On 11/23/22 4:28 AM, Barry K3EUI wrote:
Just to check ¡ª¡ª
The resonant frequency of any dipole (center fed, OCF, or end-fed) is determined only but the TOTAL length of the wire, and not be WHERE you feed it.
(And maybe how high above ground or other nearby conductors).
Altering the feed point location only changes the input IMPEDANCE.
Center feds are typically 30-80 ohms (depending on height above ground).
OCF I¡¯ve seen at 200 to 600 ohms.
End-feds can be thousands of ohms.
Typically you see 1/3, 2/3 split (45 ft, 90 ft) for a 135 ft long wire, which puts the impedance around 400 ohms.
I think some argue that for a 120-140 ft overall length, based on the harmonics but you can try different locations depending on the BALUN ratio.
Barry k3eui
Getting back to the original question - can you use the NanoVNA to determine where the feed point is in an offcenter dipole:
NanoVNA can easily determine resonant frequency (where X=0).
Once you've got that, you'll know R at the resonance.
The question is whether there is a consistent relationship between R and "split point" If R = 72, then you know it's in the middle of the dipole. But If there's droop in the dipole, that changes the R (e.g. 120 degrees gives you about 50 ohms; and diameter of the wires changes the resistance as you move out.
So the short answer might be "maybe" -
You could build a model in NEC, and then iterate it until R matches what you measured. Somewhere around, I have a plot of R vs split for a dipole from a series of modeled scenarios. I'll look for it.
Or you could use 4nec2 or one of the other tools
