting task. But assembling your own is actually
quite easy and these instructions make it nearly goof-proof. This Patch will
outperform commercially made versions that use lower efficiency circuit board
construction.
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Introduction
It is no secrete that
the most important component in your wireless video system is the antenna.
While most folks are investing in more powerful transmitters, you can obtain
additional range merely by concentrating your efforts on the antennas that
are used. For sure, in our R/C wireless camera application, where the transmitter
is a moving target, antenna choices require some care during the selection
process.
Antennas come in many
flavors, but the ones that are supplied with most imported "sardine can"
video systems are dipoles and vertical whips. These are considered to be
omni directional antennas and are fine for basic consumer applications. And
although there are better choices, they have been used with good success
on the transmitter end of an airborne R/C video system. However, these omni
antennas are the system's achille's heel when they are used on the ground
based receiver.
I have found that a
higher gain antenna with a reduced beamwidth is best for use on the
receiver's end. The extra gain extends your range and limiting the beamwidth
helps reduce multi-path interference and minimizes the capture of extraneous
signal sources (the 2.4GHz band is full of consumer RF devices).
And in order for mortals
like me to build it, home brew antenna projects must be very simple and nearly
goof proof. Better still, step-by-step instructions would be nice to have.
Up until now, that laundry list of requirements seemed like an very tall
order. But not any longer, as you will soon see.
A Patch is Hatched
Since I experienced
good results with commercially purchased Patch antennas, I decided to concentrate
my efforts on documenting a 2.4GHz (13cm) ham band Patch that others could
build for themselves. My goal was to present a set of plans that nearly any
opposing thumb equipped mammal could understand (some lower primates excluded)
and present the information in such a way that success would be nearly
guaranteed.
The Patch antenna shown
here is a good performer when used on a 2.4GHz video receiver. It has more
gain (estimated at nearly 3dB higher) than my battle worn X10 Patch. Depending
on the exact frequency used, gain is estimated to be from +5dBi to +8dBi.
The antenna is linear polarized. Beamwidth is broad enough so that antenna
aiming is minimized.
I recommend the GP
Patch (Goof Proof Patch) to anyone that is experiencing poor results using
simpler antennas, such as a factory supplied vertical whip. It is easy and
cheap to build, so there is really no down side to trying it out. If
you suspect that your antenna is not up to par then give this one a try.
What you will find
here is a design that requires basic hand tools to build. The materials are
commonly found and there is enough flexibility in their choice that obtaining
them should be uneventful. In addition, full scale templates (available as
a free download) are provided that allow you to accurately cut out the
parts.
If your video system
operates on 900Mhz or 1.2Ghz then you cannot use the plans as-is. However,
you can modify them to work using the information found here:
GP Patch FAQ.
Goof Proofing the Patch
I spent a bit of time
trying to devise a way to minimize coax related obstacles. My biggest concern
was with feedline length efficiency issues due to impedance mismatches and
RF losses. I was also concerned that most folks do not have the experience
to correctly install SMA connectors onto the coax (which requires expensive
specialty tools). In the end, I decided that the only way to keep costs down,
and to ensure absolute success, I had to eliminate the coax feedline.
The result is an antenna
that has a integral SMA connector that screws directly onto the popular video
receivers. There are no SMA connectors to crimp, no SWR problems, and no
coax losses to deal with. Frankly, this method offers an ideal low-loss antenna
feedpoint. It's inexpensive too. Did I mention it was goof proof?
Bigger and Better
Nearly all Patch antennas
are simple circuit board designs, mainly because they are cheap to mass produce.
They consist of a double sided circuit board, usually .06" thick, that has
the reflector etched on one side and the driven element on the other. A coax
is used as the feedline and the coax's connection point determines the feedpoint
impedance.
However, printed circuit
board (pcb) Patches are not as efficient as other construction techniques.
The pcb has an exaggerated dielectric value that impacts the antenna design.
