5.3. Tutorial - Designing Directional Antennas

Some of the directional antennas are :Yagi Uda, Log periodic, Helical, Travelling wave tube

Uda (Yagi-Uda) Antenna Design :

Discovered and designed by Uda, popularized by Yagi. Later Yagi & Uda together involved researched this antenna. The paper by Uda and Yagi is considered one of the classical papers in Radio Engineering, and was republished by IEEE often. Its design idea is so fundamental, that made the design so interesting and easy construction — such that it is widely used for Amateur Radio, Satellite Communication and TV reception

If you want to know any of the things provided here, we can discuss them deeply in physical meetings…..

A finished typical Yagi-Uda antenna for Wifi applications could like one of the following :


  • It is an End Fire antenna
  • It is a Travelling Wave antenna

I recommend one to read the following Radio-Electronics content on Yagi-Uda :

Things to know for Understanding Principle of Operation:

Please refer to the Antenna fundamentals for new comers section.
One can also learn themselves about :

  • Radiation,
  • Mutual coupling,
  • Resonance,
  • Phase Lagging, Phase Leading,
  • Constructive and Destructive Interference,
  • Diffraction,
  • Polarization

Resources :

What are all the design data required for constructing Yagi-Uda Antenna physically ?

  1. Frequency of Operation (f) in Hz
    • Know the Bandwidth & Channels : (ISM – Wifi band 2.4GHz)
  2. Required Gain (G) in dB
  3. Diameter (d) of Parasitic elements (Directors and Reflectors)
  4. Diameter (D) of Supporting beam

Stuff that influences Performance of Yagi-Uda Antenna:

1. Reflector & Feeder arrangement
2. Feeder itself
3. Rows of Directors (A Wave Canal)

Things we are going to use for Construction:

Upcycling would be the best way to go………. :)

  1. Need Material for Feeder, Reflector, Director
    • One can use Refrigerator condenser tubes and fins (or) 14 Gauge copper wire (or) Conductors from cloth hangers (or) Old measurement tape strip for Elements
    • One can use Wooden Stick (or) Mob stick’s tubing/available PVC tubes (or) Fiber glass (or) Plexi glass for Boom & Mast
    • One can use cable Ties instead of Clamps
  2. Need RF cable, RF connectors, Soldering tools
  3. Need Power tools for drilling, cutting

Design Procedure :

For Adhoc, fast implementation one can follow AB9IL Ham’s guide (I think its reliable)
or, can use :
The design procedure in the National Bureau of Standards could be followed.

A Typical Yagi-Uda design diagram:

  1. Find the Wavelength
  2. Find the Lengths of Parasitic Elements
  3. Find the Spacing between the Reflector and Driving Element
  4. Find the Spacing between the Director Elements
  5. Find the Length of the Overall Structure
  6. Design the Balun for Impedance matching
  7. Adjust for Optimization


Adjustment for Optimization

  1. Bandwidth coverage can be increased, by lengthening the reflector to improve operation at low frequencies and shortening the directors for higher frequencies, with a sacrifice in Gain.

Coaxial Feed & Impedance Matching
When the Radio Unit is connected to the antenna through a Transmission Line like a Coaxial Cable with its own characteristic impedance (in this case 50Ω – for RG58), the Signal to and fro between the Air Medium – Antenna – Transmission Line – Radio TX/RX must not see any change in impedance as far as possible. This is important because, the performance of the system depends upon the Standing Wave Ratio(VSWR) influenced by the Reflection Coefficient caused by the impedance mismatch that happens at the connectors, soldering ends, medium interfaces. Practical design must be taken care to construct the system, such that very less reflection – as small as possible, thus very less VSWR can be allowed.


