Monday, July 7, 2014

Double BiQuad sector antenna for 3.5 GHz / WiMAX / G4


This is basic instructions on building a double biquad antenna for 3.5 GHz frequency with approx. 14 dBi Gain. (13 dBi if without lips)

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 a big deal.

All antenna measurements based on careful analysis and testing: see here


double biquad antenna for 3.5 GHz frequency with approx. 14 dBi Gain
double biquad antenna for 3.5 GHz frequency with approx. 13 dBi Gain
Scheme for double biquad without "lips"

Wave length -
Section length - 2/√2×0.1789×86.2446≈21.820 mm ≈ 2.18 cm (analysis shows huge impact on correct size)
Element spacing -  0.0915×86.2446≈7.891 mm ≈ 0.8 cm
Reflector dimensions - 20.2x13.7 cm with ‘lips’, or 20.2x9.1 cm without ‘lips’.
For more see analysis.


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.)
  • 40cm length of copper wire for the element. 1 mm - 1.2 mm in diameter. Wider is not recommended. See my testing and analysis: here
  • 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 3.5 GHz antenna.)

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 2.18 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.
Note: You can use 1.2 mm in diameter wire to bypass small bending inaccuracies.

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! 0.8c m (or 0.7 if yours without lips)

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.

Testing Spacing and Wire thickness

As the 4nec2 modeling shows significant impact on the wire’s thickness and element spacing, it was time to see what would happen in real life with various thickness of wire and various spacing for element. Sadly I don’t have a laboratory, neither can find one around, so the testing was made in home conditions as best as I could (It took all weekend to test 4 types of wire with different polarization and spacing, repeating tests hundreds of times):
VP – Vertically polarized; HP – Horizontal polarized; H- Wire Spacing; D - Wire Diameter
CINR [dB] - Carrier to Interference + Noise Ratio (sometimes SINR: Signal to Interference + Noise ratio). Higher values are better.
RSSI [dBm] - Received Signal Strength Indicator. Higher values are better (Note: Its negative, higher is closer to zero).
The main impact is in CINR as its shows the channel quality, possibility to choose better modulation.

 For more information see analysis

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.


  •     3 Euros for antenna
  •     12 Euros for 2 meters professionally made coaxial cable SMA(m) to N(m)
  •     Time.

Two sector antennas at work

Note: Distance from Base Station and my antenna is 2.1 km, almost clear visibility.
I couldn’t test the real physical double biquad gain, but this is what I get from two external antennas (pic. above). From a handmade Double BiQuad(2) and KTB 3.5GHz 14dBi HP90VP14 WiMAX Antenna(1). Yes, I get more CINR from handmade antenna than from professionally made weather-proof outdoor antenna priced 90 Euros. Reason - badly chosen position, needs reattachment.
Burst Data shows the modulation, as you see above, I enjoy 16-QAM ¾ / 64-QAM ¾ the same I get from one external antenna. (modulation table below).


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


This 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 07 Jul 2014

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