孤城浪子的地盘

来源：http://info.awmn.net/users/images/stories/Library/Handmade/helical-helix%20antenna%202_4%20GHz%20HOWTO.htm

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Abstract  
The helix antenna, invented in the late fourties by John Kraus (W8JK), can  be considered as the genious ultimate simplicity as far as antenna design is  concerned. Especially for frequencies in the range 2 - 5 GHz this design is very  easy, practical, and, non critical. This contribution describes how to produce a  helix antenna for frequencies around 2.4 GHz which can be used for e.g. high  speed packet radio (S5-PSK, 1.288 Mbit/s), 2.4 GHz wavelans, and, amateur  satellite (AO40). Developments in wavelan equipment result in easy possibilities  for high speed wireless internet access using the 802.11b (aka WiFi) standard.  
  

Theory in a birds eye  view
The helix antenna can be considered as a spring with  N turns with a reflector. The circumference (C) of a  turn is approximately one wavelength (l), and, the distance  (d) between the turns is approx. 0.25C. The size of the  reflector (R) is equal to C or l, and can be a  circle or a square. The design yields circular polarization (CP), which can be  either 'right hand' or 'left hand' (RHCP or LHCP respectively), depending upon  how the helix is wound. To have maximum transfer of energy, both ends of the  link must use the same polarization, unless you use a (passive) reflector in the  radio path.
The gain (G) of the antenna, relative to an  isotrope (dBi), can be estimated by:  

G = 11.8 + 10 * log {(C/l)^2 * N *  d}  dBi                (1)  

According to Dr. Darrel Emerson (AA7FV) of the National Radio Astronomy  Observatory, the results from [1], also known as the 'Kraus formula',  are  4 - 5 dB too optimistic. Dr.  Ray Cross (WK0O) inserted the results from Emerson in an antenna analysis  program called 'ASAP'.  

The characteristic impedance (Z) of the resulting 'transmission  line' empirically seems to be:  

Z = 140 * (C/l)  Ohm                                      (2)
  

Practical design for 2.43 GHz (aka S-band, ISM  band, 13 cm amateur band)  

l = (0.3/2.43) = 0.1234567 m  ;-)(12.34  cm)              (3)  

The diameter (D) of one turn = (l/pi) = 39.3  mm         (4)  

Standard PVC sewer pipe with an outer diameter of 40 mm is perfect for the  job and can be obtained easily (at least in The Netherlands ;-) from a 'do it  yourself' shop or a plumber. The helix will be wound with standard wire used to  interconnect 220V AC outlets in (Dutch ;-) house holds. This wire has a  colourized PVC isolation and a 1.5 mm thick copper core. Winding it around the  PVC pipe will result in D = ca. 42 mm, due to the thickness of the  isolation.  

With D = 42 mm, C = 42*pi = 132 mm (which is  1.07 l)    (5)  

Now d = 0.25C = 0.25*132 = 33  mm                         (6)  

For distances ranging from 100 m - 2.5 km with line of sight, 12 turns  (N = 12) are sufficient. The length of the PVC pipe therefore will be 40  cm (3.24 l). Turn the wire around the PVC pipe and glue it with PVC glue  or any other glue containing tetrahydrofurane (THF). The result will be a very  solid helix wound along the pipe, see figure 1 below.  

Figure 1. Overview of some of the materials used and  dimensions.  

The impedance of the antenna, which is:  

Z = 140 * (C/l) = 140*{(42*pi)/123.4} = 150  Ohm       (7)  

requires a matching network on order to apply standard 50 Ohm UHF/SHF coax  and connectors.  

The use of a 1/4-wave matching stub with an impedance (Zs) of :  

Zs = sqrt(Z1*Z2) = sqrt(50*150) = 87  Ohm                 (8)  

is very common. Due to the helix design, this equals 1/4 turn. However, from  a mechanical point of view -bearing water proof aspects in mind when using the  antenna outdoors- there are more preferred methods to match the helix to 50 Ohm.  My first thoughts were to empirically decrease d for the first and second  turn and match the helix using the 'trial and error'-method, while measuring the  results with a directional coupler, and signal generator. Browsing the internet  for while I found helices matched this way, but surprisingly I bumped into the page of Jason  Hecker. He really used an elegant way to match his helix by using a  copper vane, referring to the ARRL Handbook. So, full credits go to the ARRL and  Jason, and I used his dimensions for the vane. To be honest, this page seems to  be a duplicate of his page, except that our helices are wound the other way  around!! Yes, and I am left handed, so, is this a coincidence? It is funny  anyway :-)) For details, see figure 2 (below).  

  
Figures 2a and 2b. The idea, the dimensions, and, mounting the stub.  The hypotenusa of the stub should follow the wire.  

Now with some luck and skills solder the stub to the helix, glue it, and  prepare the contrapsion to be inserted into
the cap, see figure 3.  

Figure 3. Almost finished helix antenna.  

And.... ready! (figure 4)  

Figure 4. Finished 12 turn 2.4 GHz helix antenna,  G = 17.5 dBi or 13.4 dBi (Kraus or Emerson respectively)
  

The antenna was sweeped an measured. The results are given below (figures 5a  and 5b)  

  
Figure 5a Return loss (dB) from 2300 - 2500  MHz              Figure 5b Smith chart 2300 - 2500 MHz
  

  
Figure 6a Measurement  setup                                  Figure 6b 'helix-in-one-hour' and Rohde & Schwarz analyser  

And... finally.... the helix 'in action'....  

  
Figure 7a Beaming to my LAP (Local Access Point  ;-)                          Figure 7b  'bottom view'  

It is really nice to receive feedback from people who are inspired by this  page. Here a contribution from Rob Jaspers who made
his helices using this  page: