Interesting results... Good, solid data confirming ON4UN's table 7-10 info.
After taking Tom's advice and choosing to measure the impedances correctly
(i.e. lashing the two wires together at each end), I have pretty convincing
evidence that my termination impedance should be about 353 ohms. Since I wound
it for 450 ohms, perhaps this also explains why my feedpoint impedance
consistantly comes in at 40 ohms at most frequencies below 8MHz.
Term Measured
Ohms Min Max Sqrt(RminRmax) Range Error
340 338 372 355 34 15
351 340 367 353 27 2
360 335 363 349 28 -11
379 327 370 348 43 -31
My best guess based on this is 353 ohms. This appears to be consistant with
Table 7-10, which used meters intead of 9 feet and 12" spacing (710 ohms)
instead of my 10" spacing (688 ohms). So my termination transformer will get
wound as close to (688 : 353) as I can get.
By the way, using the Autek VA1's 300 ohm mode, it was difficult to sweep and
document the variation in SWR since all of the above values never fell outside
the 1.14 to 1.24 SWR relative to 300 ohms. So the Square Root of the Minimum
times the Maximum reading over frequency was the best of the two approaches.
In all the measurements, I swept from 600 KHz to well over 8.4 MHz. The data
started to fall apart at about 7.5 MHz, so I limited my sweep to about that
spot. None of the above data points were any where near either frequency
extremes.
Thanks for all the help. I can now fix my beverage and forget about whether it
is 'right' or not. Problems in pattern will no-doubt be sourced to other
issues, rather than termination design.
Ford-N0FP
ford@cmgate.com
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