Teflon disc to hold center pin solder BNC nut to UG-21 nut n * Output to &QHL J meter and adjust pot.

resistor soldered to brass ring

Fig B

Cheap Microwave Detector

By John Sortor, KB3XG, 1214 N Trooper Road, Norristown, PA 19403

I recently added a YSWR circuit to my 3456 rig. I needed two detectors but didn't want to buy those expensive HP diodes. I had some cheap Schottky diodes (1N5711) in my junk box. These diodes have about 2-pF shunt capacitance to ground. At this frequency, the 2 pF looks like a pure capacitance located on the outside ring of the Smith Chart. (0.2-j20 Q) It is very difficult to match to this impedance.

Shunting the pure capacitance with a lossy element (resistor) will make impedance matching possible. I used two 100-fi chip resistors in parallel as a termination for the detector diode

(see Fig C ). The network analyzer told me that the impedance of my circuit



Fig C

was now 6-j 16 £2. This is a much easier impedance to work with. I calculated that a 0.43 wave, 10-ii transmission line would get me back to 50 Q. 0.43 wave and 10 Q at 3456 MHz works out to be 0.7 by 0.7 inches on a 0.032-inch Teflon board. The detector has better than a -20 dB return loss and I am able to detect signals down to -10 dBm without biasing the diode.

3456 MHz Isolator/Circulator

By Dave Mascaro, WA3JUF, 1603 Mink Road, Ottsville, PA 18942

Used C-Band TVRO LNAs can be found at flea markets for less than $20. Some that don't work can be had for $5. In the past, I have shown how to modify these units for amateur use on 3.4 GHz by removing the horn and attaching an SMA connector. The unit can also be used for a transmit amplifier. Usually defective units have a blown GaAsFET in the first stage, which is easily replaced by an NE720, MGF 1302 or equivalent.

For units that are to be used for parts, ie chip capacitors, GaAsFETs, bipolar transistors and trimmer capacitors, there is another application. Remove the PC board arid disassemble the isolator (Fig D). Cut the cast aluminum housing in half between the isolator and the PC board with a hacksaw. Then file the end of the unit flush. Mount an SMA for the input (Port 1) and an SMA for the output (Port 2). Use 2-56 screws to mount the connectors.

I found that the insertion loss (S2j) and return loss (Sip can be improved at 3456 MHz (below the design frequency of 3.7 to 4.2 GHz) by adding a small magnet to the existing magnet. Move the magnets around to "tune"

the isolator.

The existing load at Port 3 will handle 250 mW or less. To use the unit as a circulator (3 ports), add a third connector in piece of the load at Port 3. A high-power load can now be added.

Port 1

I.L. port 1- 2 0.65 dB SI 1 w/port 2 open 1 1 dB Sil w/port 2 term. 12 dB

Cut off with a hacksaw end file end flush. Remove isolator guts first.

Port 2

Lock tuners and mounting screws with fingernail polish after adding magnets 0.5 dB 13 dB 13.5 dB

Fig D

Hybrid Power Splitters

By Chuck Steer, WA3IAC, 3446 Shelmire Avenue, Philadelphia, PA 19136

One of the simplest and least expensive methods of making a power divider is by direct-coupled or branch line. Originally, branch couplers and hybrids were characterized as narrowband. Broader band devices are possible by using multisections. Below are two of the most fundamental dividers.

The first of these is the 90° hybrid. This is, perhaps, the most useful of all types. This is shown below (Fig E). In the configuration shown, it consists of a cross-over constructed with collinear output arms. An input signal is split equally to the two output arms with a phase quadrature relationship between the two outputs. Both the quality of the hybrid and the mismatches on the two output arms determine the amount of power which is shunted into the terminated arm, somewhere between 10 and 20 dB down from the output power.

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