定向天线资料
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:
= 140 * () = 140*{(42*pi)/123.4} = 150 Ohm (7) ZC/l
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:
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