Saturday, August 8, 2009

Absorptive SWR Bridge for the Microwave Bands


A number of years ago, I built an antenna tuner with a very simple SWR meter built in. This SWR meter worked using the Wheatstone Bridge theory, and the indicator was an LED. This meter worked well on HF using 2 watt transmitters. It had occured that since most microwave work is at similar power levels, why one of these meters wouldnt work on VHF and above.

I therefore built a meter similar to the one I used. I found an article on W1GHZ's site, that was very similar to what I was using. Here I thought this was a new idea for a microwave reflectometer, but Paul thought about it before me. I did, however use his circuit, and I also made a few modifications to his circuit to make it better. The schematic is shown. Click on the image for a larger view.
Basically, this is a wheatstone bridge. The three 50 ohm resistors, along with the output port, which if the impedance equals 50 j0 ohms, there will be a null across the 100 ohm resistor, and therefore no diode current. If however, the resistance is anything other than 50 ohms, an imbalance occurs, and RF will be present across the 100 ohm resistor, as well as across the diode. The imbalance current is read on the meter, and can be calibrated as SWR. One feature that is inherent with this type of meter is that it always presents a load near 50 ohms to the source, preventing any kind of burnout of the PA. The disadvantage of this type of circuit is that it cannot be left in the circuit, as it will have approximately a 6 dB loss.

The resistor with the "*" can be found experimentally, or replaced with a potentiometer of about 10 K or so to adjust the meter for a full scale reading with no load connected. To measure forward power to calibrate the meter, remove the antenna or output connection. To measure reflected, simply reconnect the antenna or device being measured. The power handling of this device is quite low, so don't transmit into the meter running more than a couple of watts for very long. My meter will calibrate with 1 watt applied. This meter will work using standard leaded resistors and chip capacitors up thru 432 MHz. To make the unit null on 902 MHz and above (probably thru 2304), you must replace all the resistors with chip resistors. Remember, any path carrying RF should be made with 50 ohm stripline.

My present meter is using the standard metal film leaded resistors, and the null is very good on 432 mhz. I use sma connectors for the guzinta and the guzoutta. I used what I had available as far as the resistors and caps are concerned. I have leaded resistors but no leaded caps, so I used all chip caps for mine.

This device can be used from HF - SHF, but it is a QRP meter. I wouldnt load more than 5 watts into this device.

If you have any questions on this meter, please leave a comment. I will try to answer any questions you may have. I hope that some of you may find this device useful, as it makes antenna tuneup easy, and protects the final of your radio while youre tuning!

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