I have a dipole antenna in my attic, made of bare 14 gauge copper wire. It is 49.916 feet long, and the feed point is within 1.5 inches of the center. Thus, the ¡°rule¡± that f=468/L suggests the optimal, tuned frequency should be 9.38 MHz (right?). And at that frequency X should be about 0 and R should be radiative R at about 75 ohms (right?). 55 feet of ¡°300 ohm¡± twin-lead connects this balanced antenna to my ¡°shack¡± in the basement.
In the basement, I analyzed the "twin-lead plus dipole" using my nanoVNA-H4, calibrated with 100 points each between 0.5 and 5 MHz, then 5 and 10 MHz, then 10 and 15 MHz, then 15 and 25 MHz. The outputs looked very reasonable (I think): Smith charts making nice circles, reactance X rising periodically from capacitive to inductive smoothly, then crashing very fast from inductive to capacitive, and real R and SWR lowest where X is rising and quite high where X is falling.
But here comes the question: minimum R and SWR, and X crossing 0 from capacitive to inductive, occurred at 3.1, 8.1, 11.5, 17.3, and 23.6 MHz ¡ª nowheres near the expected 9.38 MHz. Also, I can¡¯t see a regular multiple of wavelength between those frequencies. Here are possible answers:
A. My understanding is incorrect. (I am very new to RF stuff.)
B. The feed line really changes things. And do you advise some sort of balun between antenna and twin-lead or twin-lead and receiver?
C. The nanoVNA is not that accurate.
D. The 468 rule of thumb is not that accurate.
By the way, the minimum resistance measured at each apparent tuned frequency was around 30 ohms, versus the expected 76 or so. Is this significant?
Andy