Practical Design :

Frequency: ν = 2.442GHz = 2442 MHz (centre frequency from the above 2.4GHz band channel allocation diagram)
Velocity of Electromagnetic Wave: C = 2.99792458 m/s
Therefore, Wavelength: λ = C/ν = 12.276 cm
Required Gain: G = 16.35dB (at max)

  1. Elements:
    Case 1: If Refrigerator Condensor Tubes are used
    Element diameter(d) = 6.40mm = 0.640cm ~ 0.25in (measured with Vernier Caliper) {which is 2a in the above diagram}
    d/λ ratio = 0.052134245
    Case 2: If Refrigerator Condenser Fins are used
    Element diameter(d) = 1.66mm = 0.166cm ~ 0.065in (measured with Vernier Caliper)
    d/λ ratio = 0.013522319
  2. Booms:
    Case 1: If 1inch PVC Pipe is used
    Boom diameter(D) = 1in = 2.54cm
    D/λ ratio = 0.206907787
    Case 2: If 1/2inch PVC Pipe is used
    Boom diameter(D) = 0.5in = 1.27cm
    D/λ ratio = 0.103453893

For a 15 Element Yagi-Uda design : 14 Parasitic elements + 1 Driven element
Why 15 ? In practice, beyond 15 or 20 elements there is no significant change in Gain, We have document for 15 element design in our hand.

From the National Bureau of Standards document, 4.2λ is the maximum length of the Antenna

Therefore, Overall Length: L = 4.2λ = 4.2 × 12.276cm = 51.5592cm = 515.592mm = 20.298in
Physical Length: Add +3cm in front & back for mechanical support in Boom = 51.559cm + 6cm = 57.559cm

As per the design table in the document,

  1. Length of Reflector Element:
    • 0.475λ = 5.8311cm = 58.311mm = 2.295in
  2. Length of Director Element:
    1. D1: 0.424λ = 5.2050cm = 52.050mm = 2.049in
    2. D2: 0.424λ = 5.2050cm = 52.050mm = 2.049in
    3. D3: 0.420λ = 5.1559cm = 51.559mm = 2.029in
    4. D4: 0.407λ = 4.9963cm = 49.963mm = 1.967in
    5. D5: 0.403λ = 4.9472cm = 49.472mm = 1.947in
    6. D6: 0.398λ = 4.8858cm = 48.858mm = 1.923in
    7. D7: 0.394λ = 4.8367cm = 48.367mm = 1.904in
    8. D8: 0.390λ = 4.7876cm = 47.876mm = 1.884in
    9. D9: 0.390λ = 4.7876cm = 47.876mm = 1.884in
    10. D10: 0.390λ = 4.7876cm = 47.876mm = 1.884in
    11. D11: 0.390λ = 4.7876cm = 47.876mm = 1.884in
    12. D12: 0.390λ = 4.7876cm = 47.876mm = 1.884in
    13. D13: 0.390λ = 4.7876cm = 47.876mm = 1.884in
  3. Driving Element – Folded Dipole
    Why Folded Dipole ? It offers better impedance matching for Transmission Line(50Ω) and Smooth transition towards Air medium. Because of that it achieves Broadband functionality (decreased Q)
    For a good Upcycled finishing, G3SEK’s procedure is very reliable and self explanatory.
    Since it is a Half Wave dipole, the length is 0.46λ = 5.6469cm = 56.468mm = 2.223in
  4. Spacing between the Reflector and Driven Element:
    • Srd = 0.25λ = 3.069cm = 30.69mm = 1.208in
  5. Spacing between the Director Elements:
    • Sdd = 0.308λ = 3.7810cm = 37.810mm = 1.488in

How to Match the Impedance:

In otherwords, we need the signal to travel seamlessly, without any sudden change. The Transmission line and Antenna together must act as a smooth bridge between the Radio Unit and the Air medium. The following shows Balun design for a Folded Dipole Driven Element :

Please study the Baluning in the Practical VHF/UHF design document by G8EZE.
However other impedance matching techniques like, Gamma matching or T-matching techniques can be implemented.

How to Align:

Antenna Diagram:

With the above designs, assuming the Case 1 for Element and Case 2 for Boom as available resources, the following antenna diagram is depicted:

Antenna Simulation Results:

Simulation are done using xnec2ec tool.


Radiation Pattern

Frequency Plot

Front to Back Ratio