By Zack Lau KHCP

10-GHz WBFM—Improved Designs

In trying to make long-distance wide-band FM (WBFM) contacts on 10 GHz, I've discovered the shortcomings of some of the existing designs. These designs are simple to implement using devices such as the M/A-COM Gunnplexer, but simplicity sometimes means performance compromises as well. For instance, the varactor diodes used in most Gunnplexers are much more sensitive at low tuning voltages than at higher voltages. This means you will get much more deviation at lower operating frequencies than at higher frequencies unless the modulating amplitude is changed. On the lab bench, it isn't too difficult to adjust a deviation control, but such simple tasks become more difficult out in the field, where you often have to both manipulate the radio's controls and hold the antenna in exactly the right spot to dodge the trees, which are excellent attenuators of X-band signals.

Many of the existing WBFM circuits are sensitive to supply voltage and don't really provide adequate idiot protection. Ideally, a portable setup should work with nominal 13.8-V supply voltages that drop down to 10.5 V or so and should survive reverse polarity. Proper operation over a wide range of supply voltage allows the use of sealed lead-acid batteries, which are often more cost effective than NiCads. Reverse polarity protection is particularly useful out in the field, where mistakes tend to occur more frequently. On the other hand, designing the circuits for low current drain isn't all that useful, since Gunn diodes typically draw 100 to 200 mA anyway. Saving a few mA in the supporting circuitry might get you a few more percent of battery life, but you can easily lose more than that in struggling to make a difficult contact because your gear isn't up to par.

Two other important factors to consider are interference resistance and operation in extremely high or low humidity. Not surprisingly, broadcast transmitters are often located on the very same mountains used for amateur 10-GHz work. So, unless you want to listen to TV sync buzz, it's a good idea to make your equipment as resistant to high power VHF/UHF signals as possible. Here in New England, rain is expected during 10-GHz contests, so circuitry that can operate despite a bit of stray moisture is preferred. Low-impedance circuits handle stray, moisture-induced current paths better than do high-impedance circuits. On the other hand, you can also get extremely dry weather at high elevations, and it would be nice if the gear doesn't fail when zapped with a little static electricity.

Taking all these factors into consideration, I decided that you really want to use low-impedance bipolar/diode circuitry, rather than MOSFET circuitry. Plus, microwave bipolar/diode parts are easier to find these days than MOSFETs.

For the ultimate in EMI resistance, circuits should be built over a copper ground plane rather than on a printed circuit board. (No circuit board patterns are available for the HF/LF/audio circuitry described here for that reason—build them "dead bug" style.) Shielding is obtained by soldering together pieces of unetched double-sided fiberglass circuit board to form an open box, then adding a cover made out of 24-mil aluminum sheet. You can pack more into the box by building circuitry on both the top and bottom of a piece of copper-clad fiberglass and forming a box with top and bottom openings to gain access to the circuitry.

A Better Receive Mixer

The weak point of many Gunn transceivers is the receive mixer. The M/A-COM Gunnplexer with a circulator shown in Fig 1 works reasonably well, especially if the mixer injection

ANTENNA

30 MHz RECEIVER

GUNNPLEXER

GUNNPLEXER

INPUT +1 TO 20V TYPICAL

INPUT +1 TO 20V TYPICAL

Newingion, CT 06111 Fig 1—Schematic diagram of an M/A-COM Gunnplexer. This unit is optimized for email: [email protected] (Internet) communication work.

November 1994 25

is properly adjusted with a weak signal or a noise-figure (NF) meter for best performance. Weak-signal testing isn't too difficult with a WBFM

radio since quieting improves considerably over a small range of input signal level. Circulator operation is load-dependent, so optimizing the injection

| Modulator!—H Oscillator |—H Splitter |-Power Amp |-

Fig 2—Block diagram of a high-performance Gunn transceiver.

O Off

IN

OUT

U1

LM2941T

ADJUST

ON/OFF

Regulated 10 V Output

0 0

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