Both aperture size and RF medium losses are typical issues. The thin dielectric,
often barely just .01 wavelength in size, negatively reduces the overall
bandwidth due to the close proximity of the driven element and
reflector.
Even if zero loss Teflon
circuit board substrate is used, there are still gain reductions that harm
performance. That is because the circuit board material reduces the element
size (due to its impact to antenna resonance) and it increases the beamwidth.
Hence, there is usually at least a -3dB reduction in gain when pcb
designs are employed.
A much more efficient
Patch can be made if you use air (or vacuum) as the dielectric. With a dielectric
constant of one (unity), it allows for a full size Patch element and presents
a low loss interface between it and the reflector. Without the constraints
of a circuit board, it is quite easy to use optimal reflector spacing (.04
wavelength is used here).
 The
antenna element is much larger on an air dielectric Patch. If you hold it
up to a circuit board based antenna you will see that it is about 50% larger.
Larger antennas, that are driven at resonance, have higher efficiency than
those that are smaller. The photo on the right shows two 2.4GHz Patch antennas
(the size difference is obvious).
Antenna modeling suggest
that an air dielectric design can achieve gains in excess of 8dBi. This
is a +6dB improvement over a traditional dipole and at least a +3dB advantage
over most commercial Patch antennas. In a practical sense, every 3dB improvement
in antenna performance will offer the same advantages as to doubling your
transmitter's power. Now do I have your attention?
Tool Time
 Very
few tools and supplies are needed to build the Patch. You will need a 40
watt soldering iron, pair of tin snips or heavy shears, rubber cement or
glue stick, epoxy adhesive or CA (crazy glue), a flat file, and a couple
of drill bits (drill sizes shown on the downloadable plans).
Expected construction
time is about a half hour. With practice, you can build one in ten
minutes.
Three's a Crowd
There are only three
materials needed to construct the antenna:
-
5" x 7" piece of 24
to 30 gauge sheet metal (tin, brass, or galvanized).
-
2 inch long piece of
0.20" square balsa, pine, or hardwood.
-
PCB mount SMA connector,
Amphenol RFX
#901-9895-RFX. Available at
Mouser (p/n 523-901-9895-RFX
) and Digi-Key
(ARFX1229).
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I purchased the sheetmetal
at the hardware store (Lowe's). It was found in the roofing area and is sold
in a plastic bag as "Galvanized Step Flashing." A package of ten 5"x7"
pieces cost only $1.50, so that breaks down to fifteen cents an antenna.
As a side bonus, any leftovers can be used to fix your leaky roof.
I guess patching roofs
and making Patch antennas is all the same thing to the sheet metal.
Of course you do not
need to use roof flashing. Low gauge sheets of brass or tin will work too.
Avoid copper (too soft) and aluminum (can't solder to it). The local hobby
store had some nice sheets of 0.008" tin, but they were only 4" wide, so
I was unable to use it on the Patch's big reflector.
The sheet metal material
you choose should be light enough so that you can cut it with your shears.
But, it must also be sufficiently rigid so that your finished antenna holds
its shape and survives some manhandling at the field. Heck, the lid from
a large coffee can could be cut to size if you get desperate.
The wood is used as
a controlled dielectric spacer. I bought some .25" square hardwood from the
hardware store (36" long piece cost $0.60) and trimmed it to .20" thick.
You can also use Teflon, Sturdyboard foam, or "microwave safe" plastic. Do
NOT use metallic/conductive/ferrous material.
Design Documents:
The full scale sheet
metal patterns are available as a file download. Please be aware that the
information is copyright protected, so you are not authorized to republish
it, distribute it, or sell it, in any form.
If you wish to share
it, please do so only by providing a link to the RC-CAM site. You are granted
permission to post links to the web site's main page
(http://www.rc-cam.com/).
Please respect this simple request.
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Template File:
PDF file of the Patch's sheet metal parts.
Revision: Rev A, dated 10-01-2002 |
Let's Build the GP Patch
The first thing you
need to do is download the full size template and print it out. Using a foot
long ruler, confirm that the printed caliper graphic is exactly 9.0" in length.
