Finding your way on 10 Ghz WBFM or other bands

12-10-97

Bill Burns, WA6QYR

One of the tools in finding your way on the microwave bands is a wavemeter. I'm told they are showing up now at the swap meets in Southern California. It is a device to measure your frequency. NO you don't really need the megabuck counter to do the job in the field. The wavemeter has a dial with frequency marks, or it may be a micrometer dial against which you will have a paper document to relate dial reading to frequency. This is how you will know what frequency it is tuned to. If you find one of the micrometer dial kind and don't have the documentation on frequency readout, one of the SBMS Elmers could help you "calibrate" the dial by making dial setting to frequency chart. Typically the wavemeter will have two ports or openings (waveguide or coax connectors). On one of those ports you will put a small antenna, horn, probe, or just open waveguide to pick up the signal you want to measure. You can use it in the shack connected to the waveguide or coax if you desire. On the other port you place a detector. This can be a diode in a piece of waveguide, a detector mount with diode, or coaxial detector. This detector device allows the RF in the waveguide or coax to be detected or converted into electrical current. The output of the detector is usually a BNC connector. Most likely there is inside the detector mount some capacitive decoupling on the output so RF and high frequency RF is blocked. Audio and DC is about all you can expect out of this connector. To the detector output you connect a 50 to 100 microamp meter. Since there is a lot of RF on the hill tops these days, one would want to make sure all the connections are short and shielded as much as possible. The microamp meter will be the indicator of the RF picked up by the antenna. Most wave meters will allow all the frequencies to pass through to the detector except the one to which the wavemeter is tuned. On the "tuned" frequency, the wavemeter will absorb the energy and cause a "dip" in the meter reading.

To use the wavemeter, you place it in front of the dish near the feed or gunnplexer. Most of the time I have to get real close to the side of the gunn standing broadside to the dish to get a reading on my 6 mw 10 Ghz gunn. Yes there is some interaction between your wavemeter and the gunn frequency, but a few inches away should be OK. Any closer and the wavemeter will tend to pull the gunn off frequency. Usually it isn't much and you can still tell to with in a megahertz of where your gunn is tuned (depending upon the readout of the wavemeter). You can tell when your close enough by watching the meter reading come up when wavemeter is tuned off the gunn frequency as you move into position. As you tune the wavemeter to the gunn frequency you will see the meter reading dip. Yes there is a skill there in holding the wavemeter in one place long enough to get a good dip without moving it or having the meter reading change just because you moved away from the spot it was located in with respect to the gunn. You will want to tune the wavemeter slowly to get a good frequency reading as you move it in frequency. You may need to do this several times to be sure you are reading what you think you read with all the holding still as you can physically and still tuning the wavemeter. You need to consider all this when you are building the metering. Just how you will hold the wavemeter near your dish, adjust the wavemeter dial, and read the meter all at the same time. Putting the meter in a small box and bolting it to the side of the wavemeter to where you can read it takes some planning.

Hope this helped. --- Bill, WA6QYR