Build coaxial colinear, circular polarized , corner reflector, and standard gain antennas for 2.4GHz Wifi. A simple 2.4GHz preamplifier build  is described
Basically this design arose from some private research based on curiosity, ie. How small can you make the
ground plane, whilst still keeping the antenna in forward-fire mode. The design is now used commercially (ZCG-
SCALAR's model CP2400), and is very easy to build. The prototypes used a 50 mm diam. ground-plane, and the
antenna was wound with adhesive leadlighter’s foil on a 30 mm PVC or fibreglass tube. The design as automated in
the RF3.XLS, worksheet #2, when tested gave 12 dBd. (Get RF3.XLS from Excel pages on this site)
The design provides a simple antenna for the point to point wireless LAN use, being fully sealed, and very
light as having a minimum wind load. The packet rate is much improved using circular polarisation in these links
because multipath reflections have minimal effect.
If building a pair for your own interest, I suggest if you are using ‘foil on PVC’, then design for a frequency 20% high,
which the calculator in RF3.XLS does for you. (because of the velocity factor effect, this will bring down the correct
response down 20%, to 2400 MHz) If winding with wire, say1mm, design at 10-15% high would be a reasonable
starting point. Give a PVC sample 3 min. in the microwave first , and if it barely warms, its ok! Forget about the ‘first
turn close to the groundplane’ advice in the ARRL books., and wind the helical evenly from turn one. Match is
accomplished with a ¼” wide tab soldered to ground plane about 1/8 turn around from feedpoint, and standing up
beside the winding as illustrated below (determine position with your analyser!)
This match system may not be textbook, but on the Time Domain Reflectometer the response indicates a
smooth change of impedance from 50 to the expected 120 ohms by the end of turn four.
A foam "doughnut" at the front held the assembly central in the tube.
Project:  2400 MHz reduced groundplane, circular polarisation antenna, suit point-to-point data, or WiFi.
Upper Left: The rough initial concept prototype, bare wind on fibreglass tube. Later I used pvc tubing.
Upper right: Ground plane sits against mouth of SS mount tube
Lower Left: Tab match gives a smooth impedance transformation, despite not being "textbook" My suggestion to those without an analyser is to use the stub at the length shown with a right angle foot for soldering, and slide it around for maximum signal on your wi-fi card's RSSI before soldering it in the final position. Tab position is fussy, and if you do not get it right, you will be disappointed in the build.
Lower right: After the antenna is assembled, a metal ring can be slid over the PVC tube and locked in position as if it was continuous with the internal, reduced groundplane. Although no electrical continuity exists, the gain of this setup is very close to the traditional design, with a single continuous groundplane. This external ring is optional.
Project:  DIY WiFi, 2.4 GHz corner reflector
Gain was measured as slightly better than my Standard Gain 2.4 GHz antenna.
Kraus, pioneer of the Helical  published work on a helical
with groundplane replaced by a loop, this 10 turn model achieving 15 dBi.
         IEEE Antennas and Propagation, Vol 32, No.2. 2 April 1995, P45
Project:  Skirted Dipole for 2.4 GHz
The skirted dipole is an easy to reproduce antenna, even if a little tedious to build, and this version uses the slug match as explained in Rf2.xlsm workbook, and  desigh notes in RF3.xlsm. The core of the low loss RG58 is passed through the 5mm tubes to which the elements are soldered.
Project:  Standard Gain antenna, 2.4 Ghz, but very suitable for WiFi.
Design dimensions are per a calculator on worksheet in in RF3.XLS.
Easy to build. Glue a standard strawberry punnet (removed for picture) over the delicate elements for protection. The benefit of this design is that the matching is simple and the gain is certain. Using the RSSI on your wireless link, prize apart or squeeze the split balun for maximum signal, assisted by worksheet "ANTENNA GAIN per RSSI".  .
Project:  DIY directional coupler
Built from ferrite cores found in low cost TV couplers or taps as in illustration on right, and very handy for your antenna matching..The  ferrite coupler has to be removed with tweezers, and with great care. I have hand wound  couplers on suitable TV balun cores, but was never able to equal these commercial miniatures at the higher frequencies.

Project:  Using receiver RSSI for instrumentation
Placement of stubs needs the measurements of line voltages. Use your existing RSSI, but there are things like settling time, graphed on left for a Winradio card, that may trip you up if quickly stepping through values.
Project:  DIY Slotted line
A knowledge of your load reactance is essential to allow accurate stub calculations. It's very easy to build your own slotted line, and when used with your reciever RSSI and the Rf2.xls workbook, you will be able to match your antenna perfectly
Project:  DIY Slotted line 2, and modernising the HP805 slotted line
DOS program source code in C, to automate all measurements on home made slotted lines, or a modified HP805 line.
The HP805 has a ten turn pot mounted on the carriage (see below), and the rubber wheel runs along the top of the bridge to give accurate position measurements. The software gives a graphical display of reference and load traces, and analyses automatically after cursors are moved to the max and mins. If thinking of compiling this, you will need the SMPLUS library, link is on Arduino pages, as is a sample trace from this program
Project:  Scalar Analyser
"scalar.c", program source code in C, gave two options for a Scalar Analyser.
First used a bridge, fed by a SMS signal generator stepped through a selected frequency range. The Icom R7000 was stepped to match and the RSSI used to do the computes. Of course, sweeps with shorts and opens were required first, but gave very good results.