If it is off by more than 1% then change the print menu's scale factor setting
and try again. Repeat until the scale is as close to 9.0" as you can get
it.
| Note: The printed
plans must be checked for accurate scaling. Please be aware that some printers
need compensation for best accuracy. Do NOT skip this step. |
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Now cut out the
two full scale paper patterns. The red border should be fully intact (cut
on the outside edge).
Apply the rubber cement
(or paper glue stick) to the back of the patterns and then place them on
your sheet metal. If you align them as shown in the photo then only two sides
of each pattern will need to be cut.
Do not trace the patterns
onto the metal. Take the time to glue them on. You must ensure that your
final cuts will be accurate and dressmaker patterns are part of the goof-proofing
process. |
| Using your tin
snip shears, carefully cut out the two metal parts. All trimming must be
on the outside edge of the paper pattern (leave the printed perimeter line
intact). Pay extra attention when cutting the small element since its accuracy
really counts.
Using a sharp pointed
object (center-punch, nail, ice pick, etc.) carefully mark the center locations
of the six holes. Using the drill bit sizes shown on the pattern, drill all
the thru-holes. You may use a 1/16" bit for the .060" holes and a 3/32" for
the .090".
Remove the paper patterns.
By the way, the rubber cement is not permanent and your patterns will peel
off with no residue. Glue stick adhesive will clean up with water. If you
used any other adhesives then may the Glue Gods have mercy on your soul.
Now flatten and debur
all the edges. Use a flat file to clean up any ragged stuff and slightly
round all corners to prevent injury. Straighten any warps so that the panels
are as flat as possible. |
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The SMA connector
is installed on the backside of the large reflector plate. It should
drop into your drilled holes and sit perfectly flush on the surface.
Solder the four ground
legs on the front side. I found that clean galvanized steel or tin material
flowed very well using a basic 40 watt iron and decent electronic solder.
After soldering, trim
the SMA's four legs flush on the front side. Do NOT trim the center
post. |
The SMA's center post
is a bit short and needs to be longer in order to reach the driven element
when it is added in a future step. Merely solder a short piece of 24-26 AWG
solid wire (a .5" long resistor lead works well) onto the end of the center
post. When you are done it should extend the post by about a quarter
inch.
| Now clean off ALL
the solder flux. Do not omit this step. |
| Now it's time to
cut out the wood spacer blocks. You will need five pieces that are .20" square
by about .25" long (5mm x 5mm x 6mm). The critical thing is to maintain the
0.20" dimension as close as you can since it will set the element spacing
when the two metal parts are combined.
Using a small amount
of epoxy or CA adhesive, glue the wood blocks to the center and four corners
of the small element's bottom side. Place them on the sheet metal
so that the .20" dimension (not the .25" side) sets their height . Allow
the glue to dry. If you don't want to wait long then use fast cure 5-minute
epoxy or CA. |
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| Now its time for
a dry fit of the two panels. Align the hole in the element over the SMA's
extended post on the reflector. Slide the two parts together so that the
SMA's center post passes through the small element panel.
The reflector's beveled
corner (orientation mark) should be at the upper left. The element's two
beveled corners should be at the upper left and lower right. The two panels
should appear to be centered over each other with the SMA offset towards
the bottom.
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If you like the fit
then remove the element and apply a small dab of epoxy to the top of the
wood blocks. Put the two parts together again and ensure that they are perfectly
centered.
While the glue is drying,
solder the SMA's center post to the element. Be very careful or the little
wire stub you added will come free from the SMA if its solder reflows. I
suggest you solder it while the adhesive is still wet so that you can pull
the panels apart if you run into trouble. Cut off the excess wire stub when
you are done and allow the adhesive to cure.
 The antenna is now complete! Just screw it onto the SMA
of your video receiver and aim it towards your R/C model. The arrow shown
on the template is the orientation for use with transmitters with vertical
polarization (e.g., vertical Tx whips). If yours is horizontally polarized
then rotate the patch
90º.
