VK5JST Antenna Analyser: completing the build and testing.

The time had come to do the first smoke test of the analyser. I powered up my DMM and checked that the voltage regulators were putting out 8 volts and 5 volts respectively. I obtained 7.94 and 4.93 volts. Phew, all good so far.

Next I checked the signal generator section: I was to expect between 1.10 and 1.25 volts at Test Point 1. No problems there.

The centre pin of IC1 showed 1.534 volts which was just about spot on. And as the oscillator and following amplifiers are dc coupled, this reference voltage has to be correct for everything to work correctly. Now I had a problem: the collector voltage of TR3 was only 0.742 volts.

After much head scratching and tracing through the schematic, I decided to send Jim, the designer, a shout for help. Quick as a flash he responded saying that this is typical of an open circuit switch. So I pulled to offending item from the board and checked it with my DMM. All appeared fine. Jim wasn’t convinced and recommended I make my way to Jaycar to pick up a replacement. This I did, only to find the same result when I checked TR3 once more.

I sent Jim some high resolution photos of the PCB for him to run his beady eye over before resorting to packaging up the board and posting it to him for a more detailed analysis. He picked up that one of the solder connections on the variable capacitor looked a bit dodgy. So I sorted out the offending wire and tried again.

Success! I was back in business.

Next, I had to connect a good quality 50 ohm dummy load directly to the N connector of the analyser. Not having one that would connect to an N connector, I borrowed one from Wallace, VK4CBW, and proceeded to adjust the three 20K linear trimpots as described.

All other tests were well within the specified tolerances, so I was happy.

Final bench test with the dummy load connected.

I decided to check the SWR of a 6m dipole that I build last year using theory only for dimensions. And I know the antenna worked reasonably well because I used it in a VHF/UHF Field Day contest and won my category.

Connecting the analyser directly to the antenna produced a pleasing SWR reading.




3 thoughts on “VK5JST Antenna Analyser: completing the build and testing.

    • It is remarkably accurate too, Phil. The designer told me it is as good, if not better, than most commercial analysers on the market today. However, it doesn’t sweep the band like some; that is a nice feature but not necessary in my experience.

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