On 9/22/21 7:16 AM, Andrew Kurtz via groups.io wrote:
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.)
The presence of the attic materials (your roof, etc.) reduce the resonant frequency (significantly).?? A simple experiment, at a higher frequency so it's more manageable - make a dipole that's a meter or two long and suspend it in air. Then put it on the ground.
The other resonances aren't at "exact" multiples in a real dipole.
The other thing is that your feedline may be acting as part of the antenna.? And there may be other wires in your attic that are interacting.
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?
A program like SimSmith can show you what happens with the feedline.?? Short answer is that it wont change the frequencies at which X=0, but it does change the impedances a lot.
55 feet of twinlead will change the Z quite a bit - For a quarter wavelength, for instance, Zin * Zout? = Zline^2.? Your 55 feet isn't a quarter wavelength, but let's say it was, and your antenna was actually 70 ohms.? At the end of the feedline you'd see Zout = 300^2/70 = 1.3k. If your feedline were half a wavelength, then Zin=Zout , so you'd see 70 ohms.? A pretty big difference.
C. The nanoVNA is not that accurate.
Assuming you've done the cal right, the NanoVNA is probably telling you the right numbers.
D. The 468 rule of thumb is not that accurate.
468 is a rule of thumb for a dipole "in free-ish space" - Put anything that is has an epsilon that's not 1 near it, and the resonance drops.
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?ions,
Without going through a bunch of calculations for your feedline, it's hard to tell.
