Monday, July 14, 2014

Double BiQuad sector antenna for 3.65-3.7 GHz / WiMAX / WLAN


This is basic instructions for handmade 3675 MHz 13-14dBi Double BiQuad external antenna.

I used computer software 4nec2 (antenna modeler and optimizer) version 5.8.11 to model theoretically the perfect antenna. All measurements based on very careful analysis with computer software which results many times has been proven to be quite accurate and reliable in physical world.


The WiMAX 3.65 GHz uses microwave frequencies from 3650 to 3700 MHz. So the target is about in the middle - 3675 MHz.

Target frequency - 3675 MHz (for WiMAX / WLAN)

Simulation NEC Input files:

Configuration symbols in NEC Input files:

  • ED  - Element square diagonal length / 2
  • WR - Wire radius
  • S - Spacing between element and reflector
  • RH - Reflector height / 2
  • RL - Reflector  length / 2
  • LH - Height of reflector "lips"

All dimensions in .NEC data files and graphs are in wave-length ratio.

Main 4nec2 windows with parameters (antenna with lips on the left, without on the right)

Gain (antenna with lips on the left, without on the right) 

Note: The dBi scale is logarithmic in base 10, where +3 dBi is a doubling in gain! An increase in gain of +1 dBi is equivalent to an increase of 26% and +2 dBi is equivalent to an increase of 60%. This means that the increase in +1dBi is big deal.

3D Radiation pattern 


Scheme for double biquad without "lips"

Wave length (lambda)  =  \lambda = \frac{299800(SpInVacuum)}{3675MHz} = 81,6 mm \approx 8,2 cm

Calculated measurements for antenna with "lips"
Calculated measurements for antenna without "lips"
Section length - 2/√2×ratio×lamba ≈ 2.1 cm
Wire diamter - wire.radius.ratio×lambda×2 = 1.1 mm   (Wire area A = πd2/4 ≈ 0.95 mm2)
Element spacing -  element.spacing.ratio×lambda ≈ 0.7cm
Reflector height - ver.length.ratio×lambda×2 ≈ 8.4 cm
Reflector length - hor.length.ratio×lambda×2 ≈ 14.7 cm
Height of reflector "lips" - lips.length.ratio×lambda ≈ 2.4 cm
Section length - 2/√2×ratio×lamba ≈ 2.1 cm
Wire diamter - wire.radius.ratio×lambda×2 = 1.2 mm   (Wire area A = πd2/4 ≈ 1.13 mm2)
Element spacing -  element.spacing.ratio×lambda ≈ 0.7 cm
Reflector height - ver.length.ratio×lambda×2 ≈ 8.2 cm
Reflector length - hor.length.ratio×lambda×2 ≈ 14.3 cm



Parts Required

  • N-Type (female) panel mount connector
  • Reflector - Any metallic sheet like copper or Copper Plated Blank Circuit Board 1.6mm thickness
  • Cable - read this guide before doing anything else: Guide to Antenna Cables & Connectors
    (Very important to choose proper cable! the longer the cable length, the more signal loss you'll have through the cable. Long cable will defeat the purpose of the antenna. Read the guide for more info.)
  • ~45 cm length of copper wire for the element. 1 - 1.2 mm in diameter. Wider is not recommended.
  • Spacers in each end of the element to hold its position and spacing correct height between the radiator and the reflector. Spacers must be from any non-metallic material. Plastic will do alright.

N-Type (female) connector

Single Sided Copper Clad Board

Building the antenna

I will not show you picture after the picture how to drill the hole in center of reflector. How to use ruler. How to do proper soldering. What kits to use and etc. There are plenty of tutorials for these steps:, or
(Note: all calculations in links are for 2.4 GHz WiFi antenna. Use mine calculated data and measurements for 3.675 GHz antenna.)

(Note: Pictures below not to scale)

Element bending


Use this PDF file for bending wire contour. Print without changing scale and contour will be correct size on paper.

Before you start to bend the wire, make it straight as you can. 
Ensure each side of the element is as straight and symmetrical as possible.
The element sides are rectangular and widths are 2.1 cm, measured from wire center to wire center. 
The more accurate you do the better as the analysis shows a huge impact in element dimensions.

Mine wire bending technique in the crossing sections.
Note: You must leave a ~1.5 mm gap between the element wires cross overs and solders.

Guidance and recommendations

N-connector in the center of reflector. Maintain correct reflector’s dimensions.

Maintain correct spacing between the element and the reflector! 0.7 cm

You must leave approx. 1.5 mm gap between the element wires cross overs and soldered contacts.
Use spacers in each end of the element to strengthen its position and spacing proper height between the radiator and the reflector.
Spacers must be from any non-metallic material.

Finished. Lying on the belly.



This antenna should get the best performance at horizontal polarization.
Note: The antenna is horizontally polarized then reflector is held vertically.

Outdoor usage

If you intend to use this antenna outdoor, you will need to make it weather-proof to prevent corrosion.
Some people have used small tupperware containers, but it looks ugly at the top of the roof. I rather choose the corrosion… Or the good painting with some silicone on soldered contacts.

After you build your own antenna, you need proper cable with connectors to connect antenna with your modem.

Click here: Guide to Antenna Cables & Connectors


3.5 GHz antenna 4nec2 analysis and testing:
MIMO technology tests, two antennas gives better performance (Use two only if you have weak signal with one):
How-To: Build a WiFi biquad dish antenna:

Last updated on 14 Jul 2014

1 comment:

  1. why the source .nec file is not found
    can give me the file