NEC-LIST: Quadrifilar Helix Modelling on NEC

Good Day All,

This is the first time I am posting on the net so I hope I do it
correctly!

As a hobby I chase the orbiting WXsats around the skies and am always
trying out different receive antennae.

I have been modelling the QFH and have concluded that there is more to
learn than meets the eye.

I have researched many articles and handbooks on the subject and after
much modelling have a number of questions. I would certainly
appreciate to hear from anyone who has modelled AND BUILT / MEASURED
such antennae.

The QFA I am working on is a self-phased unit, ie , a small loop
inside a large loop. The frequency of operation is the 137 MHz WX-Sat
band.

The items of concern are :

 1. What is the mathematical relationship between helix diameter and
    length to radiation pattern

 2. What is the more optimal antenna shape, ie, antenna length,number
    of helix turns, helix diameter/length ratio for a reasonable
    horizon to horizon hemispherical coverage?

 3. How does one determine the optimum delta frequency above and below
    the 137MHz center frequency at which to resonate the small and
    large loop, ensuring the 90 degree phase shift between the 2
    loops?

 4. When determining loop resonance, should this be with the 'other'
    loop present? The presence of the other loop shifts the resonant
    frequency ( where reactance = 0) up by 900KHz for the small loop
    and 500KHz for the large loop 5. Is it possible to mathematically
    quantify the relationship between the element wire diameter, and
    antenna resonance as well as feed impedance at resonance.

I have,among many designs, tried the following (courtesy
R.W.Hollander, Technote 1999-1, of the group 'De Kunstmaan').

  Radiation pattern optimal at approx 30 degrees above horizon -
  supposed helix diameter/length ratio of 0.44.

  Loop resonant frequencies designed for 3.2MHz above (small loop) and
  below (large loop) 137.5MHz

  Loop sizes are therefore :

      small loop

      diameter = 310mm length = 702mm

      half turn helix

      large loop

      diameter = 326mm length =741mm

      half turn helix

 However, even though the antenna SWR is lowest at 137.5MHz, the 90
 degree phase shift is not 90 degrees!.

 When observing Real impedance (R) and Imaginary impedance(X) for the
 combined loops, the two loops should both show resonance at the
 center frequency. This design places the small loop at 137.2MHz and
 the large loop at 137.9MHz instead of at 137.5MHz.

 Modifying the loop's sizes (keeping the 0.44 ratio) to bring the
 resonant frequency to center causes the single loop SWR (with the
 other loop present) to shift asymetrically from the center frequency
 when compared to the other loop SWR.

 Iterating through this process simply seems to increase the 'drift'
 of these parameters.

 I have built a variety of different QFH antenna, some 'Tall and Thin'
 quarter turn, some squat half turn, etc. MANY hours of modelling
 preceded the implementation and MANY subsequent hours were spent
 trimming lengths/diameters to resonate the antenna.

 The measured results have proven only that it is very difficult to
 obtain the 90 degree feed phase shift required. I have also tried a
 'same sized loop' implementation using a mini circuits 90 degree
 phase shifter, but the results were generally poorer.

 I still intend to pursue the QFH since I for some reason enjoy the
 elegance of the implementation!

 Any ideas? The NEC file is as follows :

CM QFH ANTENNA
CM 137.7MHz WXsat APT receive antenna
CM located on top of a pole 3meters above ground
CE
GH 1 40 1.4 .700 .154 .154 .154 .154 .012
GM 1 1 0 0 180 0 0 0 1
GW 4,15, .154,0,0, -.154,0,0, .012
GW 5,15, .154,0,.700, -.154,0,.700, .012
GM 0 0 0 0 90 0 0 .015 0
GH 6 40 1.460 .730 .162 .162 .162 .162 .012
GM 1 1 0 0 180 0 0 0 6
GW 8,15, .162,0,0, -.162,0,0, .012
GW 9,15, .162,0,.730, -.162,0,.730, .012
GM 0 0 0 0 0 0 0 3 0
GE -1
GN 2 0 0 0 13 .005
EX 0 4 8 00 1. 0.
EX 0 8 8 00 0. 1.
FR 0 8 0 0 137.5 .5
RP 0 36 36 1000 -90 0. 5. 10.
EN
 

Joseph Noci.
Director, Research and development
Advanced Technologies and Engineering.
South Africa.
Received on Fri Jun 23 2000 - 21:58:16 EDT