Second mode used a different technique. A HP 8690B Sweeper is set up to sweep relatively slowly. The frequency is read regularly during the sweep via the serial output of an Optoelectronics 8048, and the dual directional coupler provides fwd and ref at each step via calibrated HP detectors with 100K loads. This setup is illustrated below.

This gave amazingly good results, and replicated the boss' Wiltron 54100A sweeps. He must of wept, because it had cost him an arm and a leg.
Left: SLOTTED.C also had ability to display a near field track. Antenna was mounted on wooden frame, and probe with 10 turn pot and wheel gave position data. Antenna tracked above was first look at a sleeved colinear that was not phased!
Project:  Smith Chart
DOS program source code in C, investigating drawing a Smith chart. May be useful graphical code.
Folder also contans some functions in C useful in antenna design
Cheap and easy slotted line, if you can machine or "dremel" the slot without disaster!
Project:  Alternative feed and match for 2.4 GHz WiFi yagis
Presented pictorially, a picture worth a thousand words. An analyser is the ideal tool, but these can be matched effectively by reading below.
Lambda loop (one wavelength in diam.) with a split sheath feed.
Loop, as in Corner Reflector project above. Split sheath is tuned by sliding a short along, soldering in final position.
Lambda loop feed, close up. Instead of a tab, as in the Corner Reflector project, a short is slid along the feed until match is correct, and split can be pinched or opened to assist in finding optimal match. In the absence of an analyser, you could tune for maximum signal on your wi-fi card's RSSI. The chips used to measure RSSI are highly accurate, and could be used for antenna gain comparison. If doing such comparison, calculate cable losses for each antenna, especially if your antennas have pre-attached cables, many of which are of dubious quality and have huge losses at 2.4GHz
Project:  Smith Chart
DOS program source code in C, investigating drawing a Smith chart. May be useful graphical code.
Folder also contans some functions in C useful in antenna design
Project:  Some useful understandings for antenna builders
Every antenna element needs shortening from its calculated (theoretical)  length.
You can use the tables, such as in the ARRL handbooks, or use the formula in this short note
Achieving a good VSWR is our aim in all antenna projects. Too many boast about their achievements, without any knowledge of the uncertainties involved. Read this, and download Rf2.xls (RF page) and you will be able to calculate your worst case uncertainty. (Since writing this note, I think Anritsu have adopted a RSS method in their later publications. This does not negate the need to understand what your actual uncertainties could possibly be)
Matching at UHF and above is always tricky, but I have found the slug-match the easiest of all. Download Rf2.xls.  Sheet 11 will help you calculate slug section sizes, but you need to know feed point impedences first, unless you are happy to do a lot of trial and error only using tyour RSSI indicator for max gain..
Coaxial gain antennas, and crossed cable section gain antennas are poorly understood. Download these notes and build them correctly. Whilst crossed section antennas can be easily built for 400-900 MHz antennas, in practice I have not had success  in a commercial design at 2.4GHz, probably because of effects at the crossover joints that are not understood, and because of dimensions and matching being so critical. Actually, difficult enough at 477 MHz, and I have seen an entire production run trashed because of a slight cable VF variation.
The yagi should be an ideal for 2.4 Ghz. Looks great, but this one disappointed, just over 4 dBd. Tested four, all just as bad!

DIY hint:- First quality low-loss cable is essential. Buy top brand cable and connectors, and observe "minimum bend radius", Your normal RG58 cable will not do here.
If testing on your analyser, test with any caps on. Adding them after testing will surely throw the resonance point off.
This feed for a 2.4GHz coaxial colinear has the feed made of a RG58 Low Loss cable inner fed up a brass tube. A section of dielectric is removed , and a slot cut so the "slug" can be moved to effect the match.
Download Rf3.xls, on my RF page. Sheet 1 will help you calculate design dimensions for a crossed cable section antenna, as illustrated below. Further down this page is a download, 'EASY SMITH' that has suggestions/ diagrams of matching techniques that would apply to this design also.
Also, download Rf3.xls, on my RF page. Sheet 11 will help you calculate design dimensions for your antenna elements
EASY SMITH shows how to match a known impedance on the SMITH CHART, following simple rules that are clearly explained, even if you do not fully understand the science behind its use. Unfortunately you still need to know your feed point impedance before you can use this aid.
You need to read this info, and use the referenced worksheets in Rf2.xlsm, available on the Excel pages.
Project:  Simple guaranteed gain, easy to build UHF amplifiers.
Using the MAR-2 chips is so simple in this design. A 'Dremel' grinds away the tinned outer cable to expose the core, which is split for insertion of the amplifier. Design is from Worksheet 11 of RF4.xlsm
These are great for both instrumentation, and weak signal boosting.
Where you need a tuned amplifier, a MMIC can feed a simple resonant section can be, constructed as follows.
Copper strip, 3.25mm wide x 0.25mm thick, from a leadlight craft supplier, is formed so underside of strip is 5.5mm above groundplane. A small copper disk can be used to lower resonance frequency, also can be slid slightly each way to obtain tune/match prior soldering. Input and output taps will be 1 - 3  mm from end. The charts below arise from building circuits for signal sources as discussed in 'VHF UHF Manual', Jessup 1987, Sect 9.10, 9.21, and have been used to build amplifier chains. Using 0 -10 pf capacitor at center ( between strip and earth)  instead of disk lowers resonance, and also allows a better range of adjustment.
Project:  Simple copper strip inductors for UHF amplifiers.
Side entry UHF socket is used to feed in the 12v supply for the amplifier
This version uses a slug match. It has also  been commercially used using a small, hairpin loop as a closed stub, but the placement is critical and 1.5mm too far forward or back makes the antenna impossible to tune. The slug tune is by far the easiest to implement, but needs a steady hand with the 'Dremel'

