SWR Meter for 2.4Ghz

pe2er · 04-12-2005

"Has anyone got a fairly good design for a 2.4Ghz SWR meter?"

Perhaps a little late to reply to your message, but it may interest other visitors of this forum

I have good experience using a resistor bridge to determine VSWR of an antenna.
A Complete description of how to build one can be found on one of my pages: http://home.wanadoo.nl/erwin.gijzen/wifiswr/

It descibes the low-power RF bridge and a peak-hold circuit, so you can use a standard 20dBm Access Point to drive the bridge. The peak-hold circuit is needed because the Access Point normally only transmits in short bursts (SSID Beacon transmission every 100ms) as opposed to a constant carrier normally used during VSWR measurements.

Thorn · 04-12-2005

Split from SWR Meter for 2.4Ghz?

Pe2er, thanks for the info. Those are very nice bridge and meter designs.

For future reference, if a thread hasn't had any responses for over two years, we would prefer that a new thread be opened.

yves_mag · 04-13-2005

I've build the SWR meter described in the URL. It works, but my trouble is to calibrate him.

I didnt build the hold-peak part. I've set the lower value for the beacon interval (1ms on the wrt54g or the usr2249) to have a permanent emission.
(in this case, the AP is unusable for connecting to clients).

it works fine, when i put a metal sheet in front of my canantenna, the voltage increase.

My trouble is to calibrate the SWR. I've build a N-connector with two SMD 100 ohm resistor, but i'm unsure of the 50 ohm value at 2.4GHz ! (the 3 others resistor of the SWR meter are build with the same components).

pictures here:
http://yves.maguer.free.fr/WiFi/page...bien_accordees

pe2er · 04-13-2005

@ Thorn,
Understood

@ Yves,
Nice pictures

You described that the voltage increases when you put a metal plate in front of the cantenna.

Normally, you would expect to see only lower voltages than the 'calibration' voltage. The calibration voltage is the voltage with a short circuit attached to the Zx port of the bridge.

However, this is only true if the impedance of the RF Generator (our Access Point) is close to 0 Ohm. In the real world, this output impedance is 50 Ohm. Therefore, the DC Out voltage can be higher under certain conditions. So what you see is 'normal' system behaviour.

Concerning callibration: This is indeed a problem. You can not callibrate the VSWR meter without a known good dummy load. And you can not make a dummy load without verifying its function with a known good VSWR meter. I acquired a DC-18GHz dummy load at a hamfest to calibrate mine. I Am afraid that this is the only way to go - besides borrowing one ;-)

04-12-2005	#2 (permalink)
Thorn Did you do the math? Join Date: Apr 2002 Location: Villa Straylight Posts: 10,361	Split from SWR Meter for 2.4Ghz? Pe2er, thanks for the info. Those are very nice bridge and meter designs. For future reference, if a thread hasn't had any responses for over two years, we would prefer that a new thread be opened. __________________ Thorn "Read Altas Shrugged. Compare it to today. Repeat as necessary"

04-13-2005	#4 (permalink)
pe2er Registered Member Join Date: Nov 2002 Location: Europe - Holland - Almere Posts: 4	@ Thorn, Understood @ Yves, Nice pictures You described that the voltage increases when you put a metal plate in front of the cantenna. Normally, you would expect to see only lower voltages than the 'calibration' voltage. The calibration voltage is the voltage with a short circuit attached to the Zx port of the bridge. However, this is only true if the impedance of the RF Generator (our Access Point) is close to 0 Ohm. In the real world, this output impedance is 50 Ohm. Therefore, the DC Out voltage can be higher under certain conditions. So what you see is 'normal' system behaviour. Concerning callibration: This is indeed a problem. You can not callibrate the VSWR meter without a known good dummy load. And you can not make a dummy load without verifying its function with a known good VSWR meter. I acquired a DC-18GHz dummy load at a hamfest to calibrate mine. I Am afraid that this is the only way to go - besides borrowing one ;-) Last edited by pe2er : 04-13-2005 at 07:17 AM.

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04-12-2005	#1 (permalink)
pe2er Registered Member Join Date: Nov 2002 Location: Europe - Holland - Almere Posts: 4	"Has anyone got a fairly good design for a 2.4Ghz SWR meter?" Perhaps a little late to reply to your message, but it may interest other visitors of this forum I have good experience using a resistor bridge to determine VSWR of an antenna. A Complete description of how to build one can be found on one of my pages: http://home.wanadoo.nl/erwin.gijzen/wifiswr/ It descibes the low-power RF bridge and a peak-hold circuit, so you can use a standard 20dBm Access Point to drive the bridge. The peak-hold circuit is needed because the Access Point normally only transmits in short bursts (SSID Beacon transmission every 100ms) as opposed to a constant carrier normally used during VSWR measurements.

04-13-2005	#3 (permalink)
yves_mag Registered Member Join Date: Dec 2003 Location: Toulouse France Posts: 18	I've build the SWR meter described in the URL. It works, but my trouble is to calibrate him. I didnt build the hold-peak part. I've set the lower value for the beacon interval (1ms on the wrt54g or the usr2249) to have a permanent emission. (in this case, the AP is unusable for connecting to clients). it works fine, when i put a metal sheet in front of my canantenna, the voltage increase. My trouble is to calibrate the SWR. I've build a N-connector with two SMD 100 ohm resistor, but i'm unsure of the 50 ohm value at 2.4GHz ! (the 3 others resistor of the SWR meter are build with the same components). pictures here: http://yves.maguer.free.fr/WiFi/page...bien_accordees