Last week, several hundred signal-minima observations were taken on a D-KAZ 138 x 22 ft. antenna in a quiet location in Northern Minnesota. Data was gathered mid-day on May 26, 2016 and verified by a second observation mid-day May 29, 2016. A Perseus SDR was used with its “S-Meter” in “Peak” mode.
18 stations on the “back” side of the antenna, from 540 to 1600 kHz, were measured using two null techniques: a variable resistor (pot) at the end of a 100-foot lead-in of Cat-5, and a Vactrol system using the same lead-in. For the pot system a transformer was inserted between the 900-ohm antenna null-port and the Cat-5. This provided the required impedance transformation from 900 to Cat-5’s 110 ohms and ‘balanced’ the feed-line. When the Vactrol replaced the pot the Vactrol resistive element was connected directly to the antenna null-port. “Stand-alone” (“un-nulled”) signals were derived by measuring the signals when the D-KAZ null-port was simply open. Null depths (“F/B ratios”) were calculated by comparing each of these 18 un-nulled readings to their minima obtained by nulling. Three types of null depths were generated: First, each of the 18 stations on the back side of the antenna was individually minimized and the residual signal compared to the “open-null” signal above. Next the null controls were adjusted for minima on a 50-kilowatt station 15 degrees off the back axis of the antenna and approximately 75 miles away. (Once that minima was found, the control was left alone during the measurements of the other 17 stations.) Finally, this drill was repeated for another 50-kilowatt station 20 degrees off axis, 80 miles distant and an octave above the first (830 kHz and 1500 kHz). In a parallel exercise the 18 null-depths were then averaged to provide a figure-of-merit for discussing the relative effectiveness of the two nulling approaches. THREE OBSERVATIONS (confirmed by data available upon request): 1. Average null depths for individual nulls: Here the pot performed slightly better than the Vactrol. It was less effective below 800 kHz and superior above 1400. But the pot required a good deal of readjustment from frequency to frequency. This would not be an issue for single-frequency DX-ing and in fact, from a tactile standpoint, the null pot seemed capable of better granularity. 2. Average null depths across the band for a single null setting: The Vactrol required less readjustment from frequency to frequency and delivered a more-uniform null linearity across the band. Nulls averaged 3 to 5 db better with the Vactrol: With 830 minimized, the Vactrol nulls were more uniform below 830 and above 1180 (the devices performed about equally over the middle of the MW band). With the null at 1500 the Vactrol did better than the pot when below 830; above 860 the null pot did equally well or better. 3. Overall average: (This may be a “figure-of-merit” but don’t weight it too heavily since it includes stations well off into side-lobe territory): Individual-frequency adjust: Vactrol average null depth: 28.3 db; Pot 34.1 db 830 fixed null: Vactrol average null-depth: 26.7 db; Pot 24.7 db 1500 fixed null: Vactrol average null-depth: 26.3 db; Pot 22.6 db These numbers are within statistical error boundaries and the vagaries of signal propagation (even within the short measurement-time window). Single-frequency “set and forget” null performance is of obvious interest for wide-band recording. The Vactrol was seen to be the technology of choice by a small measured margin. HOWEVER we possess insufficient comparative data as to the performance of a pot over longer lead-ins. Your writer once had misgivings about any extension of copper off the D-KAZ ends; came to find that 100 feet or so of Cat-5 seemed okay…but got into trouble with much longer lead-ins. Several weeks ago Mark Connelly proposed a series of measurements to prove/disprove the “cleanliness” of longer-lead-in null-pot arrangements. Using his format we ran a skeleton series of measurements comparing a 50-foot Cat-5 run to a 420-foot run. We learned that while Vactrol performance stayed within 1 db for each lead-in length, the pot’s null-depth average was 12 to 15 db worse on the longer run. To finish this research, more measurements will be made this week, conforming Mark’s original format to the procedures in the above report. Thus we’ll repeat the same three evaluations for null-depth averages…but we’ll do this on 100 foot, 200 foot, 300 foot and 420 foot Cat-5 pot runs, as well as the Vactrol; and all in the same time window. The writer E. L. Doctorow said “If an experiment was valid, it could be repeated by others, to yield the same results.” PLEASE LET ME KNOW ASAP IF YOU WANT ANY OTHER MEASUREMENT CRITERIA ADDED TO THE TESTS! Cheers! Mark Durenberger, CPBE _______________________________________________ IRCA mailing list [email protected] http://montreal.kotalampi.com/mailman/listinfo/irca Be sure to register now for the Joint DX Convention Kansas City, September 9 to 11. Hotel space is filling up. Registration info: http://www.nrcdxas.org Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: [email protected]