By request of my then employer, I tried unsuccessfully to incorporate extra dipole sections, but despite a lot of work was not able to succeed with phasing it correctly. Basically, the pattern and gain was a long way from what I predicted.
Coaxial design notes. Now with dimensions for this antenna, as extracted from my lab notes.
A practical application of these strip resonators was a 2.4GHz signal source, with two stages of MAR-2 amplification, from my lab notes. Interesting to note that the simple addition of two small tabs to the diode multiplier increased the final output by 5dB. Such is RF!
Project:  Coaxial Gain Antennas
Including designs for DIY WiFi antennas, based on my experience with both designing, matching, and troubleshooting them at the production level.
                                                UPDATES: 1 Nov 2015      Cable and Sleeve coaxial dipole added.
                                                                    23 Nov 2015  BG7TBL Spectrum Analyzer board controlled directly from Microsoft Excel added
                                                                                           to my Excel page, would interest visitors to this page

A FINAL WORD for those working in the DIY environment.
My hard earned advice is firstly, that whilst it is easy to build a neat looking antenna, unless you include a design feature allowing adjustment to your matching, even on PCB based designs, you may end up with a lot less signal than the design suggests. Secondly, without the right test equipment, it is almost impossible to verify phasing of elements on multi-element antennas. As soon as you get past the first couple of sections, you may find adding the next section has a big effect on the phasing and matchings of the sections you have already tested and are happy with. Thirdly, unless you decouple the line correctly, your pattern will never be what you expect.

Many published DIY designs are based on scalings from lower frequencies. Simple scaling is not possible to microwave frequencies. Choose to build designs that have been proved on the range!
An extract from Rf3.xls. Read it twice! You can waste a lot of time on this design, and still end up disappointed.
2.4 Ghz "cable and Sleeve" Coaxial Colinear Antenna
The mystery antenna of “Estqwerty's DigDice diy wifi antennas" ( Illustrated above per DigDice website photo.)

An antenna that looks so simple, but one very hard to "tame".
This design is common, and I have reverse engineered both the ZCG-Scalar  Z-150, and
Antenna Engineering's HU6. Both companies advertised the antennas as 6dBd and the patterns
show an expected down tilt. (I cannot guarantee the HU6 design is still as the one I dissected,
and the Z150 is discontinued.)
Tasked to scale this antenna to 2.4 GHz and reduce pattern down tilt to a minimum, I was given
an unlimited time budget, good test equipment, antenna range and a near field probe on a
wooden track.  My lab diary records ninety six models built and tested in the format illustrated
Basically, when I dismantled these antennas, they consisted of repeating bare coax center cable
radiating sections, with tubing as cable  “suppressors” (at least that's what the original design
engineers called them on the blueprints)
Below:- The start of my  efforts was a reverse engineering of both antennas in an attempt to understand
the significance of each build dimension. Antennas were measured, analysed on a spreadsheet (below,
or download RF3.xls) and used as a starting point for 2.4 GHz models.  Each model had to be built,
matched, electrical center found, and the pattern analyzed.
Before the exercise was over, a total of ninety six models had been built, as the bench diary shows!
YES, 96 builds!
After all this work, what can I tell you?
1. You can achieve 6dBd with about 3 degrees downtilt. Attempting to decrease to less than 3 deg   
results in a very touchy build, pattern break up, “butterfly wings.” A flat pattern is impossible in an antenna
which is fed from the base. Five sleeves are manageable, more gave no advantage.
2. Matching is easiest by a slug on wire in base tube.
3. Dimensions are critical. Sub millimeter accuracy in cutting and placement is needed.
4. Using RG213 inner, start with the dimensions shown as Build 92 above. Work with tubing element
lengths of 40 to 41.5mm. I used aluminium sleeves.