 |
 |
|
|
|
|
|
|
||||||||||||||||||
|
|
|||||||||||||||||||
|
|
|||||||||||||||||||
|
|||||
Technical Correspondence
Technical Editor, QST:
For weeks before the recent Frequency Measuring Tests I had brayed to my friends on the air about how I would make a concerted effort to place high on the FMT results list. I had built a separate mixer, fed by my Drake twins' oscillators and followed by all outboard t.r.f. amplifier. The amplifier gives out a signal on whatever frequency is being tuned, directly into my newly acquired Hewlett-Packard 5216A frequency counter. With no interpolations involved, nothing could go wrong, I asserted.
I was busy most of the day of the FMT preparing for the two runs that evening. The Tektronix 547/1A1 dual-trace scope was set up to afford envelope or sine presentations of the receiver's 50-kHz. i.f., as well as to monitor the counter's time base against WWV (from a second receiver), and to peak the signal going to the counter. It was disturbing to discover that receiver dial backlash and frequency drifting were a real problem at those last hours, when attempting to tune and maintain zero phase difference, so I haywired a multiturn vernier capacitor across the receiver p.t.o. With a large knob attached, I was able to "ride" a calibration marker quite well.
That evening, at the start of the first FMT run, I was "loaded for bear," so I thought. All equipment was properly warmed up; I had attempted to stabilize the house temperature all day and had just closed all windows to prevent drafts. I was skeptical about receiving W1AW on 80 meters while it was still daylight here on the west coast, but there it was, right on schedule. However, the received signal on the scope was all fuzzed up with noise, and was bouncing from a nearby c.w. QSO. Tightening the bandwidth and using rejection tuning helped somewhat, but I still could not discern zero frequency clearly, much less ride zero phase as I had practiced earlier with a nice clean calibration signal.
When key thumps informed me W1AW would now shift to 40 meters(1) I was horrified and thought aloud, "Wait! I'm not finished yet!" But I wheeled everything up to the next approximate frequency, and there was a beautiful, clean signal. But just as I was ready to take some readings, some clown came on with dashes in unison with W1AW, and nearly zero beat. The 20 meter run was received well, except that by then I was so spooked I may have booted that reading also.
The second FMT run at 0430 GMT that evening was also chaotic here at W6FP. I never heard W1AW on 80 meters, not one peep. I had a good crack at a 40-meter reading, but the 20-meter transmission was hopelessly covered by heavy, continuous RTTY. By the end of the second FMT run, I was crushed by the enormity of my failure.
I know I must improve my antennas in order to have more of a received signal to play with, especially on 80 meters.
This will give me a cleaner scope readout. And I must groom my receiver regularly, not just wait until total outages force me to service it. Periodic tube checks, performance tests, and realignments "by the book" were let go here to my detriment, I subsequently observed.
With a clean enough signal at the receiver i.f. output, perhaps I can phase-lock my b.f.o. I have tried mixing the received i.f. back to the signal frequency and counting this, with only marginal success. Perhaps with more work, this idea of measuring "regurgitated" received signals directly might be the answer - to use the receiver as a tunable window only. Thanks to the ARRL for the FMTs as a challenging program, and watch out for us next time! - Dick Carpenter, W6FP, 6634 Ampere Ave., North Hollywood, Calif. 91606.
W1AW does not shift frequency during the FMT. The entire transmission is sent by all transmitters simultaneously. However, the "umpire" measures the frequencies in the order indicated in advance announcements. Actually, the frequency seldom drifts more than a few cycles during the entire transmission period, so any significant differences will be noted only when precision measuring equipment is in use. - Editor