----- Original Message ----- From: "Greg D." ko6th_greg@hotmail.com To: amsat-bb@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:
- 1/4 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.
- 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
- 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