Thursday, May 28, 2009

Quadrifilar Helix Antenna for NOAA APT Reception

Since most of my previous attempts of NOAA APT live weather imagery reception have resulted in rather poor quality images, I finally decided to give it a real try this summer. I started with purchasing a serious receiver, the icom IC-R7000. With continuous coverage of 25Mhz all the way to 2Ghz, it is probably one of the best scanner / receivers I have ever used. It has the Japan built quality of electronics that you just don't find anymore and the feel of a very solid piece of electronics. Along with this I purchased a registered copy of WXtoImg, which is definitely the best APT decoding software available. I have used several of the free APT decoding applications, and while they work... they just don't have the image clarity that WXtoImg offers. WxtoImg also can control tuning of my icom IC-R7000 via rs-232 which is very nice as well.

The final missing piece I need is a good antenna. Here is an example of an APT capture using a dipole antenna with ground plane tuned to 137 Mhz:

The image is excellent just above my location ( indicated by the yellow + sign ) but image quality decreases as noise increases quickly outside of my location. What is needed is an antenna with a much better gain from a high speed moving satellite that doesn't have signal fade caused by the orientation of the propagated wavefront. While many people have had good results using basic dipole whip antennas, discone antennas, turnstile antennas, and more exotic Lindenblads, nothing seems to perform as well as the Quadrifilar Helix Antenna (QHA) .

Looking at the QHA, it is initially somewhat hard to understand it's design. It is a pair of circularly polarized, half turn, half wavelength helical antennas designed for reception of low-earth polar orbiting satellites. Reading into the documentation for the actual NOAA polar orbiting satellites, they actually use this exact same antenna design for APT transmission on the satellites themselves. After some research, I came across an excellent calculator for designing the antenna. I recently built this antenna using measurements from the calculator and ended up with this:

It's not perfect, but it is designed to specifications. I used thick 4mm copper grounding wire instead of small copper pipe that other designers of these antennas use mostly for ease of assembly. Also in the picture is the RF choke balun, which converts the balanced signal from the antenna to an unbalanced coaxial cable. It is simply four turns of RG-8 around the antenna mast as close to the feed point as possible. For extra gain, I'm using a mini-circuits ZFL-1000LN low noise amplifier between the antenna and receiver. Overall antenna parts cost was about $30 and assembly time took about two hours. I plan on testing the antenna this weekend and I'm looking forward to some good satellite imagery.


  1. I can't wait to hear about your computerized watering station!

  2. i want give full information& construction of receiver &antenna