A Built in Test Set for Microwave Stations

K0CQ

One day out of four roving on 10 GHz in 2012, I made no contacts. Part I blame on propagation, part was power supply, and part was operator errors. On the fourth day having solved the power supply problems and making contacts, I resolved to devise portable instrumentation to minimize those problems, at least to identify things like resistance in the power line and a keying cable disconnected.

What I came up with has two basic parts, a directional coupler and a diode detector. With auxiliaries these two parts can check transmitter power (thus showing bad power cables), transmitter frequency (thus showing bad power cables), receiver frequency, and receiver sensitivity. And can be used to set the drive power to the transverter to maximize talk power and minimize splatter and harmonic generation.

The first fundamental part is a directional coupler, I like -20 dB coupling for several reasons, first its only 1% of the output power and a good level for the transmitter power test and its handy for the receiver testing. 30 dB may be too much for some of the tests, and 10 dB represents 10% of the transmitter power wasted in the detector plus some insertion loss inherent in the best of directional couplers. I mount it in the antenna feed cable, close to the antenna.

The second part is a diode detector. For bench work I've been using a Narda coupler and an HP 423A diode detector. The need for an SMA to N adapter and the mass of the 423A detector hanging on that SMA adapter is not sturdy enough to suit me for roving where my 2012 experiences hint at winds strong enough to blow the rig over. I have a sturdier tripod now but the sail may still turn the rig over.

I think the best is to build a simple directional coupler with built in diode detector.

Having more than one directional coupler, I have built a diode detector. It is described in a separate paper.

For transmitter power tests, a volt meter is necessary. On the bench I used a DC coupled scope, and this is the calibration curve I made with signal generator at 500 MHz. I'm sure I could have used a digital DC meter to make it more precise, but while the HP432 is rated to about ½ dB flatness over its 10 to 12,400 MHz range, I don't have a way to prove or check that right now so I don't need to know the DC

to the 0.1 millivolt precision. With 20 dB directional coupler and the HP detector I got 1.4 volts for 3 watts. I made lots of contacts so it was getting out. I used a scope to check for clipping and adjusted the drive to eliminate clipping, subject of my another article.

There are a couple ways to check receiver sensitivity. One is to drive the diode with a VHF signal at a known frequency and look for the harmonic. If that signal has some phase noise at VHF or UHF it will be spread at 10 GHz. My HP8640B locked at 384 MHz. I hear as bursts of signal changing frequency over about a kHz. Packaged oscillators can be had for 96 and 108 GHz which will have harmonics at 10368 give or take their tolerance and drift. They probably will need a buffer stage to drive the diode hard enough. A synthesized handheld may be better. A 2m frequency of 146.030 multiplied by 71 comes out at 10368.130 MHz. I suggest 146 vs 144 so its not leaking into the IF receiver. 432 x 24 is 10368. 96 x 108 is 10368, 48 x 216 is 10368, but 48 x 2 is 144. 96x3 is 288, the image offset so its no test of 10368 image rejection with a 144 MHz IF. I'm sure many of us have synthesized 2m FM handhelds that will put out a little power with a good battery or external power but don't receive well or have no provisions for PL tones needed in most areas these days. That could make a handy frequency reference.

The HP423A will withstand 200 mw CW and the output side is a 1GHz low pass filter so the VHF signal gets to the diode unattenuated. I have checked sensitivity by adding some attenuation on the microwave side of the detector (without directional coupler) and adjusted the drive for a threshold signal. Then tested with a changed RF stage and more attenuation at 10 GHz to check for improved receiver performance. I found that improvement. It was harder to determine by varying the drive to the diode because the amplitude of the 10368 harmonic of 384 MHz varied much more rapidly than the drive level. So that gives a test for both sensitivity and for received frequency. Things we need to know to make contacts when roving solo.

The same scheme can be used with the help of an audio amplifier for making transmitter frequency checks. One needs to couple an audio amplifier (I might use a Radio Shack test amp that is a LM385 in a box with a speaker, circuit likely directly from the LM385 data sheet) and through a low pass filter to the diode the same place as the 146.030 drive is applied.Then adjust the transmitter frequency for a beatnote in the speaker. Probably will need to drive the diode harder than if checking receiver sensitivity. This will check for frequency control problems brought on by poor power key down pulling the crystal oscillator off frequency or malfunctions in the synthesizer under those same conditions.

The last test for receive sensitivity is to drive the diode into avalanche with reverse bias to make it a noise generator. I won't do that with an HP432A diode that costs too much to replace and takes more than 20 volts to go into avalanche. Typically a diode noise generator puts out 20 to 30 dB more noise than is needed to test a good receiver, so the 20 dB directional coupler is a good level of coarse attenuation. The noise generator circuit can be quite simple then because the DC side of the commercial diode is bypassed above 1 GHz and there's a 50 ohm termination on the other end of the coupled line. There needs to be some current regulation so the noise output is consistent from one test to the next.

So an auxiliary box to go with the directional coupler and diode needs a meter or other level indicator, like a string of LEDs driving bar graph style by an LM2815, a signal source, such as a 2m handheld, an audio amplifier, and a DC source to make the diode into a noise generator.

With 10 GHz directional couplers going for $25 and up (the sky is the limit, some epayers asking ten times the original price 25 years ago) and commercial diode detectors in the same price range, there is benefit to building something from parts.

The detector shown above is described in detail in another paper as is an attempt at a coupler but its not so good I'd leave it in line for operating at this point.

This is not lab quality gear to give power or sensitivity or frequency to a 0.1% accuracy, just go or no go test equipment to say I should be hearing and should be heard, what else is wrong when I'm not making contacts? When roving in a group, failing to transmit or to be on frequency is noticed instantly by the rest of the group, when roving solo that's not the case. And when the group is listening on speakers one gets to compare receiver performance. Nice diversity reception until the group switches to headphones.

73, Jerry, K0CQ