[amsat-bb] Re: CP antenna from 2 WiFi panels

i8cvs domenico.i8cvs at tin.it
Sun Jun 6 14:47:22 PDT 2010

----- Original Message -----
From: "Greg D." <ko6th_greg at hotmail.com>
To: <amsat-bb at amsat.org>
Sent: Sunday, June 06, 2010 10:15 PM
Subject: [amsat-bb] CP antenna from 2 WiFi panels
> Hi folks,
> Before I start nailing stuff together, I just want to verify what I'm
> doing...
> I want to make a 2.4ghz Right-Hand Circular antenna from two flat panel
> Wi-Fi antennas.  The idea is to mount >them at 90 degrees from each other,
> with one 1/4 wavelength in front of the other.  Combine the two feeds with
> a simple Tee (the feeds are of equal length), and into the pre-amp.  Since
> I'm not transmitting, I'm not too worried >about the resulting 25 ohm
> impedance (or should I be?).

Hi Greg, KO6TH

You should be worried because even on receiving your VSWR is 50/25 = 2 and
since the downconverter has been tuned for the lovest noise figure with a 50
ohm noise source it happens that the noise generated by your downconverter
will be greater than you should expect with an input VSWR = 1

> If it matters, the panels are from HyperLink Technologies, their model
> HG2414P, with a claimed 14dBi gain.
> So, the questions:
> 1.  1/4 wavelength at 2401 mhz is ((3 x 10**8 / 2401 x 10**6) / 4) meters,
> or about 1.23 inches.  Right?

Right, but 1.23 inches is a too short distance to mechanically separate by
1/4 wavelenght two flat panels so that if necessary it will more convenient
to use for spacing an odd numbar of 1/4 wavelenghts into free space at 2401
MHz and use two coax feed lines of the same lenght.

> 2.  Most of our 2.4 ghz satellite downlinks seem to be either linear or
> RHCP, so I'm guessing that RHCP is >probably the preferred construction.
> (Yes?)

All 2.4 GHz downlinks on the actual satellites are linear so that no matter
if you connect the antenna for RHCP or LHCP

> 3.  Looking at the Satellite Experimenter's Handbook (figure 7-10), I
> believe the panel rotated 90 degrees counter->clockwise as seen from
> behind
> the panels should be the one farther out in front, for RHCP.  (Their
> picture shows >clockwise for LHCP.)  Is this correct?

It depends if you connect the inner conductor of the coax cables to A or to
A' for the front dipole and to B or to B' for the rear dipole.

In a separate email I will send to you a drawing to explain how two linearly
polarized waves radiated as a 90° components combines each other to generate
a resultant wave that can be RHCP or LHCP.

> Thanks,
> Greg  KO6TH

73" de

i8CVS Domenico

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