Initial examination of my overnight reports suggested a poor session. WSPR spot counts were down from the previous two sessions but further review suggested many more reports at CW levels, better than -10 dB S/N over significantly greater distances. The band was very quiet here throughout the session but only WD2XSH/15 and WH2XGP were decoded overnight.
Geomagnetic conditions were calm and while solar wind velocity is currently below 400 km/s, the Bz this morning is pointing to the South. DST reports suggest diminished band conditions overall.
Phil, VE3CIQ, also reported only decoding two stations during the session and offers these reports from his station:
Neil, W0YSE/7 / WG2XSV, operated reduced power during the session and provides these details for his activity.
There was massive band data pollution by a station in Australia, initially reported by Roger, VK4YB, in the ON4KST chat/logger. I am excluding the North American and Australian map in this report because they are absolutely worthless today. Hopefully the station at fault can get his problems resolved quickly. European and Japanese WSPR breakdowns follow:
There were no WSPR reports from the trans-Atlantic or trans-African paths. UA0SNV was present but no reports were found in the WSPRnet database.
Once again, Eden, ZF1EJ, received my station as the sole 630-meters WSPR signal in the Caribbean.
Laurence, KL7L / WE2XPQ, successfully reported WH2XCR and WH2XGP overnight through the persistent ionic fog.
Merv, K9FD/KH6 / WH2XCR, was reported by a full compliment of VK stations including VK7ZL in Tasmania. Two-way reports were shared with Phil, VK3ELV, and Roger, VK4YB, and Merv successfully decoded Berndt, VK5ABN, once again. Reports from David, VK2DDI, and John, VK2XGJ, were also received including numerous post-sunrise reports by John. This was a good session for the salt water path across the Pacific. The lack of an recent open path to Japan remains a mystery.
Jim, W5EST, goes to the mailbag once again to examine modeling from another angle in this discussion entitled, “DESIGNING ATU FROM MODELING CAN CAUSE GRIEF”:
“Thanks to Joe WI2XBQ in California for today’s topic! He writes:
‘I gave EZNEC a try for a simple 13m (43’) vertical at .475Mhz… The Rr value is reasonably close to the classical formula results .16 vs eznec .19 The reactance value it reports is -j1966. This equivalent to 168pf over salt water.
My actual measured value over salt marsh with salt water within 20′ is 126pf or
-j2659. One of the popular web vertical antenna calculators reports reactance as 78pf or -j4296.
I tried changing the soil conductivity parameters, but the reported reactance did not change. That included changing ground type from perfect to real. Am I doing something wrong or is the program up against some limit?
I think EZNEC is a great way to compare performance of antennas, but using it to design a matching network is going to cause a lot of grief.’
Jim W5EST Reply:
I think this cautionary message based on measurements is sound advice. An actual 630m antenna, its ground, and its surroundings are likely to differ from our best attempts to specify them accurately to a modeling program. Modeling can be very helpful for comparative purposes.
Since actual base impedance may significantly depart from modeling predictions, better wisdom would suggest measuring the base impedance after selecting and constructing the antenna – and only then designing the ATU.
Regarding the reactance under a change of ground type, EZ-NEC’s Help section does recognize “Ground has a profound effect on the antenna pattern, and sometimes on its efficiency and feedpoint impedance.” (Help menu “Building the Model” subsection “Modeling Ground” paragraph “About Ground Models.”)
The MININEC type of real ground probably got selected. In the Help paragraph “Real Ground,” we learn that “…the MININEC-type model takes ground conductivity and dielectric constant (relative permittivity) into consideration when it calculates the pattern and gain. However, it considers the ground to be perfect when calculating the impedances and currents.” [Italics mine.] Consequently, the same impedance resulted for the vertical antenna as modeled over perfect ground and as modeled with the MININEC type of real ground.
Accordingly, in the control information window under “Ground Type,” click both “Real” and “High Accuracy.” Adjust the ground conductivity by trial and error until you get the actual antenna base resistance you measured with an antenna analyzer. You will see that the antenna reactance will have changed, but still not by much.
See if this double-header explanation-why makes sense: 1) Ground resistance is in parallel with ground capacitance and shunts it. 2) Ground capacitance is generally much higher than the antenna capacitance. Since the antenna capacitance to perfect ground together with the real ground capacitance are approximately in series, the overall antenna system capacitance and its reactance don’t change much with ground type.
Why then is the modeling program giving such different reactance results compared to those you measured? Before you give up on the program, consider a couple of possibilities.
First, even with “High Accuracy” ground is assumed level. If you have some down-sloping actual ground, for instance, I’d suggest adjusting the “Wires” specification to fictitiously set the antenna base to, say, Z=3 (a yard) above ground. Raise the antenna top by the same amount. That makes the vertical antenna conductor farther from the ground around it, and the reactance will estimate higher.
Second, double check the vertical antenna’s wire or tube diameter that’s been entered in the “Wires” specification. If the diameter in “Wires” was specified larger than actual, then the reactance will come out low for that reason as well.
Emails are greatly appreciated. We look forward to more, from all readers with good information to submit. GL!”
Additions, corrections, clarifications, etc? Send me a message on the Contact page or directly to KB5NJD <at> gmail dot (com)!