The photo on the left
shows the Patch screwed onto my video receiver. Other than the SMA coupling,
nothing is holding the Patch. I will probably add a bracket to support it
in order to prevent SMA connector damage.
By the way, the PVC
stand works great -- construction details to it are available on the
RC-CAM Field Equipment page.
*GP Patch FAQ:
It is important to
note that the success of any antenna project requires that you follow the
recipe exactly as shown. Improvising is not a good idea since everything
matters at microwave frequencies. Here are some common questions about making
changes to the design:
Q: I have
two video systems that operate at 900Mhz and 1.2Ghz. Can I use the GP
Patch?
A: You can use
it at other UHF and microwave frequencies by re-scaling the design. This
is a process where ALL the dimensions are increased to suit the required
operating frequency. It is important to know the exact frequency of your
system, so a frequency counter is often needed.
To scale the antenna
for your required frequency please use this formula:
Scale Factor = 2400Mhz
/ Required Mhz
Example: Cheap imported
"1.2Ghz" wireless video system (usually operate at 1.16Ghz):
2400Mhz / 1160Mhz = 2.069
Q: I computed
the scaling factor. My system runs exactly at 1.20Ghz so the scale is 2.00.
Just what do I do with that number?
A: You multiply
ALL the dimensions {as shown on the plans} with the scaling number. Accuracy
counts. The antenna will become larger, so you might want to use heavy gauge
sheet metal so that the bigger size survives rough handling.
Q: Ok, I
printed the plans. Then I carefully increased the sheet metal size using
the scaling number I computed. Do I also increase the size of the wood spacer
blocks?
A: Yes, you
must increase the size of EVERYTHING.
Q: I built
the stock antenna and I decided to use coax to connect the antenna to the
receiver. It seems to work. Is that OK?
A: If you are
happy then I am happy. However, you just robbed your video system's performance.
Why? Well, typical coax is VERY lossy at microwave frequencies and so your
range has been substantially compromised.
If it were me, I would
get rid of the coax and mount the receiver directly on the antenna. But,
if you like the performance with the coax then leave it in place (just keep
in mind that better range is possible with it removed).
Q: Do I really
need to use the SMA connector shown on the plans? Can I use a common TV
F-connector?
Or, can I solder
directly to the antenna with coax or other wire?
A: For 2.4Ghz
use, I highly recommend you use a microwave rated connector like the plans
call out. Everything else invites RF loss and will reduce antenna
performance. Most 2.4Ghz systems will mate with the SMA connector as shown.
Cheap imported 1.2Ghz systems often use a TV F-connector, so your GP Patch
should use its mate instead. Beyond this, if you must improvise then seek
help from someone that has experience with antennas.
Q: What is
the impedance of the GP Patch?
A: It is about
50 ohms. You can use it with a 50 to 75 ohm receiver.
Q: Can I
connect two GP Patch antennas together to obtain more gain?
A: Short answer:
No. Long answer: Patch arrays are more complicated than that. Without careful
impedance matching, then results will be dismal.
Q: I heard
that patch antennas are directional. I do not want to have to aim it at my
R/C airplane as I fly around. What do you suggest?
A: You should
find that constantly aiming the GP Patch is NOT necessary. Just point it
in the direction you fly in. There is no need to aim it like a
flashlight
Q: What do
the 45 degree cut corners on the sheetmetal do?
A: The original
goal was to try to entice the patch to be circularly polarized. Along with
the cut "dog ears," some early design attempts had 90 degree phase coherent
feedpoints. They were too complicated, easy to mess up during assembly, and
the results seemed to indicate that significant circular polarization operation
was not achieved. However, the final design's performance validations were
done with the dog eared cut corners and I did not want to go back and re-optimize
the design with them off. So, they remain in place. Folks that use the original
dimensions, or scale from them, should also cut the dog-eared corners as
shown. |
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